Source code for pytorch_lightning.strategies.ddp
# Copyright The PyTorch Lightning 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.
import logging
import os
import shutil
import signal
import tempfile
import time
from pathlib import Path
from typing import Any, Callable, Dict, List, Optional, Union
import torch
import torch.distributed
from torch.nn import Module
from torch.nn.parallel.distributed import DistributedDataParallel
from torch.optim.optimizer import Optimizer
import pytorch_lightning as pl
from pytorch_lightning.core.optimizer import LightningOptimizer
from pytorch_lightning.overrides import LightningDistributedModule
from pytorch_lightning.overrides.distributed import prepare_for_backward
from pytorch_lightning.plugins.environments.cluster_environment import ClusterEnvironment
from pytorch_lightning.plugins.io.checkpoint_plugin import CheckpointIO
from pytorch_lightning.plugins.precision import PrecisionPlugin
from pytorch_lightning.strategies.launchers.subprocess_script import _SubprocessScriptLauncher
from pytorch_lightning.strategies.parallel import ParallelStrategy
from pytorch_lightning.trainer.states import TrainerFn
from pytorch_lightning.utilities.distributed import (
_get_process_group_backend_from_env,
distributed_available,
get_default_process_group_backend_for_device,
)
from pytorch_lightning.utilities.distributed import group as _group
from pytorch_lightning.utilities.distributed import (
init_dist_connection,
ReduceOp,
register_ddp_comm_hook,
sync_ddp_if_available,
)
from pytorch_lightning.utilities.exceptions import DeadlockDetectedException
from pytorch_lightning.utilities.imports import (
_FAIRSCALE_AVAILABLE,
_IS_WINDOWS,
_TORCH_GREATER_EQUAL_1_9,
_TORCH_GREATER_EQUAL_1_10,
_TORCH_GREATER_EQUAL_1_11,
)
from pytorch_lightning.utilities.rank_zero import rank_zero_info, rank_zero_only, rank_zero_warn
from pytorch_lightning.utilities.seed import reset_seed
from pytorch_lightning.utilities.types import STEP_OUTPUT
if _FAIRSCALE_AVAILABLE:
from fairscale.optim import OSS
if _TORCH_GREATER_EQUAL_1_10:
from torch.distributed.algorithms.model_averaging.averagers import ModelAverager
log = logging.getLogger(__name__)
[docs]class DDPStrategy(ParallelStrategy):
"""Strategy for multi-process single-device training on one or multiple nodes."""
strategy_name = "ddp"
def __init__(
self,
accelerator: Optional["pl.accelerators.accelerator.Accelerator"] = None,
parallel_devices: Optional[List[torch.device]] = None,
cluster_environment: Optional[ClusterEnvironment] = None,
checkpoint_io: Optional[CheckpointIO] = None,
precision_plugin: Optional[PrecisionPlugin] = None,
ddp_comm_state: Optional[object] = None,
ddp_comm_hook: Optional[callable] = None,
ddp_comm_wrapper: Optional[callable] = None,
model_averaging_period: Optional[int] = None,
process_group_backend: Optional[str] = None,
**kwargs: Union[Any, Dict[str, Any]],
) -> None:
super().__init__(
accelerator=accelerator,
parallel_devices=parallel_devices,
cluster_environment=cluster_environment,
checkpoint_io=checkpoint_io,
precision_plugin=precision_plugin,
)
log.detail(f"{self.__class__.__name__}: initializing DDP plugin")
self._num_nodes = 1
self._ddp_kwargs = kwargs
self._ddp_comm_state = ddp_comm_state
self._ddp_comm_hook = ddp_comm_hook
self._ddp_comm_wrapper = ddp_comm_wrapper
self._model_averaging_period = model_averaging_period
self._model_averager: Optional[ModelAverager] = None
self._pids: Optional[List[int]] = None
self._sync_dir: Optional[str] = None
self._rank_0_will_call_children_scripts: bool = False
self._process_group_backend: Optional[str] = process_group_backend
@property
def is_distributed(self) -> bool:
return True
@property
def root_device(self) -> torch.device:
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):
return len(self.parallel_devices) if self.parallel_devices is not None else 0
@property
def distributed_sampler_kwargs(self):
distributed_sampler_kwargs = dict(num_replicas=(self.num_nodes * self.num_processes), rank=self.global_rank)
return distributed_sampler_kwargs
@property
def _is_single_process_single_device(self) -> bool:
return True
@property
def process_group_backend(self) -> Optional[str]:
return self._process_group_backend
def _configure_launcher(self) -> None:
if not self.cluster_environment.creates_processes_externally:
self._launcher = _SubprocessScriptLauncher(self.cluster_environment, self.num_processes, self.num_nodes)
self._rank_0_will_call_children_scripts = True
[docs] def setup(self, trainer: "pl.Trainer") -> None:
super().setup(trainer)
# share ddp pids to all processes
self._rank_0_will_call_children_scripts = self.broadcast(self._rank_0_will_call_children_scripts)
if self._should_run_deadlock_detection():
self._share_information_to_prevent_deadlock()
# move the model to the correct device
self.model_to_device()
# skip wrapping the model if we are not fitting as no gradients need to be exchanged
trainer_fn = trainer.state.fn
if trainer_fn != TrainerFn.FITTING:
return
if self._layer_sync:
self.model = self._layer_sync.apply(self.model)
self.configure_ddp()
def _setup_model(self, model: Module) -> DistributedDataParallel:
"""Wraps the model into a :class:`~torch.nn.parallel.distributed.DistributedDataParallel` module."""
device_ids = self.determine_ddp_device_ids()
log.detail(f"setting up DDP model with device ids: {device_ids}, kwargs: {self._ddp_kwargs}")
return DistributedDataParallel(module=model, device_ids=device_ids, **self._ddp_kwargs)
def setup_distributed(self):
log.detail(f"{self.__class__.__name__}: setting up distributed...")
reset_seed()
# determine which process we are and world size
self.set_world_ranks()
# set warning rank
rank_zero_only.rank = self.global_rank
self._process_group_backend = self._get_process_group_backend()
init_dist_connection(self.cluster_environment, self._process_group_backend)
def _get_process_group_backend(self) -> str:
return (
self._process_group_backend
or _get_process_group_backend_from_env()
or get_default_process_group_backend_for_device(self.root_device)
)
def set_world_ranks(self) -> None:
if self.cluster_environment is None:
return
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)
rank_zero_only.rank = self.cluster_environment.global_rank()
def pre_configure_ddp(self):
# if unset, default `find_unused_parameters` `True`
# Many models require setting this parameter to True, as there are corner cases
# when not all parameter backward hooks are fired by the autograd engine even if require_grad is set to True.
# This flag does come with a performance hit, so it is suggested to disable in cases where it is possible.
self._ddp_kwargs["find_unused_parameters"] = self._ddp_kwargs.get("find_unused_parameters", True)
def _register_ddp_hooks(self) -> None:
log.detail(f"{self.__class__.__name__}: registering ddp hooks")
# In 1.8, DDP communication hooks only work with NCCL backend and SPSD (single process single device) mode
# Since 1.9, DDP communication hooks can work on all backends.
if _TORCH_GREATER_EQUAL_1_9 or (self.root_device.type == "cuda" and self._is_single_process_single_device):
register_ddp_comm_hook(
model=self.model,
ddp_comm_state=self._ddp_comm_state,
ddp_comm_hook=self._ddp_comm_hook,
ddp_comm_wrapper=self._ddp_comm_wrapper,
)
if _TORCH_GREATER_EQUAL_1_10 and self.lightning_module.trainer.state.fn == TrainerFn.FITTING:
import torch.distributed.algorithms.ddp_comm_hooks.post_localSGD_hook as post_localSGD
if isinstance(self._ddp_comm_state, post_localSGD.PostLocalSGDState):
self._enable_model_averaging()
def _enable_model_averaging(self) -> None:
# Only called when PyTorch version >= 1.10
log.detail(f"{self.__class__.__name__}: reinitializing optimizers with post localSGD")
if self._model_averaging_period is None:
raise ValueError(
"Post-localSGD algorithm is used, but model averaging period is not provided to DDP strategy."
)
from torch.distributed.optim import DistributedOptimizer, PostLocalSGDOptimizer, ZeroRedundancyOptimizer
for optimizer in self.optimizers:
if isinstance(optimizer, LightningOptimizer):
optimizer = optimizer._optimizer
is_distributed_optimizer = isinstance(optimizer, DistributedOptimizer) if not _IS_WINDOWS else False
if (
is_distributed_optimizer
or isinstance(optimizer, ZeroRedundancyOptimizer)
or (_FAIRSCALE_AVAILABLE and isinstance(optimizer, OSS))
or isinstance(optimizer, PostLocalSGDOptimizer)
):
raise ValueError(
f"Currently model averaging cannot work with a distributed optimizer of type "
f"{optimizer.__class__.__name__}."
)
self._model_averager = torch.distributed.algorithms.model_averaging.averagers.PeriodicModelAverager(
period=self._model_averaging_period, warmup_steps=self._ddp_comm_state.start_localSGD_iter
)
[docs] def optimizer_step(
self,
optimizer: Optimizer,
opt_idx: int,
closure: Callable[[], Any],
model: Optional[Union["pl.LightningModule", Module]] = None,
**kwargs: Any,
) -> Any:
"""Performs the actual optimizer step.
Args:
optimizer: the optimizer performing the step
opt_idx: index of the current optimizer
closure: closure calculating the loss value
model: reference to the model, optionally defining optimizer step related hooks
**kwargs: Any extra arguments to ``optimizer.step``
"""
optimizer_output = super().optimizer_step(optimizer, opt_idx, closure, model, **kwargs)
if not _TORCH_GREATER_EQUAL_1_10 or self._model_averager is None:
return optimizer_output
for group in optimizer.param_groups:
for param in group["params"]:
if param.grad is None:
continue
self._model_averager.average_parameters(iter(param))
return optimizer_output
def configure_ddp(self) -> None:
log.detail(f"{self.__class__.__name__}: configuring DistributedDataParallel")
self.pre_configure_ddp()
self.model = self._setup_model(LightningDistributedModule(self.model))
self._register_ddp_hooks()
def determine_ddp_device_ids(self):
if self.root_device.type == "cpu":
return None
return [self.root_device.index]
[docs] def barrier(self, *args, **kwargs) -> None:
if not distributed_available():
return
if torch.distributed.get_backend() == "nccl":
torch.distributed.barrier(device_ids=self.determine_ddp_device_ids())
else:
torch.distributed.barrier()
[docs] def broadcast(self, obj: object, src: int = 0) -> object:
obj = [obj]
if self.global_rank != src:
obj = [None]
torch.distributed.broadcast_object_list(obj, src, group=_group.WORLD)
return obj[0]
[docs] def pre_backward(self, closure_loss: torch.Tensor) -> None:
"""Run before precision plugin executes backward."""
if not self.lightning_module.automatic_optimization:
prepare_for_backward(self.model, closure_loss)
[docs] def model_to_device(self):
log.detail(f"{self.__class__.__name__}: moving model to device [{self.root_device}]...")
self.model.to(self.root_device)
[docs] def reduce(self, tensor, group: Optional[Any] = None, reduce_op: Union[ReduceOp, str] = "mean") -> torch.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, torch.Tensor):
tensor = sync_ddp_if_available(tensor, group, reduce_op=reduce_op)
return tensor
[docs] def training_step(self, *args, **kwargs) -> STEP_OUTPUT:
with self.precision_plugin.train_step_context():
return self.model(*args, **kwargs)
[docs] def validation_step(self, *args, **kwargs) -> Optional[STEP_OUTPUT]:
with self.precision_plugin.val_step_context():
if isinstance(self.model, DistributedDataParallel):
# used when calling `trainer.fit`
return self.model(*args, **kwargs)
else:
# used when calling `trainer.validate`
return self.lightning_module.validation_step(*args, **kwargs)
[docs] def test_step(self, *args, **kwargs) -> Optional[STEP_OUTPUT]:
with self.precision_plugin.test_step_context():
return self.lightning_module.test_step(*args, **kwargs)
[docs] def predict_step(self, *args, **kwargs) -> STEP_OUTPUT:
with self.precision_plugin.predict_step_context():
return self.lightning_module.predict_step(*args, **kwargs)
def post_training_step(self):
if not self.lightning_module.automatic_optimization:
self.model.require_backward_grad_sync = True
@classmethod
def register_strategies(cls, strategy_registry: Dict) -> None:
strategy_registry.register(
"ddp_find_unused_parameters_false",
cls,
description="DDP Strategy with `find_unused_parameters` as False",
find_unused_parameters=False,
)
strategy_registry.register(
cls.strategy_name,
cls,
description=f"{cls.__class__.__name__}",
)
def _should_run_deadlock_detection(self) -> bool:
"""Determines whether the plugin will perform process reconciliation in case of errors.
If the environment variable `PL_RECONCILE_PROCESS` is set, run detection regardless of the cluster environment.
By default this is disabled. Otherwise, if the cluster environment creates the processes, allow the scheduler /
parent process to perform the process termination, external to Lightning.
"""
return os.getenv("PL_RECONCILE_PROCESS", "0") == "1" or self._rank_0_will_call_children_scripts
def _share_information_to_prevent_deadlock(self) -> None:
self._share_pids()
# there should be a unique sync_dir per nodes.
if self.local_rank == 0:
# create a temporary directory used to synchronize processes on deadlock.
self._sync_dir = tempfile.mkdtemp()
sync_dirs = []
global_node_rank_zero = 0
for _ in range(self.num_nodes):
sync_dirs.append(self.broadcast(self._sync_dir, global_node_rank_zero))
global_node_rank_zero += self.world_size // self.num_nodes
self._sync_dir = sync_dirs[self.node_rank]
def _share_pids(self) -> None:
"""Make all DDP processes aware of all processes pids."""
self.barrier()
pids = self.all_gather(torch.tensor(os.getpid(), device=self.root_device))
pids = pids.cpu().numpy().tolist()
self._pids = pids if isinstance(pids, list) else [pids]
[docs] def reconciliate_processes(self, trace: str) -> None:
if self.world_size < 2:
return
if not self._should_run_deadlock_detection():
return
sync_dir = self._sync_dir
if not sync_dir:
rank_zero_warn("Error handling mechanism for deadlock detection is uninitialized. Skipping check.")
return
# The cluster may be configured to periodically purge the `/tmp`
# directory, in which case `sync_dir` may not exist anymore at this
# point. Idempotently create it to ensure its existence.
Path(sync_dir).mkdir(parents=True, exist_ok=True)
# save a file locally.
torch.save(True, os.path.join(sync_dir, f"{self.global_rank}.pl"))
# sleep for a short time
time.sleep(3)
# return if all processes wrote a file in the `sync_dir`.
# todo (tchaton) Add support for non-shared file-system which will fail.
if len(os.listdir(sync_dir)) == (self.world_size // self.num_nodes):
return
for pid in self._pids:
if pid != os.getpid():
os.kill(pid, signal.SIGKILL)
shutil.rmtree(sync_dir)
raise DeadlockDetectedException(f"DeadLock detected from rank: {self.global_rank} \n {trace}")
[docs] def teardown(self) -> None:
log.detail(f"{self.__class__.__name__}: tearing down strategy")
super().teardown()
if isinstance(self.model, DistributedDataParallel):
if (
_TORCH_GREATER_EQUAL_1_11
and not self.model.static_graph
and self.model._get_ddp_logging_data().get("can_set_static_graph")
):
rank_zero_info(
"Your model can run with static graph optimizations. For future training runs, we suggest you"
f" pass `Trainer(..., strategy={self.__class__.__name__}(static_graph=True))` to enable them."
)
# unwrap model
self.model = self.lightning_module
if (
self.lightning_module.trainer is not None
and self.lightning_module.trainer.state.fn == TrainerFn.FITTING
and self._layer_sync
):
# `self.lightning_module.trainer` can be None if teardown gets called on an exception before
# the trainer gets set on the LightningModule
self.model = self._layer_sync.revert(self.model)
if self.root_device.type == "cuda":
# GPU teardown
log.detail(f"{self.__class__.__name__}: moving model to CPU")
self.lightning_module.cpu()
# clean up memory
torch.cuda.empty_cache()
