# Copyright (c) OpenMMLab. All rights reserved.
import functools
import pickle
import warnings
from collections import OrderedDict
import torch
import torch.distributed as dist
from mmcv.runner import OptimizerHook, get_dist_info
from torch._utils import (_flatten_dense_tensors, _take_tensors,
_unflatten_dense_tensors)
def _allreduce_coalesced(tensors, world_size, bucket_size_mb=-1):
if bucket_size_mb > 0:
bucket_size_bytes = bucket_size_mb * 1024 * 1024
buckets = _take_tensors(tensors, bucket_size_bytes)
else:
buckets = OrderedDict()
for tensor in tensors:
tp = tensor.type()
if tp not in buckets:
buckets[tp] = []
buckets[tp].append(tensor)
buckets = buckets.values()
for bucket in buckets:
flat_tensors = _flatten_dense_tensors(bucket)
dist.all_reduce(flat_tensors)
flat_tensors.div_(world_size)
for tensor, synced in zip(
bucket, _unflatten_dense_tensors(flat_tensors, bucket)):
tensor.copy_(synced)
[docs]def allreduce_grads(params, coalesce=True, bucket_size_mb=-1):
"""Allreduce gradients.
Args:
params (list[torch.Parameters]): List of parameters of a model
coalesce (bool, optional): Whether allreduce parameters as a whole.
Defaults to True.
bucket_size_mb (int, optional): Size of bucket, the unit is MB.
Defaults to -1.
"""
grads = [
param.grad.data for param in params
if param.requires_grad and param.grad is not None
]
world_size = dist.get_world_size()
if coalesce:
_allreduce_coalesced(grads, world_size, bucket_size_mb)
else:
for tensor in grads:
dist.all_reduce(tensor.div_(world_size))
[docs]class DistOptimizerHook(OptimizerHook):
"""Deprecated optimizer hook for distributed training."""
def __init__(self, *args, **kwargs):
warnings.warn('"DistOptimizerHook" is deprecated, please switch to'
'"mmcv.runner.OptimizerHook".')
super().__init__(*args, **kwargs)
[docs]def reduce_mean(tensor):
""""Obtain the mean of tensor on different GPUs."""
if not (dist.is_available() and dist.is_initialized()):
return tensor
tensor = tensor.clone()
dist.all_reduce(tensor.div_(dist.get_world_size()), op=dist.ReduceOp.SUM)
return tensor
def obj2tensor(pyobj, device='cuda'):
"""Serialize picklable python object to tensor."""
storage = torch.ByteStorage.from_buffer(pickle.dumps(pyobj))
return torch.ByteTensor(storage).to(device=device)
def tensor2obj(tensor):
"""Deserialize tensor to picklable python object."""
return pickle.loads(tensor.cpu().numpy().tobytes())
@functools.lru_cache()
def _get_global_gloo_group():
"""Return a process group based on gloo backend, containing all the ranks
The result is cached."""
if dist.get_backend() == 'nccl':
return dist.new_group(backend='gloo')
else:
return dist.group.WORLD
[docs]def all_reduce_dict(py_dict, op='sum', group=None, to_float=True):
"""Apply all reduce function for python dict object.
The code is modified from https://github.com/Megvii-
BaseDetection/YOLOX/blob/main/yolox/utils/allreduce_norm.py.
NOTE: make sure that py_dict in different ranks has the same keys and
the values should be in the same shape.
Args:
py_dict (dict): Dict to be applied all reduce op.
op (str): Operator, could be 'sum' or 'mean'. Default: 'sum'
group (:obj:`torch.distributed.group`, optional): Distributed group,
Default: None.
to_float (bool): Whether to convert all values of dict to float.
Default: True.
Returns:
OrderedDict: reduced python dict object.
"""
_, world_size = get_dist_info()
if world_size == 1:
return py_dict
if group is None:
# TODO: May try not to use gloo in the future
group = _get_global_gloo_group()
if dist.get_world_size(group) == 1:
return py_dict
# all reduce logic across different devices.
py_key = list(py_dict.keys())
py_key_tensor = obj2tensor(py_key)
dist.broadcast(py_key_tensor, src=0)
py_key = tensor2obj(py_key_tensor)
tensor_shapes = [py_dict[k].shape for k in py_key]
tensor_numels = [py_dict[k].numel() for k in py_key]
if to_float:
flatten_tensor = torch.cat(
[py_dict[k].flatten().float() for k in py_key])
else:
flatten_tensor = torch.cat([py_dict[k].flatten() for k in py_key])
dist.all_reduce(flatten_tensor, op=dist.ReduceOp.SUM)
if op == 'mean':
flatten_tensor /= world_size
split_tensors = [
x.reshape(shape) for x, shape in zip(
torch.split(flatten_tensor, tensor_numels), tensor_shapes)
]
return OrderedDict({k: v for k, v in zip(py_key, split_tensors)})