"""
"Copyright 2020 The Microsoft DeepSpeed Team.
Licensed under the MIT license.
"""

import math
from collections import OrderedDict

import torch
import torch.distributed as dist

try:
    from deepspeed.utils.debug import debug_module2name_id, debug_param2name_id, debug_param2name_id_numel, \
        debug_param2name_id_shape_device, debug_module2name_class
    from deepspeed.ops.adam import DeepSpeedCPUAdam
    from deepspeed.ops.op_builder import UtilsBuilder
    from deepspeed.runtime.swap_tensor.partitioned_optimizer_swapper import PartitionedOptimizerSwapper
    from deepspeed.runtime.swap_tensor.pipelined_optimizer_swapper import PipelinedOptimizerSwapper
    from deepspeed.runtime.utils import is_model_parallel_parameter
    from deepspeed.runtime.zero.constants import ZERO_OPTIMIZATION_WEIGHTS
    from deepspeed.runtime.zero.partition_parameters import *
    from deepspeed.runtime.zero.partition_parameters import _init_external_params
except ImportError:
    pass

from torch._six import inf
from torch.distributed.distributed_c10d import _get_global_rank
from torch.optim import Optimizer

from colossalai.core import global_context as gpc
from colossalai.utils import report_memory_usage
from .loss_scaler import LossScaler, DynamicLossScaler
from colossalai.context import ParallelMode

# Toggle this to true to enable correctness test
# with gradient partitioning and without
pg_correctness_test = False

FWD_MODULE_STACK = list()


def print_rank_0(message, debug=False, force=False):
    rank = torch.distributed.get_rank()
    if rank == 0 and (debug or force):
        print(message)
    # other variations
    # - print for all ranks w/o interleaving
    # printflock(f"[{rank}] {message}")
    # - print to log file per rank
    # log_rank_file(rank, message)


def input(msg):
    return


def split_half_float_double(tensors):
    dtypes = [
        "torch.cuda.HalfTensor",
        "torch.cuda.FloatTensor",
        "torch.cuda.DoubleTensor"
    ]
    buckets = []
    for i, dtype in enumerate(dtypes):
        bucket = [t for t in tensors if t.type() == dtype]
        if bucket:
            buckets.append(bucket)
    return buckets


def isclose(a, b, rtol=1e-09, atol=0.0):
    return abs(a - b) <= max(rtol * max(abs(a), abs(b)), atol)


def lcm(x, y):
    from fractions import gcd  # or can import gcd from `math` in Python 3
    return x * y // gcd(x, y)


def move_to_cpu(tensor_list):
    for tensor in tensor_list:
        tensor.data = tensor.data.cpu()


def get_all_parameters(sub_module, recurse=False):
    return itertools.chain(sub_module.named_parameters(recurse=recurse),
                           sub_module.ds_external_parameters())


# apply torch.autograd.Function that calls a backward_function to tensors in output
def _apply_to_tensors_only(module, functional, backward_function, outputs):
    if type(outputs) is tuple:
        touched_outputs = []
        for output in outputs:
            touched_output = _apply_to_tensors_only(module,
                                                    functional,
                                                    backward_function,
                                                    output)
            touched_outputs.append(touched_output)
        return tuple(touched_outputs)
    elif type(outputs) is torch.Tensor:
        return functional.apply(module, backward_function, outputs)
    else:
        return outputs


# for each tensor in outputs run the forward_funciton and register backward_function as hook
def _apply_forward_and_backward_to_tensors_only(module,
                                                forward_function,
                                                backward_function,
                                                outputs):
    if type(outputs) is tuple:
        touched_outputs = []
        for output in outputs:
            touched_output = _apply_forward_and_backward_to_tensors_only(
                module,
                forward_function,
                backward_function,
                output)
            touched_outputs.append(touched_output)
        return tuple(touched_outputs)
    elif type(outputs) is torch.Tensor:
        forward_function(outputs)
        if outputs.requires_grad:
            outputs.register_hook(backward_function)
        return outputs
    else:
        return outputs


class ZeROOrderedDict(OrderedDict):
    def __init__(self, parent_module, *args, **kwargs):
        """A replacement for ``collections.OrderedDict`` to detect external ZeRO params.

        Args:
            parent_module (``collections.OrderedDict``): the collection to replace
        """

        super().__init__(*args, **kwargs)
        self._parent_module = parent_module
        self._in_forward = False

    def __getitem__(self, key):
        param = super().__getitem__(key)

        # Params can be registered as None (e.g., bias)
        if param is None:
            return param

        if param.ds_status == ZeroParamStatus.NOT_AVAILABLE:
            if self._parent_module._parameters._in_forward:
                print_rank_0(f'Registering external parameter from getter {key}',
                             force=False)
                register_external_parameter(FWD_MODULE_STACK[-1], param)
                param.all_gather()

        return param


def _inject_parameters(module, cls):
    for module in module.modules():
        if cls == ZeROOrderedDict:
            new_param = cls(parent_module=module)
        else:
            new_param = cls()

        for key, param in module._parameters.items():
            new_param[key] = param
        module._parameters = new_param


# TODO Needs to be implemented
class PrefetchCoordinator(object):
    def __init__(self):
        # step_id keeps track of the number of sub-modules invoked so far
        # the step_id is tracking forward and backward sequence of sub-modules
        self.step_id = 0

        # stores the sequence of sub modules in forward+backward pass
        self.sub_module_trace = []

        # maps sub_module id to submodule objects
        self.id_to_sub_module_map = {}

        # stores the total number of parmeters in each sub_module
        self.id_to_sub_module_size_map = {}

        self.trace_completed = False

        self.most_recent_sub_module_step = {}

        # reuse distances
        self.reuse_numel_for_step_id = {}

    def record_trace(self, sub_module):
        if not self.trace_completed:
            self.sub_module_trace.append(sub_module.id)
            self.id_to_sub_module_map[sub_module.id] = sub_module

    def print_trace(self):
        print_rank_0(
            f"The module trace is : {[self.id_to_sub_module_map[module_id].id for module_id in self.sub_module_trace]}"
        )

    def increment_step(self, sub_module):
        self.most_recent_sub_module_step[sub_module.id] = self.step_id
        self.step_id += 1

    def reset_step(self):
        self.step_id = 0

    # returns the next numel parameters that will be used next but are not available or inflight
    def get_params_to_prefetch(self, sub_module, numel=2000000):

        # numel_in_sub_module = 0
        # for name, param in sub_module.named_parameters(recurse=False):
        #     numel_in_sub_module += param.ds_numel

        # #if numel_in_sub_module < (numel // 2):
        #    return []

        # tracing failed. The sub_module passed at the step_id must match with the sub_module during tracing
        if sub_module.id != self.sub_module_trace[self.step_id]:
            print_rank_0(
                f"Tracing failed. Prefetching is disabled at sub-module: {debug_module2name_id(sub_module)}"
            )
            return []

        params_to_prefetch = []
        total_numel_to_prefetch = 0

        for i in range(self.step_id, len(self.sub_module_trace)):
            module_id = self.sub_module_trace[i]
            for _, param in get_all_parameters(self.id_to_sub_module_map[module_id]):
                if param.ds_status is ZeroParamStatus.NOT_AVAILABLE and (
                        param.ds_id not in [p.ds_id for p in params_to_prefetch]):
                    params_to_prefetch.append(param)
                    total_numel_to_prefetch += param.ds_numel
                    # print_rank_0(f"Total numel to prefetch: {total_numel_to_prefetch}. Param: {param.ds_shape} and numel {param.ds_numel}, numel limit {numel}")
                    # and total_numel_to_prefetch > (numel_in_sub_module // 2):
                    if total_numel_to_prefetch >= numel:
                        return params_to_prefetch

        return params_to_prefetch

    # checks if this sub_module will be used again and if so then returns the number of elements
    # in the parameters used between this sub_module and the reuse of this sub_module
    def get_reuse_distance_in_numel(self, sub_module, sub_module_step_id=None):
        # assert is_forward is not None, "is_forward must be set to True for Forward Propagation and False for backward Propagation"
        is_there_reuse = False
        reuse_distance_in_numel = 1000000000000

        # set the appropriate trace
        trace = self.sub_module_trace
        total_steps = len(trace)
        if sub_module_step_id is None:
            sub_module_step_id = self.most_recent_sub_module_step[sub_module.id]

        # tracing failed. The sub_module passed at the step_id must match with the sub_module during tracing
        if sub_module.id != trace[sub_module_step_id]:
            print_rank_0(
                f"Tracing failed. Cannot tell if the sub_module: {sub_module.id} is reused"
            )
            return reuse_distance_in_numel

        # return cached value
        if sub_module_step_id in self.reuse_numel_for_step_id:
            return self.reuse_numel_for_step_id[sub_module_step_id]

        start_step = self.step_id
        print_rank_0(f"Step id is {self.step_id} ")
        for step_id in range(start_step, total_steps):
            print_rank_0(
                f"Trace id {trace[step_id]} and sub_module id {sub_module.id}")
            if sub_module.id == trace[step_id]:
                end_step = step_id

                is_there_reuse = True
                reuse_distance_in_numel = self._distance_in_numel(
                    start_step,
                    end_step,
                    trace)
                break

        self.reuse_numel_for_step_id[sub_module_step_id] = reuse_distance_in_numel

        return reuse_distance_in_numel

    def _distance_in_numel(self, start_step, end_step, trace):
        distance_in_numel = 0
        for step_id in range(start_step, end_step):
            module_id = trace[step_id]
            for _, param in self.id_to_sub_module_map[module_id].named_parameters(recurse=False):
                distance_in_numel += param.ds_numel
            for _, param in self.id_to_sub_module_map[module_id].ds_external_parameters():
                distance_in_numel += param.ds_numel
        return distance_in_numel


class PartitionedParameterCoordinator(object):
    def __init__(self,
                 comm_stream=None,
                 max_reuse_distance_in_numel=500000000,
                 max_available_parameters_in_numel=700000000):

        self.in_flight_handles = []
        self.params_in_flight = []
        self.comm_stream = comm_stream if comm_stream is not None else torch.cuda.current_stream(
        )
        self.prefetch_coordinator = PrefetchCoordinator()
        self.hierarchy = 0

        self.total_available_parameter_numel = 0
        self.max_available_parameters_in_numel = max_available_parameters_in_numel

        # max distance between two use of the module beyond which module is released
        self.max_reuse_distance_in_numel = max_reuse_distance_in_numel

    def _increment_available_parameter_numel(self, increment):
        self.total_available_parameter_numel += increment

    def _decrement_available_parameter_numel(self, decrement):
        self.total_available_parameter_numel -= decrement

    '''-----------------------Tracing and Prefetching ---------------'''

    def record_trace(self, sub_module):
        self.prefetch_coordinator.record_trace(sub_module)

    def finish_tracing(self, print_trace=False):
        self.prefetch_coordinator.trace_completed = True

        if print_trace:
            self.prefetch_coordinator.print_trace()

    # swap in parameter partitions from nvme for those parameters that will be used
    # after the ones that are already being prefetched into full parameters
    def _prefetch_nvme_param_partitions(self, sub_module, params_in_flight):
        numel_in_flight = sum(
            [param.ds_tensor.ds_numel for param in params_in_flight])
        upcoming_param_list = self.prefetch_coordinator.get_params_to_prefetch(
            sub_module,
            numel=2 * numel_in_flight)
        swap_in_params = []
        for param in upcoming_param_list:
            if len(swap_in_params) >= param.nvme_swapper.available_swap_in_buffers():
                break
            if param.ds_tensor.status == PartitionedParamStatus.NOT_AVAILABLE:
                swap_in_params.append(param)

        if len(swap_in_params) > 0:
            swap_in_params[0].nvme_swapper.swap_in(
                swap_in_params, async_op=True)

    # Pre fetches the parameters for sub_modules that comes after
    #  the current sub_module. This call is asynchronous
    def prefetch_next_sub_modules(self, sub_module, numel=5000000, nvme=False):

        params_to_prefetch = []
        if not self.prefetch_coordinator.trace_completed:
            return params_to_prefetch

        # prefetch if there is no current prefetching in flight
        if not self.in_flight_handles and self.total_available_parameter_numel < self.max_available_parameters_in_numel:
            params_to_prefetch = self.prefetch_coordinator.get_params_to_prefetch(
                sub_module,
                numel=numel)

            self._all_gather(params_to_prefetch, async_op=True)
            for param in params_to_prefetch:
                param.ds_status = ZeroParamStatus.INFLIGHT

                # keeping track of number of elements consumed by available parmaeters
                self._increment_available_parameter_numel(param.ds_numel)

            if nvme:
                self._prefetch_nvme_param_partitions(
                    sub_module, params_to_prefetch)

        self._print_prefetch_elements_info(sub_module, params_to_prefetch)
        print_rank_0(
            f"{'--' * self.hierarchy}--PreFetching parameters {[param.ds_id for param in params_to_prefetch]} and available {self.total_available_parameter_numel}, max limit {self.max_available_parameters_in_numel}",
            force=False)

    def _print_prefetch_elements_info(self, sub_module, params_to_prefetch):
        sub_module_numel = 0.0
        for name, param in sub_module.named_parameters(recurse=False):
            sub_module_numel += param.ds_numel
        numel_being_prefetched = 0
        for param in params_to_prefetch:
            numel_being_prefetched = param.ds_numel
        print_rank_0(
            f"{'--' * self.hierarchy}--PreFetching  {numel_being_prefetched} numels and number of numel in the next sub module is {sub_module_numel}",
            force=False)

    def increment_step(self, sub_module):
        self.prefetch_coordinator.increment_step(sub_module)

    def reset_step(self):
        self.prefetch_coordinator.reset_step()

    '''----------------------------------------------------------------------'''

    # Fetches the parameters in the sub_module
    # This call is blocking
    def fetch_sub_module(self, sub_module):
        partitioned_params = []
        params_in_flight = False
        print_rank_0(
            f"{'--' * self.hierarchy}Fetching params in module {debug_module2name_class(sub_module)}"
        )
        params_to_fetch = [
            param for _,
            param in sub_module.named_parameters(recurse=False)
        ]
        # print([n for n,p in sub_module.named_parameters(recurse=False)])

        if hasattr(sub_module, 'ds_external_parameters'):
            print_rank_0(
                f"{'--' * self.hierarchy}--Fetching external parameters {sub_module.ds_external_parameters()}"
            )
            params_to_fetch += [
                param for _,
                param in sub_module.ds_external_parameters()
            ]
        # for _, param in sub_module.named_parameters(recurse=False):
        for param in params_to_fetch:
            param.ds_active_sub_modules += 1
            print_rank_0(
                f"{'--' * self.hierarchy}--Fetching parameters {debug_param2name_id_shape(param)} with active sub modules {param.ds_active_sub_modules}"
            )

            if param.ds_status == ZeroParamStatus.AVAILABLE:
                print_rank_0(
                    f"{'--' * self.hierarchy}--Parameter {debug_param2name_id(param)} is already available"
                )

            if param.ds_status == ZeroParamStatus.NOT_AVAILABLE:
                print_rank_0(
                    f"{'--' * self.hierarchy}--Parameter {debug_param2name_id(param)} is being fetched"
                )
                partitioned_params.append(param)

                # keeping track of number of elements consumed by available parmaeters
                self._increment_available_parameter_numel(param.ds_numel)
                print_rank_0(f"Incrementing with parameter id {param.ds_id}")

            if param.ds_status == ZeroParamStatus.INFLIGHT:
                params_in_flight = True
                print_rank_0(
                    f"{'--' * self.hierarchy}--Parameters {debug_param2name_id(param)} is already in flight (prefetched)"
                )
        self.hierarchy += 1

        # parameters are partitioned and need to be allgathered
        self._all_gather(partitioned_params, async_op=True)

        # parameters are inflight and communication needs to be completed
        if partitioned_params or params_in_flight:
            self._synchronize_communication()

        for _, param in sub_module.named_parameters(recurse=False):
            param.ds_status = ZeroParamStatus.AVAILABLE
            print_rank_0(
                f"Param {debug_param2name_id_shape_device(param)} norm={param.norm()}",
                force=False)
        # print_rank_0(f"After fetching (id, shape, device): {[(param.ds_id, param.shape, param.device) for param in sub_module.named_parameters(recurse=False)]}")

    def release_sub_module(self, sub_module):
        self.hierarchy -= 1
        print_rank_0(
            f"{'--' * self.hierarchy}Releasing params in module {debug_module2name_class(sub_module)}"
        )
        params_to_release = [
            param for _,
            param in sub_module.named_parameters(recurse=False)
        ]

        if hasattr(sub_module, 'ds_external_parameters'):
            # print_rank_0(f"Releasing external parameters {sub_module.ds_external_parameters()}")
            params_to_release += [
                param for _,
                param in sub_module.ds_external_parameters()
            ]

        # for _, param in sub_module.named_parameters(recurse=False):
        for param in params_to_release:
            param.ds_active_sub_modules -= 1
            if not param.ds_active_sub_modules and not self._keep_for_later(
                    sub_module) and not param.ds_persist:

                print_rank_0(
                    f"{'--' * self.hierarchy}--Releasing parameter {debug_param2name_id_numel(param)} active sub modules {param.ds_active_sub_modules} and keep for later {self._keep_for_later(sub_module)}",
                    force=False)

                # Keeping track of number of elements that are consumed by available parameters
                self._decrement_available_parameter_numel(param.ds_numel)

                # report_memory_usage(
                #     f"Before releasing param {debug_param2name_id_numel(param)}",
                # )
                param.partition(hierarchy=self.hierarchy)

                # report_memory_usage(
                #     f"After releasing param {debug_param2name_id_numel(param)}",
                # )

                param.ds_status = ZeroParamStatus.NOT_AVAILABLE
            else:
                print_rank_0(
                    f"{'--' * self.hierarchy}--Did not release param {debug_param2name_id_numel(param)} with active sub modules {param.ds_active_sub_modules}, keep for later={self._keep_for_later(sub_module)} and persistence={param.ds_persist}",
                    force=False)

    def release_and_reset_parameter(self, param):
        param.ds_active_sub_modules = 0
        if param.ds_status == ZeroParamStatus.AVAILABLE:
            print_rank_0(
                f"Releasing unpartitioned param {debug_param2name_id_numel(param)} active sub-modules {param.ds_active_sub_modules} and persisitence {param.ds_persist}"
            )
            self._decrement_available_parameter_numel(param.ds_numel)
            param.partition()

    def _keep_for_later(self, sub_module):
        if not self.prefetch_coordinator.trace_completed:
            return False
        if self.max_reuse_distance_in_numel == 0:
            return False
        reuse_distance_in_numel = self.prefetch_coordinator.get_reuse_distance_in_numel(
            sub_module)
        # print_rank_0(f"Reuse distance and numel for sub_module id {sub_module.id} is {reuse_distance_in_numel}")
        return reuse_distance_in_numel < self.max_reuse_distance_in_numel

    def _all_gather(self, partitioned_params, async_op=False):
        with torch.cuda.stream(self.comm_stream):
            handles = partitioned_params[0].all_gather(
                param_list=partitioned_params,
                async_op=async_op,
                hierarchy=self.hierarchy) if partitioned_params else None

        if handles is not None:
            self.in_flight_handles.extend(handles)
            self.params_in_flight.extend(partitioned_params)

    def _synchronize_communication(self, synchronize_streams=True):
        assert len(self.params_in_flight) == len(self.in_flight_handles)
        for handle, param in zip(self.in_flight_handles, self.params_in_flight):
            if handle is not None:
                with torch.cuda.stream(self.comm_stream):
                    handle.wait()
            param.ds_status = ZeroParamStatus.AVAILABLE
        self.comm_stream.synchronize()
        torch.cuda.synchronize() if synchronize_streams else None
        self.in_flight_handles = []
        self.params_in_flight = []


class PreBackwardFunction(torch.autograd.Function):
    @staticmethod
    def forward(ctx, module, pre_backward_function, outputs):
        ctx.module = module
        ctx.pre_backward_function = pre_backward_function
        if not hasattr(module, "applied_pre_backward_ref_cnt"):
            module.applied_pre_backward_ref_cnt = 0
        module.applied_pre_backward_ref_cnt += 1
        # print(f"After Forward: {ctx.module.__class__.__name__}")
        outputs = outputs.detach()
        return outputs

    @staticmethod
    def backward(ctx, *args):
        # print(f"Before Backward: {ctx.module.__class__.__name__}")
        ctx.pre_backward_function(ctx.module)
        return (None, None) + args


class PostBackwardFunction(torch.autograd.Function):
    @staticmethod
    def forward(ctx, module, pre_backward_function, output):
        ctx.module = module
        if output.requires_grad:
            # TODO SOME TIMES post backward does not report_memory_usage()ered debug in detail
            # Should only cause increase in memory not correctness issue
            # if output.grad_fn.__class__.__name__ == 'ViewBackward':
            #    ctx.view=True
            #    print(f"Warning view tensor for input to module : {module.__class__.__name__}. Backward hooks may not trigger properly")
            # assert len(module.parameters(recurse=False)), "The input tensor to the module is a view, and autograd Function or register_hook is not triggered with view tensors."
            # if module.ds_grads_remaining == 0:
            #    print(f"Before Forward: {ctx.module.__class__.__name__}")
            module.ds_grads_remaining += 1
            ctx.pre_backward_function = pre_backward_function
        output = output.detach()
        return output

    @staticmethod
    def backward(ctx, *args):
        ctx.module.ds_grads_remaining = ctx.module.ds_grads_remaining - 1
        if ctx.module.ds_grads_remaining == 0:
            ctx.pre_backward_function(ctx.module)
            # print(f"After Backward: {ctx.module.__class__.__name__}")
        return (None, None) + args


INITIAL_MICRO_STEP_ID = -1


class ZeroRedundancyOptimizer_Level_3(Optimizer):
    """
    ZeroRedundancyOptimizer_Level_3 designed to reduce the memory footprint
    required for training large deep learning models.

    For more details please report_memory_usage() Optimization Towards Training A Trillion Parameter Models
    https://arxiv.org/abs/1910.02054

    """

    def __init__(self,
                 module,
                 init_optimizer,
                 dp_paralllel_mode=ParallelMode.DATA,
                 static_loss_scale=1.0,
                 dynamic_loss_scale=False,
                 dynamic_loss_args=None,
                 verbose=False,
                 contiguous_gradients=True,
                 reduce_bucket_size=500000000,
                 prefetch_bucket_size=50000000,
                 max_reuse_distance=1000000000,
                 max_live_parameters=1000000000,
                 param_persistence_threshold=100000,
                 reduce_scatter=True,
                 overlap_comm=False,
                 offload_optimizer_config=None,
                 offload_param_config=None,
                 sub_group_size=1000000000000,
                 clip_grad=0.0,
                 allreduce_always_fp32=False,
                 postscale_gradients=True,
                 gradient_predivide_factor=1.0,
                 gradient_accumulation_steps=1,
                 aio_config=None,
                 dtype=torch.half):
        # mpu = None
        # mpu is removed from the parameter list
        # tensor parallel will be automatically detected later

        # LSG: default parameter for compatibility
        elastic_checkpoint = False
        timers = None
        dp_process_group = gpc.get_group(dp_paralllel_mode)
        self.verbose = verbose

        # LSG: in deepspeed deepspeed/runtime/zero/partition_parameters.py,
        # self.local_device = torch.device('cuda:{}'.format(os.environ["LOCAL_RANK"]))
        # the local device is obtained by env var LOCAL_RANK, thus, need to change this
        # env var on the spot as LOCAL_RANK may not be present
        if not 'LOCAL_RANK' in os.environ:
            device_id = gpc.get_global_rank() % torch.cuda.device_count()
            os.environ['LOCAL_RANK'] = str(device_id)

        # self.local_device = torch.device('cuda:{}'.format(os.environ["LOCAL_RANK"]))

        if self.verbose:
            report_memory_usage("Stage 3 initialize beginning")

            if dist.get_rank() == 0:
                print(f"Reduce bucket size {reduce_bucket_size}")
                print(f"Allgather bucket size {prefetch_bucket_size}")
        # The fused optimizer does all the work. We need this layer for two reason:
        # 1. maintain same user API from apex.fp16_utils
        # 2. keep common stuff here in case we need to add ne552w fused optimizer later

        # differences from apex.fp16_utils:
        # - assume all model params in fp16
        # - assume all params requires grad
        # - flat by groups, not keeping state. TODO: remove state explicitly?
        # - master gard and unflat master weight never exist. TODO: a way to save out unflat master?
        if not torch.cuda.is_available:
            raise SystemError("Cannot use fp16 without CUDA.")
        self.optimizer = init_optimizer
        self.defaults = init_optimizer.defaults

        # Load pre-built or JIT compile (un)flatten ops
        util_ops = UtilsBuilder().load()
        self.flatten = util_ops.flatten
        self.unflatten = util_ops.unflatten
        self.dtype = dtype

        if not all(is_zero_param(p) for p in module.parameters()):
            ds_config = {
                "train_micro_batch_size_per_gpu": 1,
                "gradient_accumulation_steps": 1,
                "zero_optimization": {
                    "offload_param": offload_param_config,
                    "offload_optimizer": offload_optimizer_config,
                },
                "aio": aio_config
            }

            if offload_param_config is not None:
                remote_device = offload_param_config['device']
            else:
                remote_device = None

            if offload_optimizer_config is not None:
                pin_memory = offload_optimizer_config.get(OFFLOAD_OPTIMIZER_PIN_MEMORY, False)
            else:
                pin_memory = False

            group = None
            if gpc.is_initialized(ParallelMode.DATA):
                group = gpc.get_group(ParallelMode.DATA)
            Init(module=module, data_parallel_group=group, dtype=self.dtype,
                 remote_device=remote_device, config_dict_or_path=ds_config,
                 pin_memory=pin_memory)

        for m in module.modules():
            _init_external_params(m)

        self.module = module
        self.elastic_checkpoint = elastic_checkpoint
        self.overlap_comm = overlap_comm

        # Replace ._parameters with a new class to enable auto-registration of
        # external parameters
        _inject_parameters(module, ZeROOrderedDict)

        if self.overlap_comm:
            self.gpu_sum = torch.zeros(1, dtype=torch.float).cuda()

        ###################### offload optimizer setup ##################################
        self.optimizer_swapper = None
        self.swap_optimizer = False

        self.offload_optimizer = False
        self.offload_optimizer_pin_memory = False
        self.offload_optimizer_fast_init = False
        if offload_optimizer_config is not None:
            self.offload_optimizer = True
            self.offload_optimizer_pin_memory = offload_optimizer_config[
                OFFLOAD_OPTIMIZER_PIN_MEMORY]
            self.swap_optimizer = offload_optimizer_config[
                OFFLOAD_OPTIMIZER_DEVICE] == OFFLOAD_NVME_DEVICE
            self.offload_optimizer_fast_init = offload_optimizer_config[
                OFFLOAD_OPTIMIZER_FAST_INIT]

        ###################### offload param setup ##################################
        self.offload_param = False
        self.offload_param_pin_memory = False
        self.params_in_nvme_and_cpu = False
        self.max_params_in_cpu = 0
        if offload_param_config is not None:
            assert self.offload_optimizer, "parameter offload is only available with optimizer state offload"
            self.offload_param = True
            self.offload_param_pin_memory = offload_param_config[
                OFFLOAD_PARAM_PIN_MEMORY]
            self.params_in_nvme_and_cpu = offload_param_config[
                OFFLOAD_PARAM_DEVICE] == OFFLOAD_NVME_DEVICE
            self.max_params_in_cpu = offload_param_config[OFFLOAD_PARAM_MAX_IN_CPU]
            if self.verbose:
                print_rank_0(
                    f"FP16 params swapping is {self.params_in_nvme_and_cpu}, Max params in CPU is {self.max_params_in_cpu}",
                    force=False)

        self.deepspeed_adam_offload = (self.offload_optimizer
                                       and type(init_optimizer) == DeepSpeedCPUAdam)

        self.device = torch.cuda.current_device(
        ) if not self.offload_optimizer else OFFLOAD_CPU_DEVICE
        ############################################################################

        if self.verbose:
            report_memory_usage("Before Partitioned Parameter Coordinator")

        fetch_stream = torch.cuda.Stream() if self.overlap_comm else None
        self.param_coordinator = PartitionedParameterCoordinator(
            comm_stream=fetch_stream,
            max_reuse_distance_in_numel=int(max_reuse_distance),
            max_available_parameters_in_numel=int(max_live_parameters))

        if self.verbose:
            report_memory_usage("After Partitioned Parameter Coordinator")

        # self.param_coordinator = PartitionedParameterCoordinator(comm_stream=torch.cuda.Stream())
        # -------------Stage 3 Setup-------------------#
        # parameters smaller than the threshold will be collectively gathered at the
        # end of the optimizer step and will be kept till the end of the backward pass
        # TODO maybe worth just replicating these parameters and doing all reduce for them
        self.persistence_threshold = int(param_persistence_threshold)

        self.persistent_parameters = self.persistent_parameters()

        self.setup_zero_stage3_hooks()

        # resetting ds_tensor just in case parameters have been changed after initialization
        # example .half() or .to()
        # self.reset_ds_tensor()
        # ---------------------------------------------#

        self.timers = timers

        self.reduce_scatter = reduce_scatter

        self.dp_process_group = dp_process_group

        self.partition_count = dist.get_world_size(group=self.dp_process_group)

        if gpc.is_initialized(ParallelMode.TENSOR) is None:
            self.model_parallel_group = None
            self.model_parallel_rank = 0
        else:
            self.model_parallel_group = gpc.get_group(ParallelMode.TENSOR)
            self.model_parallel_rank = gpc.get_local_rank(ParallelMode.TENSOR)

        self.overflow = False
        self.clip_grad = clip_grad
        self.allreduce_always_fp32 = allreduce_always_fp32
        self.gradient_predivide_factor = gradient_predivide_factor
        self.postscale_gradients = postscale_gradients
        self.gradient_accumulation_steps = gradient_accumulation_steps
        self.micro_step_id = INITIAL_MICRO_STEP_ID

        if self.reduce_scatter:
            assert not self.allreduce_always_fp32, "allreduce_always_fp32 is not yet supported with ZeRO-2 with reduce scatter enabled"
            assert self.gradient_predivide_factor == 1.0, "gradient_predivide_factor != 1.0 is not yet supported with ZeRO-2 with reduce scatter enabled"
            assert self.postscale_gradients, "pre-scale gradients is not yet supported with ZeRO-2 with reduce scatter enabled"

        # Holds the mode parameter
        # The param.data may not hold any meaningful data
        # when param's status is NOT_AVAILABLE or IN_FLGHT
        self.fp16_groups = []

        # Hold partitioned parameters
        self.fp16_partitioned_groups = []

        # Holds a fused and flattened copy of the parameters
        self.fp16_partitioned_groups_flat = []
        self.fp16_partitioned_groups_flat_numel = []

        # defragmented pinned memory
        self.param_groups_fp16_flat_cpu_memory = []

        # a single 32-bit partition of the parallel partitioned parameters
        # that this process will update
        self.fp32_partitioned_groups_flat = []
        self.next_swappable_fp32_partitioned_groups = []

        # number of elements per partition in each group
        self.partition_size = []

        self.all_reduce_print = False

        self.prefetch_elements = int(prefetch_bucket_size)

        # padding on each partition for alignment purposes
        self.groups_padding = []

        self.sub_group_size = sub_group_size

        self.sub_group_to_group_id = {}

        if self.verbose:
            report_memory_usage("Before creating fp16 partitions")
        self._create_fp16_partitions_with_defragmentation()
        num_fp16_subgroups = len(self.fp16_partitioned_groups_flat)
        if self.verbose:
            report_memory_usage(
                f"After creating fp16 partitions: {num_fp16_subgroups}")

        # Optimizer ensor swapping
        if self.swap_optimizer:
            self._configure_tensor_swapping(
                offload_optimizer_config, aio_config)

        if self.verbose:
            report_memory_usage("Before creating fp32 partitions")
        self._create_fp32_partitions()
        if self.verbose:
            report_memory_usage("After creating fp32 partitions")
        dist.barrier()

        # To support pipelined optimizer swapping
        self._create_next_swappable_fp32_groups()

        if self.verbose:
            report_memory_usage("Before initializing optimizer states")
        self.initialize_optimizer_states()
        if self.verbose:
            report_memory_usage("After initializing optimizer states")
        dist.barrier()

        if dist.get_rank() == 0 and self.verbose:
            print(f"optimizer state initialized")

        self.reduce_bucket_size = int(reduce_bucket_size)

        self.reduction_event = torch.cuda.Event(
            enable_timing=False, blocking=False)

        self.reduction_stream = torch.cuda.Stream(
        ) if self.overlap_comm else torch.cuda.current_stream()
        self.callback_queued = False
        self.copy_grad_stream = torch.cuda.Stream()

        self.param_dict = {}

        # map between param_id and bool to specify if a param is in this partition
        self.is_param_in_current_partition = {}

        self.contiguous_gradients = contiguous_gradients
        self.extra_large_param_to_reduce = None
        self.grads_in_ipg_bucket = []
        self.params_in_ipg_bucket = []
        self.elements_in_ipg_bucket = 0
        self.params_already_reduced = []
        self.is_gradient_accumulation_boundary = True
        self._release_ipg_buffers()
        self.previous_reduced_grads = None

        # simplified param id
        self.param_id = {}

        count = 0
        for i, params_group in enumerate(self.fp16_groups):
            for param in params_group:
                unique_id = id(param)
                self.param_id[unique_id] = count
                self.param_dict[count] = param
                self.params_already_reduced.append(False)
                count = count + 1

        # Largest partitioned param
        largest_partitioned_param_numel = max([
            max([tensor.numel() for tensor in fp16_partitioned_group])
            for fp16_partitioned_group in self.fp16_partitioned_groups
        ])
        if self.verbose:
            print_rank_0(
                f'Largest partitioned param numel = {largest_partitioned_param_numel}',
                force=False)

        if self.verbose:
            report_memory_usage(f"Before Set Grad positions")

        self.grad_position = {}
        self.set_grad_positions()
        if self.verbose:
            report_memory_usage(f"Before CPU Offload initialization")

        self.grads_in_partition = None

        if self.offload_optimizer:
            self.accumulated_grads_in_cpu = {}
            self.norm_for_param_grads = {}
            self.local_overflow = False
            self.temp_grad_buffer_for_gpu_offload = torch.zeros(
                largest_partitioned_param_numel,
                device=torch.cuda.current_device(),
                dtype=self.dtype)
            self.temp_grad_gpu_buffer = torch.zeros(largest_partitioned_param_numel,
                                                    device=torch.cuda.current_device(),
                                                    dtype=self.dtype)

        if self.verbose:
            report_memory_usage(f"After CPU Offload initialization")

        # stores if a partition has been reduced in this step
        self.is_partition_reduced = {}

        # stores if a grad in a partition has been computed or not
        self.is_grad_computed = {}

        # will store the averaged gradients required by this parititon
        self.averaged_gradients = {}

        # creates backward hooks for gradient partitioning
        self.create_reduce_and_remove_grad_hooks()

        # exit(0)

        # we may have a way of fusing dynamic scale. Do not support for now
        if self.dtype == torch.float or not dynamic_loss_scale:
            loss_scale_value = 1.0 if self.dtype == torch.float else static_loss_scale

            self.dynamic_loss_scale = False
            self.loss_scaler = LossScaler(scale=loss_scale_value)
            cur_iter = 0
        else:
            if dynamic_loss_args is None:
                self.loss_scaler = DynamicLossScaler()
            else:
                self.loss_scaler = DynamicLossScaler(**dynamic_loss_args)

            self.dynamic_loss_scale = True

        self.debug_fp16_grads = [{} for _ in self.fp16_groups]

        if dist.get_rank(group=self.dp_process_group) == 0 and self.verbose:
            report_memory_usage(f"After initializing ZeRO optimizer")

    def _configure_tensor_swapping(self, offload_optimizer_config, aio_config):
        nvme_swap_folder = os.path.join(
            offload_optimizer_config[OFFLOAD_OPTIMIZER_NVME_PATH],
            'zero_stage_3')
        os.makedirs(nvme_swap_folder, exist_ok=True)
        if torch.distributed.get_rank() == 0 and self.verbose:
            print(f'Tensor Swapping: Adding optimizer tensors')

        swapper_type = PipelinedOptimizerSwapper if offload_optimizer_config[
            OFFLOAD_OPTIMIZER_PIPELINE] else PartitionedOptimizerSwapper

        self.optimizer_swapper = swapper_type(
            swap_config=offload_optimizer_config,
            aio_config=aio_config,
            base_folder=nvme_swap_folder,
            optimizer=self.optimizer,
            largest_numel=max(self.fp16_partitioned_groups_flat_numel),
            device=self.device,
            dtype=torch.float32,
            timers=self.timers)

    def _create_fp16_partitions(self):
        dist.barrier()
        partition_id = dist.get_rank(group=self.dp_process_group)

        # loop to deal with groups
        for j, param_group in enumerate(self.optimizer.param_groups):

            sub_groups = self._create_fp16_sub_groups(param_group['params'])
            for sub_group in sub_groups:
                i = len(self.fp16_groups)

                # push this group to list before modify
                self.fp16_groups.append(sub_group)
                self.sub_group_to_group_id[i] = j

                # These are the list of the partitioned parameters
                self.fp16_partitioned_groups.append(
                    [param.ds_tensor for param in self.fp16_groups[i]])

                if self.verbose:
                    print_rank_0(
                        f"fp16 group {i} partitioned_param norms : {[param.ds_tensor.norm().item() for param in self.fp16_groups[i]]}"
                    )

                # Record padding required to align group to world size (only applies to last rank)
                if partition_id == dist.get_world_size(group=self.dp_process_group) - 1:
                    padding = [p.padding_size() for p in self.fp16_groups[i]]
                else:
                    padding = [0] * len(self.fp16_groups[i])
                self.groups_padding.append(padding)

                # not sure why apex was cloning the weights before flattening
                # removing cloning here
                if self.verbose:
                    report_memory_usage(f"Before Flattening param group {i}")

                if not self.offload_param:
                    if self.verbose:
                        report_memory_usage(
                            f"Before moving param group {i} to CPU")
                    # move all the parameters to cpu to free up GPU space for creating flat buffer
                    move_to_cpu(self.fp16_partitioned_groups[i])
                    if self.verbose:
                        report_memory_usage(
                            f"After moving param group {i} to CPU")

                    # create flat buffer in CPU and move to GPU
                    self.fp16_partitioned_groups_flat.append(
                        self.flatten_dense_tensors_aligned(
                            self.fp16_partitioned_groups[i],
                            dist.get_world_size(group=self.dp_process_group)).cuda(
                            torch.cuda.current_device()))

                    if self.verbose:
                        report_memory_usage(
                            f"After flattening and moving param group {i} to GPU"
                        )
                else:
                    # Without the detach, report_memory_usage()lattening becomes part of the
                    # model graph causing errors downstream
                    self.fp16_partitioned_groups_flat.append(
                        self.flatten_dense_tensors_aligned(
                            self.fp16_partitioned_groups[i],
                            dist.get_world_size(
                                group=self.dp_process_group)).detach().pin_memory())

                if self.verbose:
                    report_memory_usage(f"After Flattening param group {i}")

                # set model fp16 weight to slices of flattened buffer
                updated_params = self.unflatten(self.fp16_partitioned_groups_flat[i],
                                                self.fp16_partitioned_groups[i])

                for partitioned_param, q in zip(self.fp16_partitioned_groups[i], updated_params):
                    partitioned_param.data = q.data

    def _move_to_flat_buffer(self, param_list, flat_buffer, avoid_copy=False):
        '''If flat buffer is None then the parameters in the param_list are
        not copied to the flat buffer. This is because they excede the number of max_params_in_cpu
        Some of these parameters may aready be in CPU in unflattened buffers
        or they maybe in GPU, or they maybe in NVME. If they are in NVME, then
        they will be marked as NOT_AVAILABLE, and will be moved to CPU when they are
        needed during training.'''
        if flat_buffer is None:
            # this dst buffer is on NVMe, so skip this
            return

        start = 0
        for param in param_list:
            src = param.ds_tensor
            dest = flat_buffer.narrow(0, start, src.ds_numel)
            start = start + src.ds_numel
            '''if the parameter was initialized in nvme then bring it to the destination buffer directly'''
            if src.status == PartitionedParamStatus.NOT_AVAILABLE:
                if self.verbose:
                    print_rank_0(
                        f"Swapping in {param.ds_id} with partition size {param.ds_tensor.ds_numel} permanently to CPU"
                    )
                param.nvme_swapper.swap_into_buffer(param, dest)
                src.data = dest.data
                src.status = PartitionedParamStatus.AVAILABLE
            else:
                assert src.status == PartitionedParamStatus.AVAILABLE, "Partitioned Parm must be avialable here"
                if not avoid_copy:
                    dest.data.copy_(src.data)
                src.data = dest.data

            # Final location must be gpu/cpu in this case
            param.ds_tensor.final_location = 'not-nvme'

    def _create_param_groups_fp16_flat_cpu_memory(self):

        aggregate_params_count = 0

        for j, param_group in enumerate(self.optimizer.param_groups):
            params_in_group = sum(
                [p.ds_tensor.ds_numel for p in param_group['params']])

            flat_buffer_size = params_in_group

            if self.params_in_nvme_and_cpu and \
                    aggregate_params_count + params_in_group > self.max_params_in_cpu:
                flat_buffer_size = max(0,
                                       self.max_params_in_cpu - aggregate_params_count)

            aggregate_params_count += params_in_group

            if flat_buffer_size > 0:
                if self.verbose:
                    print_rank_0(f"group {j} flat buffer size {flat_buffer_size}",
                                 force=False)
                self.param_groups_fp16_flat_cpu_memory.append(
                    torch.empty(int(flat_buffer_size),
                                dtype=self.dtype,
                                pin_memory=True))
            else:
                if self.verbose:
                    print_rank_0(
                        f"No flat buffer size. Param group size was  {params_in_group}",
                        force=False)

                self.param_groups_fp16_flat_cpu_memory.append(
                    torch.empty(1,
                                dtype=self.dtype))

    def _create_fp16_partitions_with_defragmentation(self):
        dist.barrier()
        partition_id = dist.get_rank(group=self.dp_process_group)
        create_fp16_flat_reuse_buffer = False
        largest_partition_numel = []
        max_partition_numel = 0

        # create a flat CPU memory allocation for each param group
        if self.offload_param:
            self._create_param_groups_fp16_flat_cpu_memory()

        # loop to deal with groups
        for j, param_group in enumerate(self.optimizer.param_groups):

            sub_groups = self._create_fp16_sub_groups(param_group['params'])

            if self.verbose:
                print_rank_0(
                    f'fp16 group {j} has {len(sub_groups)} subgroups', force=False)

            flat_offset = 0
            for sub_group in sub_groups:
                i = len(self.fp16_groups)

                # push this group to list before modify
                self.fp16_groups.append(sub_group)
                self.sub_group_to_group_id[i] = j

                # comment out for zero_to_fp32 debug
                # if torch.distributed.get_rank() == 0:
                #     for param in self.fp16_groups[i]:
                #         print(f"{debug_param2name_id_shape(param)} {param.ds_shape}")

                # These are the list of the partitioned parameters
                self.fp16_partitioned_groups.append(
                    [param.ds_tensor for param in self.fp16_groups[i]])

                total_elements = sum(
                    [t.ds_numel for t in self.fp16_partitioned_groups[i]])
                self.fp16_partitioned_groups_flat_numel.append(total_elements)

                if total_elements > max_partition_numel:
                    largest_partition_numel = [
                        t.ds_numel for t in self.fp16_partitioned_groups[i]
                    ]
                    max_partition_numel = total_elements

                if self.verbose:
                    print_rank_0(
                        f"fp16 group {i} partitioned_param norms : {[param.ds_tensor.norm().item() for param in self.fp16_groups[i]]}"
                    )

                # Record padding required to align group to world size (only applies to last rank)
                if partition_id == dist.get_world_size(group=self.dp_process_group) - 1:
                    padding = [p.padding_size() for p in self.fp16_groups[i]]
                else:
                    padding = [0] * len(self.fp16_groups[i])
                self.groups_padding.append(padding)

                # not sure why apex was cloning the weights before flattening
                # removing cloning here
                if self.verbose:
                    report_memory_usage(
                        f"Before Flattening param subgroup {i}")

                # all partitioned parameters remain in GPU during training
                if not self.offload_param:
                    if self.verbose:
                        report_memory_usage(
                            f"Before moving param subgroup group {i} to CPU")
                    # move all the parameters to cpu to free up GPU space for creating flat buffer
                    move_to_cpu(self.fp16_partitioned_groups[i])
                    if self.verbose:
                        report_memory_usage(
                            f"After moving param subgroup {i} to CPU")

                    # create flat buffer in CPU and move to GPU
                    self.fp16_partitioned_groups_flat.append(
                        self.flatten_dense_tensors_aligned(
                            self.fp16_partitioned_groups[i],
                            1).cuda(torch.cuda.current_device()))
                    if self.verbose:
                        report_memory_usage(
                            f"After flattening and moving param subgroup {i} to GPU")

                # all partitioned parameters are in CPU during training
                else:
                    if self.verbose:
                        print_rank_0(
                            f"Params in nvme and cpu {self.params_in_nvme_and_cpu}")
                    # Flat buffer may not be available for parameters that reside in NVME
                    if not self.params_in_nvme_and_cpu or flat_offset + total_elements <= \
                            self.param_groups_fp16_flat_cpu_memory[
                                j].numel():
                        fp16_partitioned_group_flat = self.param_groups_fp16_flat_cpu_memory[
                            j].narrow(0,
                                      flat_offset,
                                      total_elements)
                        if self.verbose:
                            print_rank_0(
                                f"Creating a flat buffer for subgroup {i} requiring {total_elements} elements, and cumulative CPU elemets {flat_offset + total_elements}",
                                force=False)
                    # these parameters reside in NVME and
                    elif self.params_in_nvme_and_cpu:
                        fp16_partitioned_group_flat = None
                        if self.verbose:
                            print_rank_0(
                                f"No flat buffer for sub group {i} of {total_elements} elements",
                                force=False)
                    else:
                        assert False, "Either params are in nvme, or they are in CPU memory. This code path should not be triggered. Please report_memory_usage()ms_in_cpu and params_in_nvme configs"

                    self.fp16_partitioned_groups_flat.append(
                        fp16_partitioned_group_flat)
                    flat_offset += total_elements

                # move param to flat buffer for both param offload on/off
                self._move_to_flat_buffer(self.fp16_groups[i],
                                          self.fp16_partitioned_groups_flat[i],
                                          avoid_copy=not self.offload_param)
                if self.verbose:
                    report_memory_usage(f"After Flattening param group {i}")

                # create a pinned memory to be used for swapping out params to NVME after optimizer step
                if self.fp16_partitioned_groups_flat[-1] is None:
                    create_fp16_flat_reuse_buffer = True

                if self.verbose:
                    report_memory_usage(f"After Flattening param subgroup {i}")

        if create_fp16_flat_reuse_buffer:
            assert len(
                largest_partition_numel) > 0, f'Unexpected that largest partition is empty'
            self.fp16_groups[0][0].nvme_swapper.reserve_partitioned_swap_space(
                largest_partition_numel)

    def _swap_in_sub_group_to_flat_buffer(self, flat_buffer, sub_group_id):
        offset = 0
        elements_in_sub_group = sum(
            [t.ds_numel for t in self.fp16_partitioned_groups[sub_group_id]])
        assert (flat_buffer.numel() == elements_in_sub_group)
        for param, partitioned_param in zip(self.fp16_groups[sub_group_id], self.fp16_partitioned_groups[sub_group_id]):
            dest = flat_buffer.narrow(0, offset, partitioned_param.ds_numel)
            if partitioned_param.status == PartitionedParamStatus.NOT_AVAILABLE:
                if self.verbose:
                    print_rank_0(
                        f"Swapping in {param.ds_id} with elements {param.ds_numel} and partition {param.ds_tensor.ds_numel}"
                    )
                param.nvme_swapper.swap_in([param], async_op=False)
                dest.data.copy_(partitioned_param.data)
                param.nvme_swapper.remove_partition_and_release_buffers([
                    param])
                if self.verbose:
                    print_rank_0(f"Swapping in {param.ds_id} done")
            else:
                dest.data.copy_(partitioned_param.data)
            offset += partitioned_param.ds_numel

    def _create_next_swappable_fp32_groups(self):
        reverse_order_indices = [
            i for i in range(len(self.fp32_partitioned_groups_flat))
        ]
        reverse_order_indices.reverse()

        next_group = None
        for i in reverse_order_indices:
            self.next_swappable_fp32_partitioned_groups.append(next_group)
            if self._swappable_optimizer_subgroup(i):
                next_group = self.fp32_partitioned_groups_flat[i]

        self.next_swappable_fp32_partitioned_groups.reverse()

    def _get_sub_group_partitions(self, sub_group_id):
        sub_group_partitions = []
        for param, partitioned_param in zip(self.fp16_groups[sub_group_id], self.fp16_partitioned_groups[sub_group_id]):
            if partitioned_param.status == PartitionedParamStatus.NOT_AVAILABLE:
                swap_path = param.nvme_swapper.get_path(param, True)
                sub_group_partitions.append((partitioned_param,
                                             param.ds_tensor.ds_numel,
                                             swap_path))
            else:
                sub_group_partitions.append((partitioned_param,
                                             partitioned_param.ds_numel,
                                             None))

        return sub_group_partitions

    def _create_fp32_partitions(self):
        cpu_memory_usage = 0
        cpu_memory_sub_groups = 0
        nvme_memory_usage = 0
        num_swappable_partitions = 0
        num_swap_from_nvme_partitions = 0
        num_swap_from_cpu_partitions = 0
        swap_from_nvme_memory_usage = 0
        swap_from_cpu_memory_usage = 0
        GIGA_BYTES = (1024 ** 3)

        swappable_fp32_tensors = []
        swappable_fp16_src_tensors = []
        nvme_fp16_partitions_info = []
        nvme_fp16_num_elems = []
        nvme_fp32_dest_tensors = []
        fp32_element_size = torch.tensor(
            [], dtype=torch.float32).element_size()

        for i, tensor in enumerate(self.fp16_partitioned_groups_flat):
            num_elements = self.fp16_partitioned_groups_flat_numel[i]

            # a partition of the fp32 master weights that will be updated by this process
            if self._swappable_optimizer_subgroup(i):
                self.fp32_partitioned_groups_flat.append(torch.Tensor())
                nvme_memory_usage += (fp32_element_size * num_elements)
                num_swappable_partitions += 1

                if self.params_in_nvme_and_cpu and tensor is None:
                    num_swap_from_nvme_partitions += 1
                    swap_from_nvme_memory_usage += (
                        fp32_element_size * num_elements)
                    if self.offload_optimizer_fast_init:
                        sub_group_partitions = self._get_sub_group_partitions(
                            i)
                        nvme_fp16_partitions_info.append(sub_group_partitions)
                        nvme_fp16_num_elems.append(num_elements)
                        nvme_fp32_dest_tensors.append(
                            self.fp32_partitioned_groups_flat[i])
                    else:
                        unpinned_fp32_buffer = torch.empty(num_elements,
                                                           device=self.device,
                                                           dtype=torch.float)
                        self._swap_in_sub_group_to_flat_buffer(
                            unpinned_fp32_buffer, i)
                        self.optimizer_swapper.initialize_parameters(
                            parameters=[self.fp32_partitioned_groups_flat[i]],
                            src_tensors=[unpinned_fp32_buffer])
                else:
                    num_swap_from_cpu_partitions += 1
                    swap_from_cpu_memory_usage += (
                        fp32_element_size * num_elements)
                    swappable_fp32_tensors.append(
                        self.fp32_partitioned_groups_flat[i])
                    swappable_fp16_src_tensors.append(
                        self.fp16_partitioned_groups_flat[i])
            else:
                cpu_memory_usage += (fp32_element_size * num_elements)
                cpu_memory_sub_groups += 1

                if self.params_in_nvme_and_cpu and tensor is None:
                    unpinned_fp32_buffer = torch.empty(num_elements,
                                                       device=self.device,
                                                       dtype=torch.float)
                    self._swap_in_sub_group_to_flat_buffer(
                        unpinned_fp32_buffer, i)
                    self.fp32_partitioned_groups_flat.append(
                        unpinned_fp32_buffer)
                else:
                    self.fp32_partitioned_groups_flat.append(
                        self.fp16_partitioned_groups_flat[i].to(
                            self.device).clone().float().detach())

            self.fp32_partitioned_groups_flat[
                i].requires_grad = True  # keep this in case internal optimizer uses it

        if len(swappable_fp32_tensors) > 0:
            self.optimizer_swapper.initialize_parameters(
                parameters=swappable_fp32_tensors,
                src_tensors=swappable_fp16_src_tensors)

        if len(nvme_fp32_dest_tensors) > 0:
            fp16_pinned_buffers = self.fp16_groups[0][
                0].nvme_swapper.reserve_available_buffers()
            assert len(fp16_pinned_buffers) > 0
            self.optimizer_swapper.initialize_from_swapped_fp16_params(
                fp16_partitions_info=nvme_fp16_partitions_info,
                fp16_num_elems=nvme_fp16_num_elems,
                fp16_pinned_buffers=fp16_pinned_buffers,
                fp32_parameters=nvme_fp32_dest_tensors)
            self.fp16_groups[0][0].nvme_swapper.release_reserved_buffers()

        nvme_gigabytes = nvme_memory_usage / GIGA_BYTES
        if self.verbose:
            print_rank_0(
                f'Swappable FP32 Partitions: count={num_swappable_partitions} size={nvme_gigabytes:5.2f} GB',
                force=False)
        if self.params_in_nvme_and_cpu:
            if self.verbose:
                print_rank_0(
                    f'Swap from NVMe Partitions: count = {num_swap_from_nvme_partitions}, size = {swap_from_nvme_memory_usage / GIGA_BYTES:5.2f}GB',
                    force=False)
                print_rank_0(
                    f'Swap from CPU Partitions: count = {num_swap_from_cpu_partitions}, size = {swap_from_cpu_memory_usage / GIGA_BYTES:5.2f}GB',
                    force=False)

        cpu_memory_gigabytes = cpu_memory_usage / GIGA_BYTES
        if self.verbose:
            print_rank_0(
                f'In-Memory FP32 Partitions: count={cpu_memory_sub_groups} size={cpu_memory_gigabytes:5.2f} GB',
                force=False)

        # Clear for on-the-fly population before the optimizer step
        for param_group in self.optimizer.param_groups:
            param_group['params'] = []

    def _create_fp16_sub_groups(self, params_group):

        params_group_numel = sum([param.partitioned_size()
                                  for param in params_group])
        sub_group_size = self.sub_group_size

        if sub_group_size is None or sub_group_size >= params_group_numel:
            return [params_group]

        sub_groups = []
        sub_group = []
        local_sub_group_size = 0
        for param in params_group:

            sub_group.append(param)
            local_sub_group_size += param.partitioned_size()

            if local_sub_group_size >= sub_group_size or id(param) == id(
                    params_group[-1]):
                sub_groups.append(sub_group)

                sub_group = []
                local_sub_group_size = 0

        return sub_groups

    # def reset_ds_tensor(self):
    #     for name, param in self.module.named_parameters(recurse=True):
    #         assert hasattr(param,'ds_id'), "Parameters have not been converted to be Zero 3 compatible"
    #         assert (param.ds_status == ZeroParamStatus.NOT_AVAILABLE), "All the parameters must have been partitioned by now"
    #         param.ds_tensor.data = param.data

    def setup_zero_stage3_hooks(self):
        self.hierarchy = 0
        self._register_hooks_recursively(self.module)

        # reset step at the beginning of forward
        def _pre_forward_hook(module, *args):
            self.param_coordinator.reset_step()

        # reset step if in inference mode
        def _end_of_forward_hook(module, *args):
            if not torch._C.is_grad_enabled():
                self.param_coordinator.reset_step()

        # likely one of them should be enough but just to be safe
        self.module.register_forward_hook(_end_of_forward_hook)
        self.module.register_forward_pre_hook(_pre_forward_hook)

        # Add top todule to stack trace
        global FWD_MODULE_STACK
        FWD_MODULE_STACK.append(self.module)

    def persistent_parameters(self):
        persistent_params = []
        total_persistent_parameters = 0
        params_count = 0
        for _, param in self.module.named_parameters(recurse=True):
            if param.ds_numel < self.persistence_threshold:
                params_count += 1
                param.ds_persist = True
                persistent_params.append(param)
                total_persistent_parameters += param.ds_numel

        if self.verbose:
            print_rank_0(
                f"ZeRO 3: Total persistent parameters: {total_persistent_parameters} in {params_count} params",
                force=False)
        return persistent_params

    def _register_hooks_recursively(self, module, count=[0]):
        my_count = count[0]
        module.id = my_count

        # print(f"{module.__class__} : {module.id}")

        for child in module.children():
            count[0] = count[0] + 1
            self._register_hooks_recursively(child, count=count)

        def _pre_forward_module_hook(module, *args):
            self.pre_sub_module_forward_function(module)

        def _post_forward_module_hook(module, input, output):
            global FWD_MODULE_STACK
            FWD_MODULE_STACK.pop()
            if output is None:
                output = []
            elif not isinstance(output, (list, tuple)):
                if torch.is_tensor(output):
                    output = [output]
                else:
                    # print(f'got UNKNOWN type {type(output)}')
                    outputs = []
                    output = output if isinstance(
                        output, dict) else vars(output)
                    for name, val in output.items():
                        if not name.startswith('__') and torch.is_tensor(val):
                            outputs.append(val)
                    output = outputs
                    # print(f'convert output to {output}')

            for item in filter(lambda item: is_zero_param(item), output):
                if not any(id(item) in m._external_params for m in FWD_MODULE_STACK):
                    item.ds_active_sub_modules += 1
                    module_to_register = FWD_MODULE_STACK[-1]

                    if self.verbose:
                        print_rank_0(
                            f'Registering dangling parameter for module {module_to_register.__class__.__name__}.',
                            force=False)
                    register_external_parameter(module_to_register, item)

                    # It's possible that the parameter was already external to the completed module. If so, remove it the
                    # registration as it will be covered by the outer module instead.
                    if id(item) in module._external_params:
                        if self.verbose:
                            print_rank_0(
                                f'  Unregistering nested dangling parameter from module {module.__class__.__name__}',
                                force=False)
                        unregister_external_parameter(module, item)

                    item.all_gather()

            self.post_sub_module_forward_function(module)

        def _pre_backward_module_hook(module, inputs, output):
            def _run_before_backward_function(sub_module):
                # some models (e.g. Albert) may run multiple forwards on the same layer in a loop
                # before doing backwards, so each backward will need a pre-fetch - using reference
                # counting to support this scenario
                # print(f"COUNTER before: {sub_module.applied_pre_backward_ref_cnt}")
                if sub_module.applied_pre_backward_ref_cnt > 0:
                    self.pre_sub_module_backward_function(sub_module)
                    sub_module.applied_pre_backward_ref_cnt -= 1
                # print(f"COUNTER after: {sub_module.applied_pre_backward_ref_cnt}")

            return _apply_to_tensors_only(module,
                                          PreBackwardFunction,
                                          _run_before_backward_function,
                                          output)

        # This is an alternate to doing _post_backward_module_hook
        # it uses tensor.register_hook instead of using torch.autograd.Function
        def _alternate_post_backward_module_hook(module, inputs):
            module.ds_grads_remaining = 0

            # print(f"Before Forward {module.__class__.__name__}")

            def _run_after_backward_hook(*unused):
                module.ds_grads_remaining = module.ds_grads_remaining - 1
                if module.ds_grads_remaining == 0:
                    # print(f"After backward {module.__class__.__name__}")
                    self.post_sub_module_backward_function(module)

            def _run_before_forward_function(input):
                if input.requires_grad:
                    module.ds_grads_remaining += 1

            return _apply_forward_and_backward_to_tensors_only(
                module,
                _run_before_forward_function,
                _run_after_backward_hook,
                inputs)

        def _post_backward_module_hook(module, inputs):
            module.ds_grads_remaining = 0

            def _run_after_backward_function(sub_module):
                if sub_module.ds_grads_remaining == 0:
                    self.post_sub_module_backward_function(sub_module)

            return _apply_to_tensors_only(module,
                                          PostBackwardFunction,
                                          _run_after_backward_function,
                                          inputs)

        # Pre forward hook
        module.register_forward_pre_hook(_pre_forward_module_hook)
        # Post forward hook
        module.register_forward_hook(_post_forward_module_hook)

        # Pre backward hook
        module.register_forward_hook(_pre_backward_module_hook)

        # post backward hook
        module.register_forward_pre_hook(_post_backward_module_hook)

    def pre_sub_module_forward_function(self, sub_module):
        if self.verbose:
            report_memory_usage(
                f"Before sub module function {sub_module.__class__.__name__}")

        global FWD_MODULE_STACK
        FWD_MODULE_STACK.append(sub_module)

        self.param_coordinator.record_trace(sub_module)

        self.param_coordinator.fetch_sub_module(sub_module)
        if self.verbose:
            report_memory_usage(
                f"Before sub module function {sub_module.__class__.__name__} after fetch")

        self.param_coordinator.prefetch_next_sub_modules(
            sub_module,
            numel=self.prefetch_elements,
            nvme=self.params_in_nvme_and_cpu)
        if self.verbose:
            report_memory_usage(
                f"Before sub module function {sub_module.__class__.__name__} after prefetch")

        self.param_coordinator.increment_step(sub_module)

    def post_sub_module_forward_function(self, sub_module):
        if self.verbose:
            report_memory_usage(
                f"After sub module function {sub_module.__class__.__name__} {sub_module.id} before release")

        self.param_coordinator.release_sub_module(sub_module)
        if self.verbose:
            report_memory_usage(
                f"After sub module function {sub_module.__class__.__name__}  {sub_module.id} after release")

    def pre_sub_module_backward_function(self, sub_module):
        self.param_coordinator.record_trace(sub_module)

        self.param_coordinator.fetch_sub_module(sub_module)

        self.param_coordinator.prefetch_next_sub_modules(sub_module,
                                                         numel=self.prefetch_elements)

        self.param_coordinator.increment_step(sub_module)

    def post_sub_module_backward_function(self, sub_module):
        if self.verbose:
            report_memory_usage(
                f"After sub module backward function {sub_module.__class__.__name__} {sub_module.id} before release")
        self.param_coordinator.release_sub_module(sub_module)

        if self.verbose:
            report_memory_usage(
                f"After sub module backward function {sub_module.__class__.__name__} {sub_module.id} after release")

    def _release_ipg_buffers(self):
        if self.contiguous_gradients:
            self.ipg_buffer = None
            if not self.offload_optimizer and self.is_gradient_accumulation_boundary:
                self.grads_in_partition = None

            self.grads_in_partition_offset = 0

    def _optimizer_step(self, sub_group_id):
        param_group_id = self.sub_group_to_group_id[sub_group_id]
        fp32_param = self.fp32_partitioned_groups_flat[sub_group_id]
        fp16_param = self.fp16_partitioned_groups_flat[sub_group_id]
        self.optimizer.param_groups[param_group_id]['params'] = [fp32_param]

        self.optimizer.step()
        self.optimizer.param_groups[param_group_id]['params'] = []

    def _swappable_optimizer_subgroup(self, sub_group_id):
        if not self.swap_optimizer:
            return False

        return self.optimizer_swapper.swappable_tensor(
            None,
            numel=self.fp16_partitioned_groups_flat_numel[sub_group_id])

    def _partitioned_params_swap_out(self, i):
        offset = 0
        fp32_param = self.fp32_partitioned_groups_flat[i]
        assert fp32_param is not None, \
            f'fp32 parameters of sub_group {i} is None'

        swap_fp16_params = []
        swap_fp32_params = []
        for param, partitioned_param in zip(self.fp16_groups[i], self.fp16_partitioned_groups[i]):
            src = fp32_param.narrow(0, offset, partitioned_param.ds_numel)
            if partitioned_param.status == PartitionedParamStatus.AVAILABLE:
                partitioned_param.data.copy_(src.data)
            else:
                swap_fp32_params.append(src)
                swap_fp16_params.append(param)
            offset += partitioned_param.ds_numel

        if len(swap_fp16_params):
            swap_fp16_params[0].nvme_swapper.swap_out_partitioned_params(
                dst_fp16_params=swap_fp16_params,
                src_fp32_params=swap_fp32_params)

    def initialize_optimizer_states(self):
        num_subgroups = len(self.fp16_groups)

        largest_numel = max(
            [sum([p.ds_numel for p in psg]) for psg in self.fp16_partitioned_groups])
        gradient_dtype = self.fp32_partitioned_groups_flat[0].dtype
        gradient_buffer = torch.zeros(int(largest_numel),
                                      dtype=gradient_dtype,
                                      device=self.device)

        timers = self.timers
        timer_names = set()

        if self.swap_optimizer:
            self.optimizer_swapper.init_timers()

        INIT_OPTIMIZER_TIMER = 'init_optimizer_state'
        timer_names.add(INIT_OPTIMIZER_TIMER)
        self.start_timers([INIT_OPTIMIZER_TIMER])

        for i, group in enumerate(self.fp16_groups):
            swappable_optimizer_subgroup = self._swappable_optimizer_subgroup(
                i)
            swappable_param_subgroup = self.fp16_partitioned_groups_flat[i] is None

            num_elements = int(self.fp16_partitioned_groups_flat_numel[i])

            if self.verbose:
                report_memory_usage(
                    f'[Begin] Initialize optimizer states {i} / {num_subgroups} subgroups, num_elems: {num_elements}, swappable opt/param:{swappable_optimizer_subgroup}/{swappable_param_subgroup}')

            if swappable_optimizer_subgroup:
                self._optimizer_states_and_gradient_swap_in(i, timer_names)

            if self.offload_optimizer and not swappable_optimizer_subgroup:
                subgroup_gradient_buffer = torch.zeros(num_elements,
                                                       dtype=gradient_dtype,
                                                       device=self.device)
                if self.offload_optimizer_pin_memory:
                    subgroup_gradient_buffer = subgroup_gradient_buffer.pin_memory()

                self.fp32_partitioned_groups_flat[i].grad = subgroup_gradient_buffer
            else:
                self.fp32_partitioned_groups_flat[i].grad = gradient_buffer.narrow(
                    0,
                    0,
                    num_elements)

            self._optimizer_step(i)

            if swappable_param_subgroup:
                self._partitioned_params_swap_out(i)

            if swappable_optimizer_subgroup:
                self._optimizer_states_and_gradient_swap_out(i, timer_names)

            if self.verbose:
                report_memory_usage(
                    f'[End] Initialize optimizer states {i} / {num_subgroups} subgroups, num_elems: {num_elements}, swappable opt/param:{swappable_optimizer_subgroup}/{swappable_param_subgroup}')

        self.stop_timers([INIT_OPTIMIZER_TIMER])
        self.log_timers(timer_names)

        if self.swap_optimizer:
            self.optimizer_swapper.log_timers()

        if not self.offload_optimizer:
            for group in self.fp32_partitioned_groups_flat:
                group.grad = None

        # Reset steps
        return

    #########################################################################
    #########################ZeRO Partition Gradients########################
    #########################################################################

    def get_first_param_index(self, group_id, param_group, partition_id):
        for index, param in enumerate(param_group):
            param_id = self.get_param_id(param)
            if partition_id in self.param_to_partition_ids[group_id][param_id]:
                return index
        return None

    def initialize_gradient_partitioning_data_structures(self):

        total_partitions = dist.get_world_size(group=self.dp_process_group)

        for i, param_group in enumerate(self.fp16_groups):

            self.param_to_partition_ids[i] = {}
            self.is_partition_reduced[i] = {}
            self.total_grads_in_partition[i] = {}
            self.remaining_grads_in_partition[i] = {}
            self.is_grad_computed[i] = {}
            self.grad_partition_insertion_offset[i] = {}
            self.grad_start_offset[i] = {}
            self.first_param_index_in_partition[i] = {}

            for partition_id in range(total_partitions):
                self.is_grad_computed[i][partition_id] = {}
                self.grad_partition_insertion_offset[i][partition_id] = {}
                self.grad_start_offset[i][partition_id] = {}
                self.initialize_gradient_partition(
                    i, param_group, partition_id)
                self.is_partition_reduced[i][partition_id] = False
                self.first_param_index_in_partition[i][
                    partition_id] = self.get_first_param_index(
                    i,
                    param_group,
                    partition_id)

    def independent_gradient_partition_epilogue(self):
        if self.verbose:
            self.report_ipg_memory_usage(
                f"In ipg_epilogue before reduce_ipg_grads", 0)
        self.reduce_ipg_grads()
        if self.verbose:
            self.report_ipg_memory_usage(
                f"In ipg_epilogue after reduce_ipg_grads", 0)

        if self.overlap_comm:
            self.reduction_stream.synchronize()

        with torch.cuda.stream(self.reduction_stream):
            self.partition_previous_reduced_grads()

        # if dist.get_rank() == 0:
        #    print()("Params already reduced %s", self.params_already_reduced)
        for i in range(len(self.params_already_reduced)):
            self.params_already_reduced[i] = False

        # in case of cpu offload, averaged gradients are already in fp32_partitioned_groups_flat.grad
        # TODO: use a similar code path for both cpu_offload and non-cpu offload
        if not self.offload_optimizer:
            for i, sub_group in enumerate(self.fp16_groups):
                self.averaged_gradients[i] = [
                    torch.zeros_like(param.ds_tensor) if param.grad is None else
                    param.grad.data.narrow(0,
                                           0,
                                           param.ds_tensor.numel())
                    for param in sub_group
                ]
                # self.averaged_gradients[i] = self.get_flat_partition(
                #     self.fp16_groups[i],
                #     0,
                #     self.fp32_partitioned_groups_flat[i].numel(),
                #     return_tensor_list=True)

        self._release_ipg_buffers()

        if self.verbose:
            report_memory_usage(f"End ipg_epilogue")

    # resets all partition to no reduced
    # sets remianing grads to the total number of grads in each partition
    # set is grad computed to false for all grads in partition
    def reset_partition_gradient_structures(self):
        total_partitions = dist.get_world_size(group=self.dp_process_group)
        for i, _ in enumerate(self.fp16_groups):
            for partition_id in range(total_partitions):
                self.is_partition_reduced[i][partition_id] = False
                self.remaining_grads_in_partition[i][
                    partition_id] = self.total_grads_in_partition[i][partition_id]

                for param_id in self.is_grad_computed[i][partition_id]:
                    self.is_grad_computed[i][partition_id][param_id] = False

    def initialize_gradient_partition(self, i, param_group, partition_id):
        def set_key_value_list(dictionary, key, value):
            if key in dictionary:
                dictionary[key].append(value)
            else:
                dictionary[key] = [value]

        def increment_value(dictionary, key):
            if key in dictionary:
                dictionary[key] += 1
            else:
                dictionary[key] = 1

        partition_size = self.partition_size[i]

        start_index = partition_size * partition_id
        end_index = partition_size * (partition_id + 1)

        current_index = 0
        first_offset = 0

        for param in param_group:

            param_size = param.numel()
            param_id = self.get_param_id(param)

            if (current_index >= start_index and current_index < end_index):
                set_key_value_list(self.param_to_partition_ids[i],
                                   param_id,
                                   partition_id)
                increment_value(self.total_grads_in_partition[i], partition_id)

                self.is_grad_computed[i][partition_id][param_id] = False

                self.grad_partition_insertion_offset[i][partition_id][
                    param_id] = current_index - start_index
                self.grad_start_offset[i][partition_id][param_id] = 0

            elif start_index > current_index and start_index < (current_index +
                                                                param_size):
                assert (
                    first_offset == 0), "This can happen either zero or only once as this must be the first tensor in the partition"
                first_offset = start_index - current_index

                set_key_value_list(self.param_to_partition_ids[i],
                                   param_id,
                                   partition_id)
                increment_value(self.total_grads_in_partition[i], partition_id)

                self.is_grad_computed[i][partition_id][param_id] = False

                self.grad_partition_insertion_offset[i][partition_id][param_id] = 0
                self.grad_start_offset[i][partition_id][param_id] = first_offset

            current_index = current_index + param_size

    def overlapping_partition_gradients_reduce_epilogue(self):
        self.independent_gradient_partition_epilogue()
        self.zero_grad()

    def create_reduce_and_remove_grad_hooks(self):
        if self.verbose:
            print_rank_0(f'[Begin] Create gradient reduction hooks')
        self.grad_accs = []
        for i, param_group in enumerate(self.fp16_groups):
            for param in param_group:
                if param.requires_grad:
                    # print_rank_0(f" Before all gather {param.device}, {param.shape}")

                    # The hook must be created in un-partitioned parameter
                    param.all_gather()

                    # print(f"After all gather {param.device}, {param.shape}")
                    def wrapper(param, i):
                        param_tmp = param.expand_as(param)
                        grad_acc = param_tmp.grad_fn.next_functions[0][0]

                        def reduce_partition_and_remove_grads(*notneeded):
                            self.reduce_ready_partitions_and_remove_grads(
                                param, i)

                        grad_acc.register_hook(
                            reduce_partition_and_remove_grads)
                        self.grad_accs.append(grad_acc)

                    # print(f"param grad fn {param.expand_as(param).grad_fn}")
                    wrapper(param, i)

                    # Partition the parameter after creating the hook
                    param.partition()
        if self.verbose:
            print_rank_0(f'[End] Create gradient reduction hooks')

    def get_param_id(self, param):
        unique_id = id(param)
        return self.param_id[unique_id]

    def report_ipg_memory_usage(self, tag, param_elems):
        elem_count = self.elements_in_ipg_bucket + param_elems
        percent_of_bucket_size = (
            100.0 * elem_count) // self.reduce_bucket_size
        report_memory_usage(
            f"{tag}: elems in_bucket {self.elements_in_ipg_bucket} param {param_elems} max_percent {percent_of_bucket_size}")

    ###############Idependent Partition Gradient ########################
    def reduce_independent_p_g_buckets_and_remove_grads(self, param, i):
        # print_rank_0(f"Inside reduce ipg buckets. {debug_param2name_id_shape(param)}, ipg elements {self.elements_in_ipg_bucket}, reduce bucket size {self.reduce_bucket_size}", force=True)

        # Because the ipg bucket is initialized with a random place holder tensor, we must
        # explicitly check that the bucket has any real data in it (self.elements_in_ipg_bucket >
        # 0). Otherwise if the incoming param.ds_numel is large, this branch may get triggered on a
        # garbage data and `self.average_tensor()` will crash because its params_to_reduce will be
        # empty, while reduction_list will have that garbage data.
        if self.elements_in_ipg_bucket > 0 and self.elements_in_ipg_bucket + param.ds_numel > self.reduce_bucket_size:
            if self.verbose:
                self.report_ipg_memory_usage("In ipg_remove_grads before reduce_ipg_grads",
                                             param.ds_numel)

            self.reduce_ipg_grads()

            if self.contiguous_gradients and self.overlap_comm:
                # Swap ipg_index between 0 and 1
                self.ipg_index = 1 - self.ipg_index
            if self.verbose:
                self.report_ipg_memory_usage("In ipg_remove_grads after reduce_ipg_grads",
                                             param.ds_numel)

        param_id = self.get_param_id(param)
        assert self.params_already_reduced[param_id] == False, \
            f"The parameter {param_id} has already been reduced. \
            Gradient computed twice for this partition. \
            Multiple gradient reduction is currently not supported"

        # keeping the gradients contiguous to prevent memory fragmentation, and avoid flattening
        if param.ds_numel > self.reduce_bucket_size:
            self.extra_large_param_to_reduce = param

        elif self.contiguous_gradients:
            # print_rank_0("before new grad tensor move")
            new_grad_tensor = self.ipg_buffer[self.ipg_index].narrow(
                0,
                self.elements_in_ipg_bucket,
                param.ds_numel)
            # print_rank_0("after new grad tensor move")
            new_grad_tensor.copy_(param.grad.view(-1))
            param.grad.data = new_grad_tensor.data.view_as(param.grad)

        self.elements_in_ipg_bucket += param.ds_numel
        self.grads_in_ipg_bucket.append(param.grad)
        self.params_in_ipg_bucket.append((i, param, param_id))
        if self.verbose:
            self.report_ipg_memory_usage("End ipg_remove_grads", 0)

    def gradient_reduction_w_predivide(self, tensor):
        dp_world_size = dist.get_world_size(group=self.dp_process_group)

        tensor_to_allreduce = tensor

        if self.allreduce_always_fp32:
            tensor_to_allreduce = tensor.float()

        if self.postscale_gradients:
            if self.gradient_predivide_factor != 1.0:
                tensor_to_allreduce.mul_(1. / self.gradient_predivide_factor)

            dist.all_reduce(tensor_to_allreduce, group=self.dp_process_group)

            if self.gradient_predivide_factor != dp_world_size:
                tensor_to_allreduce.mul_(
                    self.gradient_predivide_factor / dp_world_size)
        else:
            tensor_to_allreduce.div_(dp_world_size)
            dist.all_reduce(tensor_to_allreduce, group=self.dp_process_group)

        if self.allreduce_always_fp32 and tensor is not tensor_to_allreduce:
            tensor.copy_(tensor_to_allreduce)

        return tensor

    def average_tensor(self, tensors, params_to_reduce):
        with torch.cuda.stream(self.reduction_stream):
            if not self.reduce_scatter:
                for tensor in tensors:
                    self.gradient_reduction_w_predivide(tensor)
                return

            for tensor in tensors:
                tensor.div_(dist.get_world_size(group=self.dp_process_group))

            # reduction resulting with each rank only holding the gradient partition it owns
            # This could either be a reduce scatter or a reduce op depending on how
            # parameters are partitionied. The method is implemented by the
            # DeepSpeed param extensions to the pytorch parameter, so its up to
            # the extension to define what happens here
            params_to_reduce[0].reduce_gradients_at_owner(
                param_list=params_to_reduce,
                hierarchy=self.param_coordinator.hierarchy)

    def set_grad_positions(self):
        for i, group in enumerate(self.fp16_groups):
            current_offset = 0
            for param in group:
                param_id = self.get_param_id(param)
                num_elements = param.ds_tensor.ds_numel

                self.grad_position[param_id] = [
                    int(i),
                    int(current_offset),
                    int(num_elements)
                ]
                # print(f"param id {param_id} i:{i}, ds_tensor {num_elements} numel {param.numel()}")
                current_offset += num_elements

    def async_accumulate_grad_in_cpu_via_gpu(self, param, acc_grad_cpu_partition):

        # copy to a preexisiting buffer to avoid memory allocation penalty
        dest_buffer = self.temp_grad_buffer_for_gpu_offload.view(-1).narrow(
            0,
            0,
            param.ds_tensor.ds_numel)

        if self.micro_step_id > 0:
            dest_buffer.copy_(
                acc_grad_cpu_partition.view(-1), non_blocking=True)
            param.grad.data.view(-1).add_(dest_buffer)

        # at the boundary we will send 32bit directly
        if not self.is_gradient_accumulation_boundary:
            acc_grad_cpu_partition.data.copy_(param.grad.data.view(-1),
                                              non_blocking=True)

    def _constant_buffered_norm2(self, input, buffer_size=250000000):
        norm = None
        for part in input.view(-1).split(buffer_size):
            if norm is None:
                norm = part.data.double().norm(2) ** 2.0
            else:
                norm += part.data.double().norm(2) ** 2.0
        return norm ** 0.5

    def set_norm_for_param_grad_in_gpu(self, param):
        param_id = self.get_param_id(param)
        # self.norm_for_param_grads[param_id] = param.grad.data.double().norm(2)
        # Using a more memory efficient version
        self.norm_for_param_grads[param_id] = self._constant_buffered_norm2(
            param.grad)

    def update_overflow_tracker_for_param_grad(self, param):
        # Credit to our user David Minn
        if param.grad is not None:
            if self.overlap_comm:
                self.gpu_sum = self.gpu_sum + param.grad.data.float().sum()
            elif self._has_inf_or_nan(param.grad.data):
                self.local_overflow = True

    def async_inplace_copy_grad_to_fp32_buffer_from_gpu(self, param, fp32_grad_tensor):
        with torch.cuda.stream(self.copy_grad_stream):
            param_id = self.get_param_id(param)
            src_tensor = param.grad.view(-1).float()
            # print(f"src_tensor {src_tensor.size()} and fp32 grad {fp32_grad_tensor.size()}")
            fp32_grad_tensor.copy_(src_tensor, non_blocking=True)
            param.grad = None

    def complete_grad_norm_calculation_for_cpu_offload(self, params):
        total_norm = 0.0
        norm_type = 2.0
        for p in params:
            if is_model_parallel_parameter(p) or (self.model_parallel_rank == 0):
                param_id = self.get_param_id(p)
                if param_id in self.norm_for_param_grads.keys():
                    param_norm = self.norm_for_param_grads[param_id]
                    total_norm += param_norm.item() ** 2

        # Sum across all model parallel GPUs.
        total_norm_cuda = torch.cuda.FloatTensor([float(total_norm)])

        torch.distributed.all_reduce(total_norm_cuda,
                                     op=torch.distributed.ReduceOp.SUM,
                                     group=self.dp_process_group)

        self._model_parallel_all_reduce(tensor=total_norm_cuda,
                                        op=torch.distributed.ReduceOp.SUM)

        total_norm = total_norm_cuda[0].item() ** (1. / norm_type)

        if total_norm == float(
                'inf') or total_norm == -float('inf') or total_norm != total_norm:
            total_norm = -1

        return total_norm

    def partition_previous_reduced_grads(self):
        if not self.previous_reduced_grads:
            return

        if self.offload_optimizer:
            allocate_grads_in_partition = self.grads_in_partition is None \
                and self.gradient_accumulation_steps > 1
        else:
            allocate_grads_in_partition = self.grads_in_partition is None

        if allocate_grads_in_partition:
            self.grads_in_partition = []

            for i, group in enumerate(self.fp16_groups):
                total_size = 0
                for param_in_partition in group:
                    total_size += param_in_partition.ds_tensor.ds_numel

                if self.verbose:
                    report_memory_usage(
                        f"group {i} before creating {total_size} reduced gradients into partition")
                if self.offload_param_pin_memory:
                    self.grads_in_partition.append(
                        torch.zeros(int(total_size),
                                    dtype=self.dtype,
                                    device=self.device).pin_memory())
                else:
                    self.grads_in_partition.append(
                        torch.zeros(int(total_size),
                                    dtype=self.dtype,
                                    device=self.device))
                if self.verbose:
                    report_memory_usage(
                        f"group {i} after creating {total_size} reduced gradients into partition")

        if self.offload_optimizer:
            offload_fp32_gradients = {}
            offload_fp32_offsets = {}

        with torch.cuda.stream(self.copy_grad_stream):
            self.reduction_stream.synchronize()
            for param in self.previous_reduced_grads:

                [i,
                 dest_offset,
                 num_elements] = self.grad_position[self.get_param_id(param)]

                if self.offload_optimizer:
                    param.partition_gradients(
                        partition_buffers=self.temp_grad_gpu_buffer)
                    # with torch.cuda.stream(self.copy_grad_stream):
                    #    self.reduction_stream.synchronize()

                    if self.gradient_accumulation_steps > 1:
                        # The allreduce buffer will be rewritted. Copy the gradients in partition to a new buffer
                        fp16_grad_tensor = self.grads_in_partition[i].narrow(
                            0,
                            dest_offset,
                            num_elements)
                        self.async_accumulate_grad_in_cpu_via_gpu(
                            param,
                            fp16_grad_tensor)

                    if self.is_gradient_accumulation_boundary:

                        self.set_norm_for_param_grad_in_gpu(param)

                        self.update_overflow_tracker_for_param_grad(param)

                        if self._swappable_optimizer_subgroup(i):
                            if not i in offload_fp32_gradients.keys():
                                offload_fp32_gradients[i] = []
                                offload_fp32_offsets[i] = []

                            offload_fp32_gradients[i].append(
                                param.grad.view(-1).float())
                            param.grad = None
                            offload_fp32_offsets[i].append(dest_offset)
                        else:
                            fp32_grad_tensor = self.fp32_partitioned_groups_flat[
                                i].grad.narrow(0,
                                               dest_offset,
                                               num_elements)

                            self.async_inplace_copy_grad_to_fp32_buffer_from_gpu(
                                param,
                                fp32_grad_tensor)
                else:
                    # The allreduce buffer will be rewritted. Copy the gradients in partition to a new buffer
                    fp16_grad_tensor = self.grads_in_partition[i].narrow(
                        0,
                        dest_offset,
                        num_elements)
                    param.partition_gradients(
                        partition_buffers=fp16_grad_tensor,
                        accumulate=True if self.micro_step_id > 0 else False)

            if self.offload_optimizer and self.swap_optimizer:
                for i in offload_fp32_gradients.keys():
                    self.optimizer_swapper.swap_out_gradients(
                        parameter=self.fp32_partitioned_groups_flat[i],
                        gradient_offsets=offload_fp32_offsets[i],
                        gradient_tensors=offload_fp32_gradients[i])

        self.previous_reduced_grads = []

    def reduce_ipg_grads(self, extra_param=None):
        if self.overlap_comm:
            self.reduction_stream.synchronize()

        with torch.cuda.stream(self.reduction_stream):
            self.partition_previous_reduced_grads()

        params_to_reduce = [param for i, param,
                            param_id in self.params_in_ipg_bucket]
        # print(f"Params in ipg bucket {self.params_in_ipg_bucket}")
        # print(f"Reducing {[(debug_param2name_id_shape(param), param.grad) for param in params_to_reduce]}")
        # exit(0)
        if self.contiguous_gradients:
            reduction_list = [self.ipg_buffer[self.ipg_index]]
            if self.extra_large_param_to_reduce is not None:
                reduction_list.append(self.extra_large_param_to_reduce.grad)
                self.extra_large_param_to_reduce = None
            self.average_tensor(reduction_list, params_to_reduce)
        else:
            self.buffered_reduce_fallback(
                None,
                self.grads_in_ipg_bucket,
                elements_per_buffer=self.elements_in_ipg_bucket)

        for _, param, param_id in self.params_in_ipg_bucket:
            self.params_already_reduced[param_id] = True

        self.previous_reduced_grads = params_to_reduce

        self.grads_in_ipg_bucket = []
        self.params_in_ipg_bucket = []
        self.elements_in_ipg_bucket = 0
        #####################################################################

    def reduce_ready_partitions_and_remove_grads(self, param, i):
        # print_rank_0(f"Backward {debug_param2name_id_shape(param)}", force=True)
        self.reduce_independent_p_g_buckets_and_remove_grads(param, i)

    def zero_reduced_gradients(self, partition_id, i):
        def are_all_related_partitions_reduced(params_id):
            for partition_id in self.param_to_partition_ids[i][params_id]:
                if not self.is_partition_reduced[i][partition_id]:
                    return False
            return True

        for params_id in self.is_grad_computed[i][partition_id]:
            if are_all_related_partitions_reduced(params_id):
                self.param_dict[params_id].grad = None

    def flatten_and_print(self, message, tensors, start=0, n=5):
        flatten_tensor = self.flatten(tensors)

        def print_func():
            print(flatten_tensor.contiguous().view(-1).narrow(0, start, n))

        self.sequential_execution(print_func, message)

    def get_grads_to_reduce(self, i, partition_id):
        def get_reducable_portion(key):
            grad = self.param_dict[key].grad
            total_elements = grad.numel()
            start = self.grad_start_offset[i][partition_id][key]
            num_elements = min(
                total_elements - start,
                self.partition_size[i] -
                self.grad_partition_insertion_offset[i][partition_id][key])
            if not pg_correctness_test:
                if num_elements == total_elements:
                    return grad
                else:
                    return grad.contiguous().view(-1).narrow(0,
                                                             int(start),
                                                             int(num_elements))
            else:
                if num_elements == total_elements:
                    return grad.clone()
                else:
                    return grad.clone().contiguous().view(-1).narrow(
                        0,
                        int(start),
                        int(num_elements))

        grads_to_reduce = []
        for key in self.is_grad_computed[i][partition_id]:
            grad = get_reducable_portion(key)
            grads_to_reduce.append(grad)
        return grads_to_reduce

    def sequential_execution(self, function, message, group=None):
        if group is None:
            group = self.dp_process_group
        if dist.get_rank(group=group) == 0:
            print(message)
        for id in range(dist.get_world_size(group=group)):
            if id == dist.get_rank(group=group):
                function()
            dist.barrier(group=group)

    def set_none_gradients_to_zero(self, i, partition_id):
        for param_id in self.is_grad_computed[i][partition_id]:
            param = self.param_dict[param_id]
            if param.grad is None:
                param.grad = torch.zero_like(param)

    ######################Reduction Related Methods##############################

    def allreduce_bucket(self, bucket, allreduce_always_fp32=False, rank=None, log=None):
        rank = None
        tensor = self.flatten(bucket)

        tensor_to_allreduce = tensor

        if pg_correctness_test:
            allreduce_always_fp32 = True

        if allreduce_always_fp32:
            tensor_to_allreduce = tensor.float()

        tensor_to_allreduce.div_(
            dist.get_world_size(group=self.dp_process_group))

        if rank is None:
            #    "All Reducing"
            dist.all_reduce(tensor_to_allreduce, group=self.dp_process_group)
        else:
            global_rank = _get_global_rank(self.dp_process_group, rank)
            dist.reduce(tensor_to_allreduce, global_rank,
                        group=self.dp_process_group)

        if allreduce_always_fp32 and tensor is not tensor_to_allreduce:
            if rank is None or rank == dist.get_rank(group=self.dp_process_group):
                tensor.copy_(tensor_to_allreduce)

        return tensor

    # if rank is specified do a reduction instead of an allreduce
    def allreduce_and_copy(self, small_bucket, rank=None, log=None):
        with torch.cuda.stream(self.reduction_stream):
            allreduced = self.allreduce_bucket(
                small_bucket, rank=rank, log=log)
            if rank is None or rank == dist.get_rank(group=self.dp_process_group):
                for buf, synced in zip(small_bucket, self.unflatten(allreduced, small_bucket)):
                    buf.copy_(synced)

    def allreduce_no_retain(self,
                            bucket,
                            numel_per_bucket=500000000,
                            rank=None,
                            log=None):
        small_bucket = []
        numel = 0
        for tensor in bucket:
            small_bucket.append(tensor)
            numel = numel + tensor.numel()
            if numel > numel_per_bucket:
                self.allreduce_and_copy(small_bucket, rank=rank, log=None)
                small_bucket = []
        if len(small_bucket) > 0:
            self.allreduce_and_copy(small_bucket, rank=rank, log=log)

    # allows using reduction of gradients instead of using all_reduce
    def buffered_reduce_fallback(self,
                                 rank,
                                 grads,
                                 elements_per_buffer=500000000,
                                 log=None):
        split_buckets = split_half_float_double(grads)

        for i, bucket in enumerate(split_buckets):
            self.allreduce_no_retain(bucket,
                                     numel_per_bucket=elements_per_buffer,
                                     rank=rank,
                                     log=log)

    #############################################################################
    #############################################################################
    #############################################################################

    # views the tensor as multiple partitions and returns
    # those partitions
    def get_data_parallel_partitions(self, tensor):
        partitions = []

        dp = dist.get_world_size(group=self.dp_process_group)
        dp_id = dist.get_rank(group=self.dp_process_group)

        total_num_elements = tensor.numel()

        base_size = total_num_elements // dp
        remaining = total_num_elements % dp

        start = 0
        for id in range(dp):
            partition_size = base_size
            if id < remaining:
                partition_size = partition_size + 1
            partitions.append(tensor.narrow(0, start, partition_size))
            start = start + partition_size
        return partitions

    def get_partition_info(self, tensor_list, partition_size, partition_id):
        params_in_partition = []
        params_not_in_partition = []

        start_index = partition_size * partition_id
        end_index = partition_size * (partition_id + 1)

        current_index = 0
        first_offset = 0

        for tensor in tensor_list:

            tensor_size = tensor.numel()

            if (current_index >= start_index and current_index < end_index):
                params_in_partition.append(tensor)

            elif start_index > current_index and start_index < (current_index +
                                                                tensor_size):
                params_in_partition.append(tensor)

                assert (
                    first_offset == 0), "This can happen either zero or only once as this must be the first tensor in the partition"
                first_offset = start_index - current_index

            else:
                params_not_in_partition.append(tensor)

            current_index = current_index + tensor_size

        return params_in_partition, params_not_in_partition, first_offset

    def zero_grad(self, set_grads_to_None=True):
        """
        Zero FP16 parameter grads.
        """
        # FP32 grad should never exist.
        # For speed, set model fp16 grad to None by default
        for group in self.fp16_groups:
            for p in group:
                if set_grads_to_None:
                    p.grad = None
                else:
                    if p.grad is not None:
                        p.grad.detach_()
                        p.grad.zero_()

    def _model_parallel_all_reduce(self, tensor, op):
        """ Perform all reduce within model parallel group, if any.
        """
        if self.model_parallel_group is None:
            pass
        else:
            torch.distributed.all_reduce(tensor=tensor,
                                         op=op,
                                         group=self.model_parallel_group)

    def clip_grad_norm(self, *args, **kwargs):
        # dummy function to retain the same function interface
        # as ColossalaiOptimizer for compatibility
        pass

    def get_grad_norm_direct(self, gradients, params, norm_type=2):
        """Clips gradient norm of an iterable of parameters.

        This is adapted from ``torch.nn.utils.clip_grad.clip_grad_norm_`` and
        added functionality to handle model parallel parameters. Note that
        the gradients are modified in place.

        Arguments:
            parameters (Iterable[Tensor] or Tensor): an iterable of Tensors or a
                single Tensor that will have gradients normalized
            max_norm (float or int): max norm of the gradients
            norm_type (float or int): type of the used p-norm. Can be ``'inf'`` for
                infinity norm.

        Returns:
            Total norm of the parameters (viewed as a single vector).
        """
        norm_type = float(norm_type)
        if norm_type == inf:
            total_norm = max(g.data.abs().max() for g in gradients)
            total_norm_cuda = torch.cuda.FloatTensor([float(total_norm)])
            torch.distributed.all_reduce(total_norm_cuda,
                                         op=torch.distributed.ReduceOp.MAX,
                                         group=self.dp_process_group)

            # Take max across all GPUs.
            self._model_parallel_all_reduce(tensor=total_norm_cuda,
                                            op=torch.distributed.ReduceOp.MAX)
            total_norm = total_norm_cuda[0].item()
        else:
            total_norm = 0.0
            # if dist.get_rank() == 0:
            #    print()(f"Total Norm begining {total_norm}")
            for g, p in zip(gradients, params):
                if is_model_parallel_parameter(p) or (self.model_parallel_rank == 0):
                    param_norm = g.data.double().norm(2)
                    total_norm += param_norm.item() ** 2
            # Sum across all model parallel GPUs.
            total_norm_cuda = torch.cuda.FloatTensor([float(total_norm)])

            torch.distributed.all_reduce(total_norm_cuda,
                                         op=torch.distributed.ReduceOp.SUM,
                                         group=self.dp_process_group)

            self._model_parallel_all_reduce(tensor=total_norm_cuda,
                                            op=torch.distributed.ReduceOp.SUM)

            total_norm = total_norm_cuda[0].item() ** (1. / norm_type)

        if total_norm == float(
                'inf') or total_norm == -float('inf') or total_norm != total_norm:
            total_norm = -1

        return total_norm

    # creates a flat fused tensor from the tensor list starting at the first_offset
    # in the first tensor of the list. If there are not enough elements in the tensor
    # list then the flat tensor will be padded with zeros
    def get_flat_partition(self,
                           tensor_list,
                           first_offset,
                           partition_size,
                           return_tensor_list=False):
        flat_tensor_list = []
        current_size = 0
        for i, tensor in enumerate(tensor_list):
            if tensor.grad is None:
                tensor.grad = torch.zeros_like(tensor)

            tensor = tensor.grad
            num_elements = tensor.numel()
            tensor_offset = 0

            # we need to offset to get to the right element
            if i == 0 and first_offset > 0:
                tensor_offset = first_offset
                num_elements = num_elements - tensor_offset

            # we dont need all elements of the tensor
            if num_elements > (partition_size - current_size):
                num_elements = partition_size - current_size

            # we need a narrow view of the tensor based on the tensor offset and number of elements that
            # we need from this tensor
            if tensor_offset > 0 or num_elements < tensor.numel():
                flat_tensor_list.append(tensor.contiguous().view(-1).narrow(
                    0,
                    int(tensor_offset),
                    int(num_elements)))
            else:
                flat_tensor_list.append(tensor)

            current_size = current_size + num_elements

        # this means its the last partition and does not align with the dp boundary. We need to pad before flattening
        if current_size < partition_size:
            flat_tensor_list.append(
                torch.zeros(int(partition_size - current_size),
                            dtype=tensor_list[0].dtype,
                            device=tensor_list[0].device))

        if return_tensor_list:
            return flat_tensor_list

        return self.flatten(flat_tensor_list)

    def free_grad_in_param_list(self, param_list):
        for p in param_list:
            p.grad = None

    def reset_cpu_buffers(self):
        self.norm_for_param_grads = {}
        self.local_overflow = False

    def log_timers(self, timer_names):
        if self.timers is None:
            return

        self.timers.log(names=list(timer_names))

    def start_timers(self, timer_names):
        if self.timers is None:
            return

        for name in timer_names:
            self.timers(name).start()

    def stop_timers(self, timer_names):
        if self.timers is None:
            return

        for name in timer_names:
            self.timers(name).stop()

    def _pre_step(self):
        self.micro_step_id = INITIAL_MICRO_STEP_ID

        if self.verbose:
            print_rank_0(f"Inside Step function")
            report_memory_usage(f"In step before checking overflow")
            print_rank_0("Finished Tracing at Beginning of Step")
        self.param_coordinator.hierarchy = 0
        self.param_coordinator.finish_tracing(print_trace=True)

        self.param_coordinator.reset_step()

        if self.verbose:
            print_rank_0("Finished Tracing at Beginning of Step")

    def _get_norm_groups(self):
        norm_groups = []
        for i, group in enumerate(self.fp16_groups):
            if self.offload_optimizer:
                norm_groups.append(
                    self.complete_grad_norm_calculation_for_cpu_offload(
                        self.fp16_groups[i]))
            else:
                norm_groups.append(
                    self.get_grad_norm_direct(self.averaged_gradients[i],
                                              self.fp16_groups[i]))
        return norm_groups

    def _prepare_fp32_grad_for_sub_group(self, sub_group_id):
        partition_id = dist.get_rank(group=self.dp_process_group)

        single_grad_partition = self.flatten(self.averaged_gradients[sub_group_id]).to(
            self.fp32_partitioned_groups_flat[sub_group_id].dtype)

        assert single_grad_partition.numel() == self.fp32_partitioned_groups_flat[sub_group_id].numel(), \
            "averaged gradients have different number of elements that partition size {} {} {} {}".format(
                single_grad_partition.numel(
                ), self.fp32_partitioned_groups_flat[sub_group_id].numel(), sub_group_id,
                partition_id)

        self.fp32_partitioned_groups_flat[sub_group_id].grad = single_grad_partition

        # release all the gradient since we have already created a necessary copy in dp_grad_partition
        self.zero_grad()

        self.averaged_gradients[sub_group_id] = None

    def _prepare_sub_group(self, sub_group_id, timer_names=set()):
        if self.verbose:
            report_memory_usage(
                f'Before prepare optimizer sub group {sub_group_id}')
        if self._swappable_optimizer_subgroup(sub_group_id):
            self._optimizer_states_and_gradient_swap_in(
                sub_group_id, timer_names)
        elif not self.offload_optimizer:
            self._prepare_fp32_grad_for_sub_group(sub_group_id)
        if self.verbose:
            report_memory_usage(
                f'After prepare optimizer sub group {sub_group_id}')

    def _optimizer_states_and_gradient_swap_in(self, sub_group_id, timer_names=set()):
        param_length = self.fp16_partitioned_groups_flat_numel[sub_group_id]
        fp32_param_id = id(self.fp32_partitioned_groups_flat[sub_group_id])
        assert self._swappable_optimizer_subgroup(sub_group_id), \
            f'Parameter {fp32_param_id} of numel={param_length} is not swappable'

        OPTIMIZER_SWAP_IN_STATE = 'optimizer_swap_in_state'
        if self.verbose:
            report_memory_usage(
                f'pre-step Before swapping in optimizer tensors {sub_group_id}')
        self.start_timers([OPTIMIZER_SWAP_IN_STATE])

        self.optimizer_swapper.swap_in_optimizer_state(
            parameter=self.fp32_partitioned_groups_flat[sub_group_id],
            async_parameter=self.next_swappable_fp32_partitioned_groups[sub_group_id])

        self.stop_timers([OPTIMIZER_SWAP_IN_STATE])
        timer_names.add(OPTIMIZER_SWAP_IN_STATE)
        if self.verbose:
            report_memory_usage(
                f'pre-step After swapping in optimizer tensors {sub_group_id}')

    def _release_sub_group(self, sub_group_id, timer_names=set()):
        if self.verbose:
            report_memory_usage(
                f'Before release optimizer sub group {sub_group_id}')
        # get rid of the fp32 gradients. Not needed anymore
        if not self.offload_optimizer:
            self.fp32_partitioned_groups_flat[sub_group_id].grad = None

        if self._swappable_optimizer_subgroup(sub_group_id):
            self._optimizer_states_and_gradient_swap_out(
                sub_group_id, timer_names)
        if self.verbose:
            report_memory_usage(
                f'After release optimizer sub group {sub_group_id}')

    # create a flat tensor aligned at the alignment boundary
    def flatten_dense_tensors_aligned(self, tensor_list, alignment):
        num_elements = 0
        for tens in tensor_list:
            num_elements = num_elements + tens.numel()

        remaining = num_elements % alignment

        if remaining:
            elements_to_add = alignment - remaining
            pad_tensor = torch.zeros(elements_to_add,
                                     device=tensor_list[0].device,
                                     dtype=tensor_list[0].dtype)
            padded_tensor_list = tensor_list + [pad_tensor]

            num_elements = num_elements + elements_to_add
        else:
            padded_tensor_list = tensor_list

        return self.flatten(padded_tensor_list)

    def _optimizer_states_and_gradient_swap_out(self, sub_group_id, timer_names=set()):
        param_length = self.fp16_partitioned_groups_flat_numel[sub_group_id]
        fp32_param_id = id(self.fp32_partitioned_groups_flat[sub_group_id])
        assert self._swappable_optimizer_subgroup(sub_group_id), \
            f'Parameter {fp32_param_id} of numel={param_length} is not swappable'

        OPTIMIZER_SWAP_OUT_STATE = 'optimizer_swap_out_state'
        if self.verbose:
            report_memory_usage(
                f'post-step Before swapping out optimizer tensors {sub_group_id}')
        self.start_timers([OPTIMIZER_SWAP_OUT_STATE])

        self.optimizer_swapper.swap_out_optimizer_state(
            parameter=self.fp32_partitioned_groups_flat[sub_group_id],
            async_swap=self.next_swappable_fp32_partitioned_groups[sub_group_id] is
            not None)

        self.stop_timers([OPTIMIZER_SWAP_OUT_STATE])
        if self.verbose:
            report_memory_usage(
                f'post-step After swapping out optimizer tensors {sub_group_id}')
        timer_names.add(OPTIMIZER_SWAP_OUT_STATE)

        # get rid of the fp32 gradients. Not needed anymore
        self.fp32_partitioned_groups_flat[sub_group_id].grad = None

    def _unflatten_partitioned_parameters(self, sub_group_id):
        updated_params = self.unflatten(self.fp16_partitioned_groups_flat[sub_group_id],
                                        self.fp16_partitioned_groups[sub_group_id])

        for partitioned_param, q in zip(self.fp16_partitioned_groups[sub_group_id], updated_params):
            partitioned_param.data = q.data

    def _overflow_clean_up(self, prev_scale):
        if self.verbose:
            report_memory_usage('After overflow before clearing gradients')
        self.zero_grad()

        if self.offload_optimizer:
            self.reset_cpu_buffers()
        else:
            self.averaged_gradients = {}

        if self.verbose:
            report_memory_usage('After overflow after clearing gradients')

        if torch.distributed.get_rank() == 0:
            print(
                "[deepscale] OVERFLOW! Rank {} Skipping step. Attempted loss scale: {}, "
                "reducing to {}".format(dist.get_rank(),
                                        prev_scale,
                                        self.loss_scale))

    def _overflow_check_and_loss_scale_update(self):

        # First compute norm for all group so we know if there is overflow
        self.check_overflow()

        # loss scaling related computation
        prev_scale = self.loss_scale
        self._update_scale(self.overflow)

        if self.overflow:
            self._overflow_clean_up(prev_scale)

        return self.overflow

    def _post_step(self, timer_names=set()):
        if self.offload_optimizer:
            self.reset_cpu_buffers()

        # Gathering persisting parameters
        if len(self.persistent_parameters) > 0:
            self.persistent_parameters[0].all_gather(
                self.persistent_parameters)

        if self.swap_optimizer:
            self.optimizer_swapper.log_timers()

        self.log_timers(timer_names)

        if self.verbose:
            report_memory_usage('After zero_optimizer step')
            print_rank_0(
                f"------------------Finishing Step-----------------------")

    def _reassign_or_swap_out_partitioned_parameters(self, sub_group_id):
        if self.fp16_partitioned_groups_flat[sub_group_id] is not None:
            self.fp16_partitioned_groups_flat[sub_group_id].data.copy_(
                self.fp32_partitioned_groups_flat[sub_group_id].data)

            # unflatten fp16 parameter subgroup
            self._unflatten_partitioned_parameters(sub_group_id)
        else:
            self._partitioned_params_swap_out(sub_group_id)

    def allreduce_gradients(self):
        self.overlapping_partition_gradients_reduce_epilogue()

    def step(self, closure=None):
        """
            Not supporting closure.
            """
        self._pre_step()

        # checks for overflow, adjust the loss scale accordingly
        if self._overflow_check_and_loss_scale_update():
            if self.swap_optimizer:
                self.optimizer_swapper.log_timers()
            return

        norm_groups = self._get_norm_groups()

        timer_names = set()

        timer_names.add('optimizer_step')
        self.start_timers(['optimizer_step'])

        # update parameters one sub group at a time
        for sub_group_id, group in enumerate(self.fp16_groups):
            # prepare optimizer states, gradients and fp32 parameters for update
            self._prepare_sub_group(sub_group_id, timer_names)

            # scale the fp32 gradients
            self.unscale_and_clip_grads(sub_group_id, norm_groups)

            # apply the optimizer step on the sub group and copy fp32 parameters to fp16
            self._optimizer_step(sub_group_id)

            # put fp16 parameters in appropriate location
            self._reassign_or_swap_out_partitioned_parameters(sub_group_id)

            # release memory or swap out optimizer states of fp32 parameters
            self._release_sub_group(sub_group_id, timer_names)

        self.stop_timers(['optimizer_step'])

        self._post_step(timer_names)
        return

    def dump_pre_step_gradients(self, debug_fp32_grads):
        # Dump gradient norms for debbuging
        for i, _ in enumerate(self.fp16_groups):
            if self.verbose:
                print(
                    f'Pre-Step Dump Norms for Group {i} FP16P, FP16G, FP32G, FP32GUC')
            for fp16_param, fp32_grad in zip(self.fp16_groups[i], debug_fp32_grads[i]):
                param_id = self.get_param_id(fp16_param)
                fp16_grad_norm = self.debug_fp16_grads[i][param_id]

                fp32_grad_norm = [float(t.data.float().norm(2))
                                  for t in fp32_grad]
                norm_list = [fp16_grad_norm, fp32_grad_norm]
                if self.verbose:
                    print(f'Pre-Step Norms {i} {param_id} = {norm_list}')

    def dump_post_step_gradients(self):
        # Dump gradient norms for debbuging
        for i, group in enumerate(self.fp16_groups):
            if self.verbose:
                print(
                    f'Post-Step Dump Norms for Group {i} FP16P, FP16DS, FP16FLAT, FP32FLAT')
            unflat_fp16 = self.unflatten(
                self.fp16_groups_flat[i], self.fp16_groups[i])
            unflat_fp32 = self.unflatten(self.fp32_partitioned_groups_flat[i],
                                         self.fp16_groups[i])
            for j, p in enumerate(self.fp16_groups[i]):
                param_id = self.get_param_id(p)
                param_norm = float(p.data.float().norm(2))
                ds_norm = float(p.ds_tensor.data.float().norm(2))

                unflat_norm = [
                    float(t.data.float().norm(2))
                    for t in [unflat_fp16[j],
                              unflat_fp32[j]]
                ]
                norm_list = [param_norm, ds_norm] + unflat_norm
                if self.verbose:
                    print(f'Post-Step Norms {i} {param_id} = {norm_list}')

    def unscale_and_clip_grads(self, sub_group_id, norm_groups):

        grad_groups_flat = [
            self.fp32_partitioned_groups_flat[sub_group_id].grad]

        total_norm = 0.0
        for norm in norm_groups:
            total_norm += norm ** 2.0
        total_norm = math.sqrt(total_norm)

        # compute combined scale factor for this group
        combined_scale = self.loss_scale
        if self.clip_grad > 0.:
            # norm is in fact norm*scale
            clip = ((total_norm / self.loss_scale) + 1e-6) / self.clip_grad
            if clip > 1:
                combined_scale = clip * self.loss_scale

        for grad in grad_groups_flat:
            if isinstance(grad, list):
                sub_partitions = grad
                for g in sub_partitions:
                    g.data.mul_(1. / combined_scale)
            else:
                grad.data.mul_(1. / combined_scale)

    def _check_overflow(self, partition_gradients=True):
        self.overflow = self.has_overflow(partition_gradients)

    # `params` is a list / generator of torch.Variable
    def has_overflow_serial(self, params, is_grad_list=False):
        for p in params:
            if p.grad is not None and self._has_inf_or_nan(p.grad.data):
                return True

        return False

    def has_overflow_partitioned_grads_serial(self):
        for i in range(len(self.fp16_groups)):
            for j, grad in enumerate(self.averaged_gradients[i]):
                if grad is not None and self._has_inf_or_nan(grad.data, j):
                    return True
        return False

    def has_overflow(self, partition_gradients=True):
        if partition_gradients:
            if self.overlap_comm:
                self.local_overflow = self._has_inf_or_nan(self.gpu_sum)
                self.gpu_sum = torch.zeros(1, dtype=torch.float).cuda()

            overflow = self.local_overflow if self.offload_optimizer else self.has_overflow_partitioned_grads_serial(
            )
            # overflow = self.has_overflow_partitioned_grads_serial()
            overflow_gpu = torch.cuda.ByteTensor([overflow])
            torch.distributed.all_reduce(overflow_gpu,
                                         op=torch.distributed.ReduceOp.MAX,
                                         group=self.dp_process_group)

        else:
            params = []
            for group in self.fp16_groups:
                for param in group:
                    params.append(param)

            overflow = self.has_overflow_serial(
                params, is_grad_list=partition_gradients)
            overflow_gpu = torch.cuda.ByteTensor([overflow])

        # Since each model parallel GPU carries only part of the model,
        # make sure overflow flag is synced across all the model parallel GPUs
        self._model_parallel_all_reduce(tensor=overflow_gpu,
                                        op=torch.distributed.ReduceOp.MAX)

        overflow = overflow_gpu[0].item()
        return bool(overflow)

    # `x` is a torch.Tensor
    @staticmethod
    def _has_inf_or_nan(x, j=None):
        try:
            # if x is half, the .float() incurs an additional deep copy, but it's necessary if
            # Pytorch's .sum() creates a one-element tensor of the same type as x
            # (which is true for some recent version of pytorch).
            cpu_sum = float(x.float().sum())
            # More efficient version that can be used if .sum() returns a Python scalar
            # cpu_sum = float(x.sum())
        except RuntimeError as instance:
            # We want to check if inst is actually an overflow exception.
            # RuntimeError could come from a different error.
            # If so, we still want the exception to propagate.
            if "value cannot be converted" not in instance.args[0]:
                raise
            return True
        else:
            if cpu_sum == float('inf') or cpu_sum == -float('inf') or cpu_sum != cpu_sum:
                return True
            return False

    def backward(self, loss, retain_graph=False):
        """
        :attr:`backward` performs the following steps:

        1. fp32_loss = loss.float()
        2. scaled_loss = fp32_loss*loss_scale
        3. scaled_loss.backward(), which accumulates scaled gradients into the ``.grad`` attributes of the model's fp16 leaves
        """
        self.micro_step_id += 1
        if self.verbose:
            print_rank_0(
                f"Total fully available parameters {self.param_coordinator.total_available_parameter_numel}"
            )

        if self.swap_optimizer:
            self.optimizer_swapper.pre_backward()

        if self.verbose:
            report_memory_usage(f"Before backward")

        if self.contiguous_gradients:
            self.ipg_buffer = []
            buf_0 = torch.empty(self.reduce_bucket_size,
                                dtype=self.dtype,
                                device=torch.cuda.current_device())
            self.ipg_buffer.append(buf_0)

            # Use double buffers to avoid data access conflict when overlap_comm is enabled.
            if self.overlap_comm:
                buf_1 = torch.empty(self.reduce_bucket_size,
                                    dtype=self.dtype,
                                    device=torch.cuda.current_device())
                self.ipg_buffer.append(buf_1)
            self.ipg_index = 0

        self.loss_scaler.backward(loss.float(), retain_graph=retain_graph)
        '''Partitioning Parameters that were not partitioned
        Usually if parameters of modules whose input parameters do not require
        grad computation do not trigger post call and will therefore will remain unpartitioned '''
        self._partition_all_parameters()

        if self.swap_optimizer:
            self.optimizer_swapper.post_backward()

    def _partition_all_parameters(self):
        for name, param in self.module.named_parameters(recurse=True):
            self.param_coordinator.release_and_reset_parameter(param)

    def check_overflow(self, partition_gradients=True):
        self._check_overflow(partition_gradients)

    def _update_scale(self, has_overflow=False):
        self.loss_scaler.update_scale(has_overflow)

    # Promote state so it can be retrieved or set via "fp16_optimizer_instance.state"
    def _get_state(self):
        return self.optimizer.state

    def _set_state(self, value):
        self.optimizer.state = value

    state = property(_get_state, _set_state)

    # Promote param_groups so it can be retrieved or set via "fp16_optimizer_instance.param_groups"
    # (for example, to adjust the learning rate)
    def _get_param_groups(self):
        return self.optimizer.param_groups

    def _set_param_groups(self, value):
        self.optimizer.param_groups = value

    param_groups = property(_get_param_groups, _set_param_groups)

    # Promote loss scale so it can be retrieved or set via "fp16_optimizer_instance.loss_scale"
    def _get_loss_scale(self):
        return self.loss_scaler.loss_scale

    def _set_loss_scale(self, value):
        self.loss_scaler.cur_scale = value

    loss_scale = property(_get_loss_scale, _set_loss_scale)
    cur_scale = property(_get_loss_scale, _set_loss_scale)

    def _get_lean_tensors(self, padded_flattened_tensor, group_tensors, paddings):
        # Remove paddings from flattened tensor
        individual_tensors = self.unflatten(
            padded_flattened_tensor, group_tensors)
        lean_lengths = [t.numel() - pad for t,
                        pad in zip(group_tensors, paddings)]
        lean_tensors = [t[:len]
                        for t, len in zip(individual_tensors, lean_lengths)]
        # print()(f'rank {dist.get_rank()}: lean_tensors = {[t.numel() for t in lean_tensors]}')
        return lean_tensors

    # TODO REVISIT this for stage 3
    def get_lean_optimizer_state(self):
        # Return optimizer states after removing paddings.
        # This method assumes that each param group contains a single flattened tensor.
        optimizer_groups_state = []

        for i, group in enumerate(self.optimizer.param_groups):
            p = group['params'][0]
            lean_state = {}
            for key, value in self.optimizer.state[p].items():
                if torch.is_tensor(value):
                    padded_lens = [t.numel()
                                   for t in self.fp16_partitioned_groups[i]]
                    lean_state[key] = self._get_lean_tensors(
                        value,
                        self.fp16_partitioned_groups[i],
                        self.groups_padding[i])
                    lean_flat_len = sum([t.numel() for t in lean_state[key]])
                else:
                    lean_state[key] = value

            optimizer_groups_state.append(lean_state)

        return optimizer_groups_state

    def get_groups_without_padding(self, groups_with_padding):
        # Return group tensor after removing paddings added for alignment to DP world size.
        groups_without_padding = []
        for i, group in enumerate(groups_with_padding):
            lean_group = self._get_lean_tensors(group,
                                                self.fp16_partitioned_groups[i],
                                                self.groups_padding[i])
            groups_without_padding.append(lean_group)

        return groups_without_padding

    def _set_fp32_optimizer_param_groups(self):
        for sub_group_id, _ in enumerate(self.fp16_groups):
            param_group_id = self.sub_group_to_group_id[sub_group_id]
            self.optimizer.param_groups[param_group_id]['params'].append(
                self.fp32_partitioned_groups_flat[sub_group_id])

    def _clear_fp32_optimizer_param_groups(self):
        for param_group in self.optimizer.param_groups:
            param_group['params'] = []

    def _rigid_state_dict(self):
        state_dict = {}
        state_dict['zero_stage'] = ZERO_OPTIMIZATION_WEIGHTS
        state_dict['loss_scaler'] = self.loss_scaler
        state_dict['dynamic_loss_scale'] = self.dynamic_loss_scale
        state_dict['overflow'] = self.overflow
        state_dict['partition_count'] = self.partition_count

        self._set_fp32_optimizer_param_groups()
        state_dict['optimizer_state_dict'] = self.optimizer.state_dict()
        state_dict['fp32_flat_groups'] = self.fp32_partitioned_groups_flat
        self._clear_fp32_optimizer_param_groups()

        return state_dict

    def state_dict(self):
        """
        Returns a dict containing the current state of this :class:`FP16_Optimizer` instance.
        This dict contains attributes of :class:`FP16_Optimizer`, as well as the state_dict
        of the contained Pytorch optimizer.

        Example::

            checkpoint = {}
            checkpoint['model'] = model.state_dict()
            checkpoint['optimizer'] = optimizer.state_dict()
            torch.save(checkpoint, "saved.pth")
        """
        if self.elastic_checkpoint:
            raise NotImplementedError(
                "ZeRO-3 does not yet support elastic checkpointing, please disable for now."
            )

        if self.swap_optimizer or self.params_in_nvme_and_cpu:
            raise NotImplementedError(
                "ZeRO-3 does not yet support checkpointing with NVMe offloading, please disable for now."
            )

        return self._rigid_state_dict()

    # Restore base optimizer fp32 weights from checkpoint by:
    # 1) Merging fp32 weights from checkpoints of all partitions
    # 2) Extracting fp32 weights for current partition from merged weights
    # 3) Using extracted weights to update base optimizer weights directly.

    def _restore_from_fp32_weights(self, all_state_dict):

        flat_local_partition = []
        for i in range(len(self.fp32_partitioned_groups_flat)):
            merged_partitions = [sd['fp32_groups'][i] for sd in all_state_dict]
            flat_local_partition.append(
                self._get_flattened_partition(merged_partitions))

        for current, saved in zip(self.fp32_partitioned_groups_flat, flat_local_partition):
            current.data.copy_(saved.data)

    # Restore base optimizer fp32 weights from ZeRO fp16 weights
    def _restore_from_fp16_weights(self):
        for fp16_partitions, fp32_partition in zip(self.fp16_partitioned_groups_flat,
                                                   self.fp32_partitioned_groups_flat):
            fp32_partition.data.copy_(fp16_partitions.data)

    # Refresh the fp32 master params from the fp16 copies.
    def refresh_fp32_params(self):
        self._restore_from_fp16_weights()

    # Extract flattened partion for current rank from all partitions
    def _get_flattened_partition(self, all_partition_states):
        partition_id = dist.get_rank(group=self.dp_process_group)
        alignment = dist.get_world_size(group=self.dp_process_group)

        param_partitions = [[] for _ in range(len(all_partition_states[0]))]
        for i, partition in enumerate(all_partition_states):
            for j, param in enumerate(partition):
                param_partitions[j].append(param)

        local_state_partitions = []
        for param_index, param_slices in enumerate(param_partitions):
            flattened_merged_tensor = self.flatten_dense_tensors_aligned(
                param_slices,
                alignment)
            new_partitions = self.get_data_parallel_partitions(
                flattened_merged_tensor)
            local_state_partitions.append(new_partitions[partition_id])

        if torch.is_tensor(local_state_partitions[0]):
            return self.flatten_dense_tensors_aligned(local_state_partitions, alignment)

        # Assume non-tensor states are not partitioned and equal across ranks, so return first one
        return local_state_partitions[0]

    # Restore base optimizer state from checkpoint by
    # 1) Merging optimizer state from checkpoints of all partitions
    # 2) Extracting optimizer state for current partition from the merged state
    # 3) Using the extracted value to directly update the base optimizer.
    def _restore_base_optimizer_state(self, all_state_dict):
        base_optimizer_group_states = []
        for i in range(len(self.optimizer.param_groups)):
            partition_states = {}
            all_partition_group_states = [
                sd['base_optimizer_state'][i] for sd in all_state_dict
            ]
            for key in all_partition_group_states[0].keys():
                all_partition_states = [
                    all_states[key] for all_states in all_partition_group_states
                ]
                partition_states[key] = self._get_flattened_partition(
                    all_partition_states)
            base_optimizer_group_states.append(partition_states)

        for i, group in enumerate(self.optimizer.param_groups):
            p = group['params'][0]
            for key, saved in base_optimizer_group_states[i].items():
                if torch.is_tensor(self.optimizer.state[p][key]):
                    self.optimizer.state[p][key].data.copy_(saved.data)
                else:
                    self.optimizer.state[p][key] = saved

    def _rigid_load_state_dict(self, state_dict, load_optimizer_states=True):
        # I think it should actually be ok to reload the optimizer before the model.
        self.loss_scaler = state_dict['loss_scaler']
        self.dynamic_loss_scale = state_dict['dynamic_loss_scale']
        self.overflow = state_dict['overflow']

        if load_optimizer_states:
            self._set_fp32_optimizer_param_groups()
            self.optimizer.load_state_dict(state_dict['optimizer_state_dict'])
            self._clear_fp32_optimizer_param_groups()

        # restore fp32 partitions
        for curr_param, saved_param in zip(self.fp32_partitioned_groups_flat, state_dict['fp32_flat_groups']):
            curr_param.data.copy_(saved_param.data)

        # restore fp16 partitions from fp32
        for sub_group_id in range(len(self.fp32_partitioned_groups_flat)):
            fp32_param = self.fp32_partitioned_groups_flat[sub_group_id]
            fp16_param = self.fp16_partitioned_groups_flat[sub_group_id]
            fp16_param.data.copy_(fp32_param.data)

        # update fp16 unflattened params
        for sub_group_id in range(len(self.fp16_partitioned_groups_flat)):
            updated_params = self.unflatten(
                self.fp16_partitioned_groups_flat[sub_group_id],
                self.fp16_partitioned_groups[sub_group_id])

            for partitioned_param, q in zip(self.fp16_partitioned_groups[sub_group_id], updated_params):
                partitioned_param.data = q.data

    # TODO: Support different/changing load/save DP degree.
    def load_state_dict(self,
                        state_dict_list,
                        load_optimizer_states=True,
                        load_from_fp32_weights=False):
        r"""Loading a ZeRO checkpoint

        Loads a state_dict created by an earlier call to state_dict().
        If ``fp16_optimizer_instance`` was constructed from some ``init_optimizer``,
        whose parameters in turn came from ``model``, it is expected that the user
        will call ``model.load_state_dict()`` before
        ``fp16_optimizer_instance.load_state_dict()`` is called.

        Arguments:
            state_dict_list: List of all saved ZeRO checkpoints, one for each saved partition.
                Note that the number of saved partitions may differ from number of loading partitions to support
                changing GPU count, specifically DP world size, between saving and loading checkpoints.
            load_optimizer_states: Boolean indicating whether or not to load base optimizer states
            load_from_fp32_weights: Boolean indicating whether to initialize fp32 master weights from fp32
            copies in checkpoints (no precision loss) or from model's fp16 copies (with precision loss).

        Example::

            model = torch.nn.Linear(D_in, D_out).cuda().half()
            optimizer = torch.optim.SGD(model.parameters(), lr=1e-3)
            optimizer = FP16_Optimizer(optimizer, static_loss_scale = 128.0)
            ...
            checkpoint = torch.load("saved.pth")
            model.load_state_dict(checkpoint['model'])
            optimizer.load_state_dict(checkpoint['optimizer'])
        """

        if self.elastic_checkpoint:
            raise NotImplementedError(
                "ZeRO-3 does not yet support elastic checkpointing, please disable for now."
            )

        if self.swap_optimizer or self.params_in_nvme_and_cpu:
            raise NotImplementedError(
                "ZeRO-3 does not yet support checkpointing with NVMe offloading, please disable for now."
            )

        self._rigid_load_state_dict(
            state_dict_list[dist.get_rank(group=self.dp_process_group)],
            load_optimizer_states=load_optimizer_states)

        if len(self.persistent_parameters) > 0:
            self.persistent_parameters[0].partition(self.persistent_parameters)
            self.persistent_parameters[0].all_gather(
                self.persistent_parameters)

    def save_checkpoint_prologue(self):
        self._partition_all_parameters()

    def save_checkpoint_epilogue(self):
        if len(self.persistent_parameters) > 0:
            self.persistent_parameters[0].all_gather(
                self.persistent_parameters)


def _handle_overflow(cpu_sum, x, i):
    import math
    rank = torch.distributed.get_rank()
    if rank == 0:
        t_i = -1
        for v_i, v in enumerate(x.data.contiguous().view(-1)):
            if not math.isfinite(float(v)):
                t_i = v_i
                break
        print(
            f"rank {rank} detected overflow {cpu_sum} in tensor {i}:{t_i} shape {x.shape}"
        )


def estimate_zero3_model_states_mem_needs(total_params,
                                          largest_layer_params,
                                          num_gpus_per_node=1,
                                          num_nodes=1,
                                          cpu_offload=True,
                                          cpu_offload_params=True,
                                          zero_init=True,
                                          additional_buffer_factor=1.5):
    total_gpus = num_nodes * num_gpus_per_node
    gpus_factor = 1 / num_nodes
    largest_layer_memory = (4 * largest_layer_params)

    if cpu_offload:
        if cpu_offload_params:
            gpu_mem = largest_layer_memory

            if zero_init:
                cpu_mem = total_params * 18 * gpus_factor * additional_buffer_factor
            else:
                cpu_mem = total_params * max(4 * num_gpus_per_node,
                                             18 * gpus_factor) * additional_buffer_factor

        else:
            gpu_mem = largest_layer_memory + int(2 * total_params / total_gpus)

            if zero_init:
                cpu_mem = total_params * 16 * gpus_factor * additional_buffer_factor
            else:
                cpu_mem = total_params * max(4 * num_gpus_per_node,
                                             16 * gpus_factor) * additional_buffer_factor
    else:
        gpu_mem = largest_layer_memory + int(18 * total_params / total_gpus)
        if zero_init:
            cpu_mem = largest_layer_params * 4 * num_gpus_per_node * additional_buffer_factor
        else:
            cpu_mem = total_params * 4 * num_gpus_per_node * additional_buffer_factor

    return int(cpu_mem), int(gpu_mem), largest_layer_memory


def model_to_params(model):
    # shared params calculated only once
    total_params = sum(
        dict((p.data_ptr(),
              p.numel()) for p in model.parameters()).values())

    largest_layer_params = 0
    for m in model.modules():
        # assuming no shared params within a single layer
        layer_params = sum(p.numel() for p in m.parameters(recurse=False))
        largest_layer_params = max(largest_layer_params, layer_params)

    return total_params, largest_layer_params


def estimate_zero3_model_states_mem_needs_all_live(model,
                                                   num_gpus_per_node=1,
                                                   num_nodes=1,
                                                   additional_buffer_factor=1.5):
    """
    Print out estimates on memory usage requirements for ZeRO 3 params, optim states and gradients
    for a given ``model`` and hardware setup.

    If you have an actual model object, use this function and everything will be derived
    automatically.

    If it's a hypothetical model, use ``estimate_zero3_model_states_mem_needs_all_cold`` where you have to pass
    the ``total_params`` and ``largest_layer_params`` explicitly.

    Args:
        - ``model``: ``nn.Module`` object
        - ``num_gpus_per_node``: how many gpus per node (defaults to 1)
        - ``num_nodes``: how many nodes (defaults to 1),
        - ``additional_buffer_factor``: estimation factor (defaults to 1.5):

    """

    total_params, largest_layer_params = model_to_params(model)

    estimate_zero3_model_states_mem_needs_all_cold(
        total_params=total_params,
        largest_layer_params=largest_layer_params,
        num_gpus_per_node=num_gpus_per_node,
        num_nodes=num_nodes,
        additional_buffer_factor=additional_buffer_factor)


def estimate_zero3_model_states_mem_needs_all_cold(total_params,
                                                   largest_layer_params,
                                                   num_gpus_per_node=1,
                                                   num_nodes=1,
                                                   additional_buffer_factor=1.5):
    """
    Print out estimates on memory usage requirements for ZeRO 3 params, optim states and gradients
    for a given ``model`` and hardware setup.

    If it's a hypothetical model, use this function where you have to pass
    the ``total_params`` and ``largest_layer_params`` explicitly.

    If you have an actual model object, use ``estimate_zero3_model_states_mem_needs_all_live`` and everything
    will be derived automatically.

    Args:
        - ``total_params``: total  model params
        - ``largest_layer_params``: largest layer's params
        - ``num_gpus_per_node``: how many gpus per node (defaults to 1)
        - ``num_nodes``: how many nodes (defaults to 1),
        - ``additional_buffer_factor``: estimation factor (defaults to 1.5):

    """

    def format_options(cpu_offload, cpu_offload_params, zero_init):
        enabled = []
        enabled.append(f"cpu_offload={1 if cpu_offload else 0}")
        enabled.append(f"cpu_offload_params={1 if cpu_offload_params else 0}")
        enabled.append(f"zero_init={1 if zero_init else 0}")
        return ", ".join(enabled)

    nodes_str = "nodes" if num_nodes > 1 else "node"
    gpus_str = "GPUs" if num_gpus_per_node > 1 else "GPU"
    print(
        "Estimated memory needed for params, optim states and gradients for a:\n"
        f"HW: Setup with {num_nodes} {nodes_str}, {num_gpus_per_node} {gpus_str} per node.\n"
        f"SW: Model with {int(total_params / 1e6)}M total params, {int(largest_layer_params / 1e6)}M largest layer params."
    )
    print("  per CPU  |  per GPU |   Options")
    for cpu_offload in [True, False]:
        for cpu_offload_params in [True, False]:
            if not cpu_offload and cpu_offload_params:
                continue
            for zero_init in [True, False]:
                cpu_mem, gpu_mem, largest_layer_memory = estimate_zero3_model_states_mem_needs(
                    total_params=total_params,
                    largest_layer_params=largest_layer_params,
                    num_gpus_per_node=num_gpus_per_node,
                    num_nodes=num_nodes,
                    cpu_offload=cpu_offload,
                    cpu_offload_params=cpu_offload_params,
                    zero_init=zero_init,
                    additional_buffer_factor=additional_buffer_factor
                )

                options_str = format_options(cpu_offload=cpu_offload,
                                             cpu_offload_params=cpu_offload_params,
                                             zero_init=zero_init)
                print(
                    f" {cpu_mem / 2 ** 30:7.2f}GB | {gpu_mem / 2 ** 30:6.2f}GB | {options_str}")
