# Copyright The Lightning AI team.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from contextlib import nullcontext
from datetime import timedelta
from typing import Any, ContextManager, Dict, List, Literal, Optional, Union
import torch
import torch.distributed
from lightning_utilities.core.rank_zero import rank_zero_only as utils_rank_zero_only
from torch import Tensor
from torch.nn import Module
from torch.nn.parallel.distributed import DistributedDataParallel
from typing_extensions import override
from lightning.fabric.accelerators.accelerator import Accelerator
from lightning.fabric.plugins.collectives.torch_collective import default_pg_timeout
from lightning.fabric.plugins.environments.cluster_environment import ClusterEnvironment
from lightning.fabric.plugins.io.checkpoint_io import CheckpointIO
from lightning.fabric.plugins.precision import Precision
from lightning.fabric.strategies.launchers.multiprocessing import _MultiProcessingLauncher
from lightning.fabric.strategies.launchers.subprocess_script import _SubprocessScriptLauncher
from lightning.fabric.strategies.parallel import ParallelStrategy
from lightning.fabric.strategies.registry import _StrategyRegistry
from lightning.fabric.strategies.strategy import TBroadcast, _BackwardSyncControl
from lightning.fabric.utilities.distributed import (
ReduceOp,
_distributed_is_initialized,
_get_default_process_group_backend_for_device,
_init_dist_connection,
_sync_ddp_if_available,
)
from lightning.fabric.utilities.distributed import group as _group
from lightning.fabric.utilities.rank_zero import rank_zero_only
_DDP_FORK_ALIASES = (
"ddp_fork",
"ddp_notebook",
)
[docs]class DDPStrategy(ParallelStrategy):
"""Strategy for multi-process single-device training on one or multiple nodes."""
def __init__(
self,
accelerator: Optional[Accelerator] = None,
parallel_devices: Optional[List[torch.device]] = None,
cluster_environment: Optional[ClusterEnvironment] = None,
checkpoint_io: Optional[CheckpointIO] = None,
precision: Optional[Precision] = None,
process_group_backend: Optional[str] = None,
timeout: Optional[timedelta] = default_pg_timeout,
start_method: Literal["popen", "spawn", "fork", "forkserver"] = "popen",
**kwargs: Any,
) -> None:
super().__init__(
accelerator=accelerator,
parallel_devices=parallel_devices,
cluster_environment=cluster_environment,
checkpoint_io=checkpoint_io,
precision=precision,
)
self._num_nodes = 1
self._process_group_backend: Optional[str] = process_group_backend
self._timeout: Optional[timedelta] = timeout
self._start_method = start_method
self._backward_sync_control = _DDPBackwardSyncControl()
self._ddp_kwargs = kwargs
@property
@override
def root_device(self) -> torch.device:
assert self.parallel_devices is not None
return self.parallel_devices[self.local_rank]
@property
def num_nodes(self) -> int:
return self._num_nodes
@num_nodes.setter
def num_nodes(self, num_nodes: int) -> None:
# note that world ranks is related to num_nodes, when resetting it, need to reset world ranks
self._num_nodes = num_nodes
@property
def num_processes(self) -> int:
return len(self.parallel_devices) if self.parallel_devices is not None else 0
@property
@override
def distributed_sampler_kwargs(self) -> Dict[str, Any]:
return {"num_replicas": (self.num_nodes * self.num_processes), "rank": self.global_rank}
@property
def process_group_backend(self) -> Optional[str]:
return self._process_group_backend
[docs] @override
def setup_environment(self) -> None:
super().setup_environment()
self._setup_distributed()
[docs] @override
def setup_module(self, module: Module) -> DistributedDataParallel:
"""Wraps the model into a :class:`~torch.nn.parallel.distributed.DistributedDataParallel` module."""
device_ids = self._determine_ddp_device_ids()
# https://pytorch.org/docs/stable/notes/cuda.html#id5
ctx = torch.cuda.stream(torch.cuda.Stream()) if device_ids is not None else nullcontext()
with ctx:
return DistributedDataParallel(module=module, device_ids=device_ids, **self._ddp_kwargs)
[docs] @override
def module_to_device(self, module: Module) -> None:
module.to(self.root_device)
[docs] @override
def all_reduce(
self, tensor: Tensor, group: Optional[Any] = None, reduce_op: Optional[Union[ReduceOp, str]] = "mean"
) -> Tensor:
"""Reduces a tensor from several distributed processes to one aggregated tensor.
Args:
tensor: the tensor to sync and reduce
group: the process group to gather results from. Defaults to all processes (world)
reduce_op: the reduction operation. Defaults to 'mean'/'avg'.
Can also be a string 'sum' to calculate the sum during reduction.
Return:
reduced value, except when the input was not a tensor the output remains is unchanged
"""
if isinstance(tensor, Tensor):
return _sync_ddp_if_available(tensor, group, reduce_op=reduce_op)
return tensor
[docs] @override
def barrier(self, *args: Any, **kwargs: Any) -> None:
if not _distributed_is_initialized():
return
if torch.distributed.get_backend() == "nccl":
torch.distributed.barrier(device_ids=self._determine_ddp_device_ids())
else:
torch.distributed.barrier()
[docs] @override
def broadcast(self, obj: TBroadcast, src: int = 0) -> TBroadcast:
if not _distributed_is_initialized():
return obj
obj = [obj]
torch.distributed.broadcast_object_list(obj, src, group=_group.WORLD)
return obj[0]
[docs] @override
def get_module_state_dict(self, module: Module) -> Dict[str, Union[Any, Tensor]]:
if isinstance(module, DistributedDataParallel):
module = module.module
return super().get_module_state_dict(module)
[docs] @override
def load_module_state_dict(
self, module: Module, state_dict: Dict[str, Union[Any, Tensor]], strict: bool = True
) -> None:
if isinstance(module, DistributedDataParallel):
module = module.module
super().load_module_state_dict(module=module, state_dict=state_dict, strict=strict)
@classmethod
@override
def register_strategies(cls, strategy_registry: _StrategyRegistry) -> None:
entries = (
("ddp", "popen"),
("ddp_spawn", "spawn"),
("ddp_fork", "fork"),
("ddp_notebook", "fork"),
)
for name, start_method in entries:
strategy_registry.register(
name,
cls,
description=f"DDP strategy with `start_method={start_method!r}`",
start_method=start_method,
)
strategy_registry.register(
"ddp_find_unused_parameters_true",
cls,
description="Alias for `find_unused_parameters_true` and `start_method='popen'`",
find_unused_parameters=True,
start_method="popen",
)
def _setup_distributed(self) -> None:
self._set_world_ranks()
self._process_group_backend = self._get_process_group_backend()
assert self.cluster_environment is not None
_init_dist_connection(self.cluster_environment, self._process_group_backend, timeout=self._timeout)
def _get_process_group_backend(self) -> str:
return self._process_group_backend or _get_default_process_group_backend_for_device(self.root_device)
def _set_world_ranks(self) -> None:
if self.cluster_environment is not None:
self.cluster_environment.set_global_rank(self.node_rank * self.num_processes + self.local_rank)
self.cluster_environment.set_world_size(self.num_nodes * self.num_processes)
# `LightningEnvironment.set_global_rank` will do this too, but we cannot rely on that implementation detail
# additionally, for some implementations, the setter is a no-op, so it's safer to access the getter
rank_zero_only.rank = utils_rank_zero_only.rank = self.global_rank
def _determine_ddp_device_ids(self) -> Optional[List[int]]:
return None if self.root_device.type == "cpu" else [self.root_device.index]
class _DDPBackwardSyncControl(_BackwardSyncControl):
@override
def no_backward_sync(self, module: Module, enabled: bool) -> ContextManager:
"""Blocks gradient synchronization inside the :class:`~torch.nn.parallel.distributed.DistributedDataParallel`
wrapper."""
if not enabled:
return nullcontext()
if not isinstance(module, DistributedDataParallel):
raise TypeError(
"Blocking backward sync is only possible if the module passed to"
f" `{self.__class__.__name__}.no_backward_sync` is wrapped in `DistributedDataParallel`."
f" Got: {module.__class__.__name__}."
)
return module.no_sync()
