fp8.py

# Copyright (c) 2022-2023, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
#
# See LICENSE for license information.

"""FP8 utilities for TransformerEngine"""
from contextlib import contextmanager
from collections import deque
from typing import Callable, List, Optional, Dict, Any, Tuple, Union

import torch
import transformer_engine_extensions as tex
from transformer_engine.common.recipe import DelayedScaling, Format

from .constants import dist_group_type
from .utils import get_device_compute_capability

_FP8_ENABLED = False
_FP8_CALIBRATION = False
_FP8_RECIPE = None
_FP8_DISTRIBUTED_GROUP = None
_IS_FIRST_FP8_MODULE = False
_FP8_AUTOCAST_COUNTER = 0
_FP8_CURRENT_CONTEXT_ID = 0
_FP8_AUTOCAST_DEPTH = 0
_global_fp8_buffer = {}
_fp8_tensors_recompute_buffer = []
_amax_forward_global_reduce_func = None
_buffer_delete_key_fwd = None
_buffer_delete_key_bwd = None
_amax_reduce_handle_fwd = None
_is_fp8_available = None
_reason_for_no_fp8 = ""


def _check_fp8_support() -> Tuple[bool, str]:
    """Return if fp8 support is available"""
    if get_device_compute_capability() >= 9.0: # hopper and above
        return True, ""
    if get_device_compute_capability() < 8.9: # pre-ada
        return False, "Device compute capability 8.9 or higher required for FP8 execution."
    if tex.get_cublasLt_version() < 120103:
        return False, "CublasLt version 12.1.3.x or higher required for FP8 execution on Ada."
    if float(torch.version.cuda) < 12.1:
        return False, "Cuda version 12.1 or higher required for FP8 execution on Ada."
    return True, ""


def is_fp8_available() -> Tuple[bool, str]:
    """Return if fp8 support is available"""
    global _is_fp8_available, _reason_for_no_fp8
    if _is_fp8_available is None:
        _is_fp8_available, _reason_for_no_fp8 = _check_fp8_support()
    return _is_fp8_available, _reason_for_no_fp8


def get_meta_tensor_key(forward: bool = True) -> str:
    """Returns scaling key in `fp8_meta`."""
    if forward:
        return "scaling_fwd"
    return "scaling_bwd"


def get_buffer_position_key(forward: bool = True) -> str:
    """Returns module position key in `fp8_meta`."""
    if forward:
        return "global_fp8_buffer_pos_fwd"
    return "global_fp8_buffer_pos_bwd"


def get_autocast_key(forward: bool = True) -> str:
    """Returns module position key in `fp8_meta`."""
    if forward:
        return "autocast_id_fwd"
    return "autocast_id_bwd"


def get_amax_reduce_handle_fwd() -> Union[bool, None]:
    """Return AMAX reduction wait handle of forward prop."""
    global _amax_reduce_handle_fwd
    return _amax_reduce_handle_fwd


def get_global_fp8_buffer() -> Dict[str, List[torch.Tensor]]:
    """Returns global fp8 buffer."""
    return _global_fp8_buffer


def set_global_fp8_buffer(buffer: Dict[str, List[torch.Tensor]]) -> None:
    """Sets global fp8 buffer."""
    global _global_fp8_buffer

    # Map all tensors back to GPU.
    for k, v in buffer.items():
        buffer[k] = [tensor.cuda() for tensor in v]

    _global_fp8_buffer = buffer


def setup_amax_forward_global_reduce_func(f: Callable) -> None:
    """Sets up the function to call during autocast exit."""
    global _amax_forward_global_reduce_func
    _amax_forward_global_reduce_func = f


def get_amax_buffer_key(fp8_meta: Dict[str, Any], forward: bool = True) -> str:
    """Return a key in `_global_fp8_buffer` for the AMAX storage."""
    if forward:
        return f"FWD_AMAX_{fp8_meta['autocast_id_fwd']}"
    return f"BWD_AMAX_{fp8_meta['autocast_id_bwd']}"


def add_amax_to_global_buffer(fp8_meta: Dict[str, Any], forward: bool = True) -> None:
    """Append 1D tensor `amax` to global buffer."""
    global _global_fp8_buffer
    buffer_key = get_amax_buffer_key(fp8_meta, forward=forward)
    fp8_meta_tensor_key = get_meta_tensor_key(forward=forward)
    buffer_position_key = get_buffer_position_key(forward=forward)

    if buffer_key not in _global_fp8_buffer:
        _global_fp8_buffer[buffer_key] = [fp8_meta[fp8_meta_tensor_key].amax_history[0]]
    else:
        _global_fp8_buffer[buffer_key].append(
            fp8_meta[fp8_meta_tensor_key].amax_history[0]
        )

    if buffer_position_key not in fp8_meta:
        fp8_meta[buffer_position_key] = len(_global_fp8_buffer[buffer_key]) - 1

    # Catch incorrect fp8_autocast usage.
    assert fp8_meta[buffer_position_key] == len(_global_fp8_buffer[buffer_key]) - 1, \
        "Same module is being invoked more than once inside an `fp8_autocast` region when using " \
        "FP8 with amax reduction. This behavior is currently unsupported. For more details and " \
        "correct usage, please see https://github.com/NVIDIA/TransformerEngine/pull/93."


def copy_forward_fp8_meta_tensors_for_recompute(fp8_meta: Dict[str, Any]) -> None:
    """Copy the scaling factors and amaxes for recompute forward phase
    to ensure both forward steps are numerically same.
    """
    global _fp8_tensors_recompute_buffer
    buffer_position_key = "global_fp8_buffer_pos_fwd_recompute"

    to_copy = [
        fp8_meta["scaling_fwd"].amax_history.clone(),
        fp8_meta["scaling_fwd"].scale.clone(),
        fp8_meta["scaling_fwd"].scale_inv.clone(),
    ]

    if buffer_position_key in fp8_meta:
        _fp8_tensors_recompute_buffer[fp8_meta[buffer_position_key]].append(to_copy)
    else:
        if len(_fp8_tensors_recompute_buffer) == 0:
            _fp8_tensors_recompute_buffer = [deque()]
        else:
            _fp8_tensors_recompute_buffer.append(deque())
        _fp8_tensors_recompute_buffer[-1].append(to_copy)
        fp8_meta[buffer_position_key] = len(_fp8_tensors_recompute_buffer) - 1


def get_old_fp8_meta_tensors_for_recompute(fp8_meta: Dict[str, Any]) -> None:
    """Switch to the copied scaling factors and amaxes from phase
    1 forward for indentical numerical outputs.
    """

    # Store updated amaxes and scales from phase 1 post forward.
    fp8_meta["updated_amax_history_fwd"] = fp8_meta["scaling_fwd"].amax_history
    fp8_meta["updated_scale_fwd"] = fp8_meta["scaling_fwd"].scale
    fp8_meta["updated_scale_inv_fwd"] = fp8_meta["scaling_fwd"].scale_inv

    # Retrieve stashed amaxes and scales from phase 1 pre forward.
    buffer_position_key = "global_fp8_buffer_pos_fwd_recompute"
    stashed_fp8_meta = _fp8_tensors_recompute_buffer[
        fp8_meta[buffer_position_key]
    ].popleft()

    # Replace amaxes and scales with stashed values for phase 2 forward
    fp8_meta["scaling_fwd"].amax_history = stashed_fp8_meta[0]
    fp8_meta["scaling_fwd"].scale = stashed_fp8_meta[1]
    fp8_meta["scaling_fwd"].scale_inv = stashed_fp8_meta[2]


def restore_fp8_meta_tensors(fp8_meta: Dict[str, Any]) -> None:
    """Restore latest scaling factors and amaxes after recompute forward run."""
    fp8_meta["scaling_fwd"].amax_history = fp8_meta["updated_amax_history_fwd"]
    fp8_meta["scaling_fwd"].scale = fp8_meta["updated_scale_fwd"]
    fp8_meta["scaling_fwd"].scale_inv = fp8_meta["updated_scale_inv_fwd"]


def copy_amax_from_global_buffer(
    fp8_meta: Dict[str, Any], forward: bool = True
) -> None:
    """Populate current amax with the correct location from buffer."""
    fp8_meta_tensor_key = get_meta_tensor_key(forward=forward)
    buffer_position_key = get_buffer_position_key(forward=forward)
    if buffer_position_key not in fp8_meta:
        return

    amax_buffer_key = get_amax_buffer_key(fp8_meta, forward=forward)
    assert amax_buffer_key in _global_fp8_buffer, "TE internal error."

    fp8_meta[fp8_meta_tensor_key].amax_history[0] = _global_fp8_buffer[amax_buffer_key][
        fp8_meta[buffer_position_key]
    ]


def set_amax_buffer_key_deletion(
    fp8_meta: Dict[str, Any], forward: bool = True
) -> None:
    """Delete this amax key from global buffer during autocast end."""
    if get_autocast_key(forward=forward) not in fp8_meta:
        return
    global _buffer_delete_key_fwd, _buffer_delete_key_bwd
    if forward:
        _buffer_delete_key_fwd = get_amax_buffer_key(fp8_meta, forward=forward)
    else:
        _buffer_delete_key_bwd = get_amax_buffer_key(fp8_meta, forward=forward)


def get_default_fp8_recipe() -> DelayedScaling:
    """FP8 recipe if not provided by user
    Margin = 0, interval = 1, E4M3
    """
    return DelayedScaling()


@contextmanager
def fp8_autocast(
    enabled: bool = False,
    calibrating: bool = False,
    fp8_recipe: Optional[DelayedScaling] = None,
    fp8_group: Optional[dist_group_type] = None,
) -> None:
    """
    Context manager for FP8 usage.

    .. code-block:: python

        with fp8_autocast(enabled=True):
            out = model(inp)

    .. note::

        Support for FP8 in the Linear layer of Transformer Engine is currently limited to tensors
        with shapes where both dimensions are divisible by 16. In terms of the input to the full
        Transformer network, this typically requires padding sequence length to be multiple of 16.

    .. note::

        When :attr:`fp8_recipe.reduce_amax==True`, any module must not be invoked more than once
        inside a single `fp8_autocast` region. This is unsupported behavior because the amax
        reduction is handled during the exit of the `fp8_autocast` context. Calling the same
        module more than once inside an `fp8_autocast` region overrides the amax tensors
        before reduction can occur.

    Parameters
    ----------
    enabled: bool, default = `False`
             whether or not to enable fp8
    calibrating: bool, default = `False`
                 calibration mode allows collecting statistics such as amax and scale
                 data of fp8 tensors even when executing without fp8 enabled. This is
                 useful for saving an inference ready fp8 checkpoint while training
                 using a higher precision.
    fp8_recipe: recipe.DelayedScaling, default = `None`
                recipe used for FP8 training.
    fp8_group: torch._C._distributed_c10d.ProcessGroup, default = `None`
               distributed group over which amaxes for the fp8 tensors
               are reduced at the end of each training step.
    """

    global _FP8_ENABLED, _FP8_CALIBRATION,  _FP8_RECIPE, _FP8_DISTRIBUTED_GROUP, _FP8_AUTOCAST_DEPTH
    global _IS_FIRST_FP8_MODULE, _FP8_AUTOCAST_COUNTER
    global _global_fp8_buffer, _buffer_delete_key_fwd
    global _amax_reduce_handle_fwd
    fp8_state = (_FP8_ENABLED, _FP8_CALIBRATION, _FP8_RECIPE, _FP8_DISTRIBUTED_GROUP)
    try:
        _FP8_ENABLED = enabled
        _FP8_CALIBRATION = calibrating
        _FP8_RECIPE = get_default_fp8_recipe() if fp8_recipe is None else fp8_recipe
        _FP8_DISTRIBUTED_GROUP = fp8_group

        if _FP8_AUTOCAST_DEPTH == 0:
            _IS_FIRST_FP8_MODULE = True
            _FP8_AUTOCAST_COUNTER += 1
        _FP8_AUTOCAST_DEPTH += 1

        if enabled:
            fp8_available, reason_for_no_fp8 = is_fp8_available()
            assert fp8_available, reason_for_no_fp8
        yield
    finally:
        _FP8_ENABLED,_FP8_CALIBRATION, _FP8_RECIPE, _FP8_DISTRIBUTED_GROUP = fp8_state
        _IS_FIRST_FP8_MODULE = False
        _FP8_AUTOCAST_DEPTH -= 1

        if _FP8_AUTOCAST_DEPTH == 0:
            if callable(_amax_forward_global_reduce_func):
                _amax_reduce_handle_fwd = _amax_forward_global_reduce_func()
            delete_key_from_amax_buffer(forward=True)


def get_fp8_context_id() -> int:
    """Returns an ID for the current FP8 context."""
    return _FP8_CURRENT_CONTEXT_ID


def set_fp8_context_id(ctx_id: int) -> None:
    """Sets the current FP8 context."""
    global _FP8_CURRENT_CONTEXT_ID
    _FP8_CURRENT_CONTEXT_ID = ctx_id


def new_fp8_context_id() -> int:
    """Returns global autocast counter as a proxy to be used
    as the autocast ID for FP8 modules.
    """
    return _FP8_AUTOCAST_COUNTER


def is_fp8_enabled() -> bool:
    """Is FP8 enabled"""
    return _FP8_ENABLED


def is_fp8_calibration() -> bool:
    """Is FP8 calibration"""
    return _FP8_CALIBRATION


def is_first_fp8_module():
    """Returns `True` only the first time when called multiple
    times from within the same `fp8_autocast` context.
    """
    global _IS_FIRST_FP8_MODULE
    tmp = _IS_FIRST_FP8_MODULE
    _IS_FIRST_FP8_MODULE = False
    return tmp


def get_fp8_recipe() -> DelayedScaling:
    """Return the fp8 recipe"""
    return _FP8_RECIPE


def get_fp8_group() -> Union[dist_group_type, None]:
    """Return the fp8 group for scale/amax comm"""
    return _FP8_DISTRIBUTED_GROUP


def update_amax_history(amax_history: torch.Tensor) -> torch.Tensor:
    """Update amax history and set next amax to zero."""
    if amax_history.shape[0] > 1:
        amax_history = torch.roll(amax_history, -1, 0)
    amax_history[0].fill_(0.0)
    return amax_history


@torch.jit.script
def _default_get_amax(
    amax_history: torch.Tensor,
    amax_compute_algo: str,
) -> Tuple[torch.Tensor, torch.Tensor]:
    """Default function to obtain amax from history."""
    if amax_compute_algo == "max":
        amax = torch.max(amax_history, dim=0).values
    else:  # amax_compute_algo == "most_recent"
        amax = amax_history[0].clone()

    amax_history = update_amax_history(amax_history)
    return amax_history, amax


@torch.jit.script
def _default_sf_compute(
    amax: torch.Tensor,
    scale: torch.Tensor,
    fp8_max: float,
    margin: int,
) -> torch.Tensor:
    """Default function to convert amax to scaling factor."""
    exp = torch.floor(torch.log2(fp8_max / amax)) - margin
    sf = torch.round(torch.pow(2, torch.abs(exp)))
    sf = torch.where(amax > 0.0, sf, scale)
    sf = torch.where(torch.isfinite(amax), sf, scale)
    sf = torch.where(exp < 0, 1 / sf, sf)

    return sf


@torch.jit.script
def _compute_scaling_factor_inverse(
    scale: torch.Tensor,
    scale_inv: torch.Tensor,
    non_weight_mask: torch.Tensor,
    update_weight_scale_inv: bool,
) -> torch.Tensor:
    """Compute inverse of scaling factor."""
    if update_weight_scale_inv:
        return 1.0 / scale
    return torch.where(non_weight_mask, 1.0 / scale, scale_inv)


@torch.jit.script
def fused_amax_and_scale_update(
    amax_history: torch.Tensor,
    scale: torch.Tensor,
    scale_inv: torch.Tensor,
    fp8_max: float,
    margin: int,
    amax_compute_algo: str,
    non_weight_mask: torch.Tensor,
    update_weight_scale_inv: bool,
) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]:
    """Amax to scale conversion."""

    # Get amax from history.
    amax_history, amax = _default_get_amax(
        amax_history,
        amax_compute_algo,
    )

    # Calculate new scaling factor.
    scale = _default_sf_compute(
        amax,
        scale,
        fp8_max,
        margin,
    )

    # Calculate new inverse of scaling factor.
    scale_inv = _compute_scaling_factor_inverse(
        scale,
        scale_inv,
        non_weight_mask,
        update_weight_scale_inv,
    )

    return amax_history, scale, scale_inv


def _compute_amax(
    amax_history: torch.Tensor,
    recipe: DelayedScaling,
) -> Tuple[torch.Tensor, torch.Tensor]:
    """Obtain the amax from the history."""

    if callable(recipe.amax_compute_algo):
        amax = recipe.amax_compute_algo(amax_history)
        amax_history = update_amax_history(amax_history)
        return amax_history, amax
    return _default_get_amax(
        amax_history,
        recipe.amax_compute_algo,
    )


def _compute_scaling_factor(
    amax: torch.Tensor,
    scale: torch.Tensor,
    fp8_max: float,
    recipe: DelayedScaling,
) -> torch.Tensor:
    """Convert amax to scaling factor."""

    if recipe.scaling_factor_compute_algo is None:
        return _default_sf_compute(
            amax,
            scale,
            fp8_max,
            recipe.margin,
        )
    return recipe.scaling_factor_compute_algo(amax, scale, fp8_max, recipe)


def amax_and_scale_update(
    fp8_meta: Dict[str, Any],
    fwd_update: bool,
    update_weight_scale_inv: bool = True,
) -> None:
    """Updates fp8 amaxes/scales for fwd | bwd."""
    amax_compute = fp8_meta["recipe"].amax_compute_algo
    sf_compute = fp8_meta["recipe"].scaling_factor_compute_algo
    fp8_meta_tensor_key = "scaling_fwd" if fwd_update else "scaling_bwd"
    fp8_max_key = "fp8_max_fwd" if fwd_update else "fp8_max_bwd"

    if not callable(amax_compute) and sf_compute is None:
        (
            fp8_meta[fp8_meta_tensor_key].amax_history,
            fp8_meta[fp8_meta_tensor_key].scale,
            fp8_meta[fp8_meta_tensor_key].scale_inv,
        ) = fused_amax_and_scale_update(
            fp8_meta[fp8_meta_tensor_key].amax_history,
            fp8_meta[fp8_meta_tensor_key].scale,
            fp8_meta[fp8_meta_tensor_key].scale_inv,
            fp8_meta[fp8_max_key],
            fp8_meta["recipe"].margin,
            fp8_meta["recipe"].amax_compute_algo,
            fp8_meta[fp8_meta_tensor_key + "_non_weight_mask"],
            update_weight_scale_inv,
        )
    else:
        fp8_meta[fp8_meta_tensor_key].amax_history, amax = _compute_amax(
            fp8_meta[fp8_meta_tensor_key].amax_history,
            fp8_meta["recipe"],
        )
        fp8_meta[fp8_meta_tensor_key].scale = _compute_scaling_factor(
            amax,
            fp8_meta[fp8_meta_tensor_key].scale,
            fp8_meta[fp8_max_key],
            fp8_meta["recipe"],
        )
        fp8_meta[fp8_meta_tensor_key].scale_inv = _compute_scaling_factor_inverse(
            fp8_meta[fp8_meta_tensor_key].scale,
            fp8_meta[fp8_meta_tensor_key].scale_inv,
            fp8_meta[fp8_meta_tensor_key + "_non_weight_mask"],
            update_weight_scale_inv,
        )


def get_fp8_te_dtype(
    fp8_recipe: DelayedScaling, fprop_tensor: bool = True
) -> tex.DType:
    """Get fp8 data type according to recipe and tensor"""
    if fp8_recipe.fp8_format == Format.E4M3 or (
        fp8_recipe.fp8_format == Format.HYBRID and fprop_tensor
    ):
        return tex.DType.kFloat8E4M3
    return tex.DType.kFloat8E5M2


def reduce_tensor_across_group_op_max(
    tensor: torch.Tensor, group: dist_group_type, async_op: bool
) -> None:
    """Reduce tensor across given group."""
    if torch.distributed.is_initialized():
        wait_handle = torch.distributed.all_reduce(
            tensor,
            op=torch.distributed.ReduceOp.MAX,
            group=group,
            async_op=async_op,
        )
        return wait_handle
    return None


def global_amax_reduction(
    fp8_meta: Dict[str, Any],
    forward: bool = True,
) -> None:
    """Concatenate, reduce, and split amaxes in the global buffer."""
    global _global_fp8_buffer
    amax_buffer_key = get_amax_buffer_key(fp8_meta, forward=forward)

    # Key already deleted.
    if amax_buffer_key not in _global_fp8_buffer:
        return None

    chunk_sizes = [x.numel() for x in _global_fp8_buffer[amax_buffer_key]]
    contiguous_amax = torch.cat(_global_fp8_buffer[amax_buffer_key])

    wait_handle = reduce_tensor_across_group_op_max(
        contiguous_amax,
        fp8_meta["fp8_group"],
        fp8_meta["async_amax_reduction"],
    )

    _global_fp8_buffer[amax_buffer_key] = list(contiguous_amax.split(chunk_sizes))
    return wait_handle


def delete_key_from_amax_buffer(forward: bool = True) -> None:
    """Delete the key from global amax buffer."""

    global _global_fp8_buffer, _buffer_delete_key_fwd, _buffer_delete_key_bwd
    if forward:
        if (
            _buffer_delete_key_fwd is not None
            and _buffer_delete_key_fwd in _global_fp8_buffer
        ):
            del _global_fp8_buffer[_buffer_delete_key_fwd]
    else:
        if (
            _buffer_delete_key_bwd is not None
            and _buffer_delete_key_bwd in _global_fp8_buffer
        ):
            del _global_fp8_buffer[_buffer_delete_key_bwd]