Source code for mmdet.core.utils.dist_utils

# 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)


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))


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)


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


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. Currently only supports
    nccl backend.

    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.
    """
    warnings.warn(
        'group` is deprecated. Currently only supports NCCL backend.')
    _, world_size = get_dist_info()
    if world_size == 1:
        return py_dict

    # all reduce logic across different devices.
    py_key = list(py_dict.keys())
    if not isinstance(py_dict, OrderedDict):
        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:
        warnings.warn('Note: the "to_float" is True, you need to '
                      'ensure that the behavior is reasonable.')
        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)
    ]
    out_dict = {k: v for k, v in zip(py_key, split_tensors)}
    if isinstance(py_dict, OrderedDict):
        out_dict = OrderedDict(out_dict)
    return out_dict