hooks.py

import copy

import torch
import torch.nn as nn
from mmcv.runner import OptimizerHook

from ..utils.dist_utils import allreduce_grads
from .utils import cast_tensor_type


class Fp16OptimizerHook(OptimizerHook):
    """FP16 optimizer hook.

    The steps of fp16 optimizer is as follows.
    1. Scale the loss value.
    2. BP in the fp16 model.
    2. Copy gradients from fp16 model to fp32 weights.
    3. Update fp32 weights.
    4. Copy updated parameters from fp32 weights to fp16 model.

    Refer to https://arxiv.org/abs/1710.03740 for more details.

    Args:
        loss_scale (float): Scale factor multiplied with loss.
    """

    def __init__(self,
                 grad_clip=None,
                 coalesce=True,
                 bucket_size_mb=-1,
                 loss_scale=512.,
                 distributed=True):
        self.grad_clip = grad_clip
        self.coalesce = coalesce
        self.bucket_size_mb = bucket_size_mb
        self.loss_scale = loss_scale
        self.distributed = distributed

    def before_run(self, runner):
        # keep a copy of fp32 weights
        runner.optimizer.param_groups = copy.deepcopy(
            runner.optimizer.param_groups)
        # convert model to fp16
        wrap_fp16_model(runner.model)

    def copy_grads_to_fp32(self, fp16_net, fp32_weights):
        """Copy gradients from fp16 model to fp32 weight copy."""
        for fp32_param, fp16_param in zip(fp32_weights, fp16_net.parameters()):
            if fp16_param.grad is not None:
                if fp32_param.grad is None:
                    fp32_param.grad = fp32_param.data.new(fp32_param.size())
                fp32_param.grad.copy_(fp16_param.grad)

    def copy_params_to_fp16(self, fp16_net, fp32_weights):
        """Copy updated params from fp32 weight copy to fp16 model."""
        for fp16_param, fp32_param in zip(fp16_net.parameters(), fp32_weights):
            fp16_param.data.copy_(fp32_param.data)

    def after_train_iter(self, runner):
        # clear grads of last iteration
        runner.model.zero_grad()
        runner.optimizer.zero_grad()
        # scale the loss value
        scaled_loss = runner.outputs['loss'] * self.loss_scale
        scaled_loss.backward()
        # copy fp16 grads in the model to fp32 params in the optimizer
        fp32_weights = []
        for param_group in runner.optimizer.param_groups:
            fp32_weights += param_group['params']
        self.copy_grads_to_fp32(runner.model, fp32_weights)
        # allreduce grads
        if self.distributed:
            allreduce_grads(fp32_weights, self.coalesce, self.bucket_size_mb)
        # scale the gradients back
        for param in fp32_weights:
            if param.grad is not None:
                param.grad.div_(self.loss_scale)
        if self.grad_clip is not None:
            self.clip_grads(fp32_weights)
        # update fp32 params
        runner.optimizer.step()
        # copy fp32 params to the fp16 model
        self.copy_params_to_fp16(runner.model, fp32_weights)


def wrap_fp16_model(model):
    # convert model to fp16
    model.half()
    # patch the normalization layers to make it work in fp32 mode
    patch_norm_fp32(model)
    # set `fp16_enabled` flag
    for m in model.modules():
        if hasattr(m, 'fp16_enabled'):
            m.fp16_enabled = True


def patch_norm_fp32(module):
    if isinstance(module, (nn.modules.batchnorm._BatchNorm, nn.GroupNorm)):
        module.float()
        module.forward = patch_forward_method(module.forward, torch.half,
                                              torch.float)
    for child in module.children():
        patch_norm_fp32(child)
    return module


def patch_forward_method(func, src_type, dst_type, convert_output=True):
    """Patch the forward method of a module.

    Args:
        func (callable): The original forward method.
        src_type (torch.dtype): Type of input arguments to be converted from.
        dst_type (torch.dtype): Type of input arguments to be converted to.
        convert_output (bool): Whether to convert the output back to src_type.

    Returns:
        callable: The patched forward method.
    """

    def new_forward(*args, **kwargs):
        output = func(*cast_tensor_type(args, src_type, dst_type),
                      **cast_tensor_type(kwargs, src_type, dst_type))
        if convert_output:
            output = cast_tensor_type(output, dst_type, src_type)
        return output

    return new_forward