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import warnings
from abc import ABC, abstractmethod
from typing import Union, Iterable, Dict
import torch
import torch.distributed as dist
import torch.distributed.algorithms.model_averaging.utils as utils
__all__ = ['ModelAverager', 'PeriodicModelAverager']
class ModelAverager(ABC):
r"""Base class for all model averagers.
Args:
process_group: The process group to be used for all-reduce.
If ``None``, the default process group, which
is created by :func:`torch.distributed.init_process_group`,
will be used. (default: ``None``)
"""
def __init__(self, process_group=None):
self.process_group = (
process_group if process_group is not None else dist.group.WORLD
)
self.step = 0
@abstractmethod
def average_parameters(self, params):
raise NotImplementedError
class PeriodicModelAverager(ModelAverager):
r"""
Averages parameters periodically after the warm-up stage.
This can be used for running `post-local SGD <https://arxiv.org/abs/1808.07217>`_,
by running :class:`~torch.nn.DistributedDataParallel` (DDP)
using the subgroups created by :meth:`~torch.distributed.new_subgroups`.
Args:
period (int): The number of steps per model averaging.
Usually the period should be greater than ``1`` to reduce the communication cost.
Otherwise, only DDP needs to be used.
warmup_steps (int): The number of warm-up steps. During this stage,
model averaging is skipped.
process_group: The process group to be used for all-reduce.
If ``None``, the default process group, which
is created by :func:`torch.distributed.init_process_group`,
will be used. (default: ``None``)
Example::
>>> # xdoctest: +SKIP("undefined variables")
>>> import torch
>>> import torch.distributed as dist
>>> import torch.distributed.algorithms.ddp_comm_hooks.post_localSGD_hook as post_localSGD
>>> import torch.distributed.algorithms.model_averaging.averagers as averagers
>>> import torch.nn as nn
>>>
>>> dist.init_process_group("nccl", rank=rank, world_size=16)
>>> torch.cuda.set_device(rank)
>>> module = nn.Linear(1, 1, bias=False).cuda()
>>> model = nn.parallel.DistributedDataParallel(
>>> module, device_ids=[rank], output_device=rank
>>> )
>>> # Register a post-localSGD communication hook.
>>> state = PostLocalSGDState(process_group=None, subgroup=None, start_localSGD_iter=100)
>>> model.register_comm_hook(state, post_localSGD_hook)
>>>
>>> # In the first 100 steps, run global gradient averaging like normal DDP at every step.
>>> # After 100 steps, run model averaging every 4 steps.
>>> # Note that ``warmup_steps`` must be the same as ``start_localSGD_iter`` used in ``PostLocalSGDState``.
>>> averager = averagers.PeriodicModelAverager(period=4, warmup_steps=100)
>>> for step in range(0, 200):
>>> optimizer.zero_grad()
>>> loss = loss_fn(output, labels)
>>> loss.backward()
>>> optimizer.step()
>>> # Will average model parameters globally every 4 steps. Thus,
>>> # inter-node communication only occurs every 4 iterations after
>>> # the initial ``warmup_steps`` period.
>>> averager.average_parameters(model.parameters())
"""
def __init__(
self,
period,
warmup_steps=0,
process_group=None
):
super().__init__(process_group)
if warmup_steps < 0:
raise ValueError("Arg ``warmup_steps`` must be a non-negative number.")
self.warmup_steps = warmup_steps
if period < 1:
raise ValueError("Arg ``period`` must be a positive value.")
elif period == 1:
warnings.warn(
"When period is 1, no need to use model averaging because the communication cost "
"of all-reducing parameters will be no less than the cost of all-reducing gradients "
"by DistributedDataParallel in the backward pass. Therefore, only "
"DistributedDataParallel should be used for this case."
)
self.period = period
def average_parameters(self, params: Union[Iterable[torch.nn.Parameter], Iterable[Dict[str, torch.nn.Parameter]]]):
"""
Averages parameters or parameter groups of an optimizer if ``step`` is no less than ``warmup_steps``
and it can be divided by ``period``, where ``step`` is increased by 1
at each iteration in the training loop.
Args:
params: The parameters of a model or parameter groups of an optimizer.
"""
if (
self.step >= self.warmup_steps
and (self.step - self.warmup_steps) % self.period == 0
):
utils.average_parameters_or_parameter_groups(params, self.process_group)
self.step += 1
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