Source code for lightning.pytorch.strategies.parallel

# 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 abc import ABC, abstractmethod
from collections.abc import Generator
from contextlib import contextmanager
from typing import Any, Optional

import torch
from torch import Tensor
from typing_extensions import override

import lightning.pytorch as pl
from lightning.fabric.plugins import CheckpointIO, ClusterEnvironment
from lightning.fabric.utilities.distributed import ReduceOp, _all_gather_ddp_if_available
from lightning.pytorch.plugins import LayerSync
from lightning.pytorch.plugins.precision import Precision
from lightning.pytorch.strategies.strategy import Strategy


class ParallelStrategy(Strategy, ABC):
    """Strategy for training with multiple processes in parallel."""

    def __init__(
        self,
        accelerator: Optional["pl.accelerators.Accelerator"] = None,
        parallel_devices: Optional[list[torch.device]] = None,
        cluster_environment: Optional[ClusterEnvironment] = None,
        checkpoint_io: Optional[CheckpointIO] = None,
        precision_plugin: Optional[Precision] = None,
    ):
        super().__init__(accelerator=accelerator, checkpoint_io=checkpoint_io, precision_plugin=precision_plugin)
        self.parallel_devices = parallel_devices
        self.cluster_environment: Optional[ClusterEnvironment] = cluster_environment
        self._layer_sync: Optional[LayerSync] = None

    @property
    @abstractmethod
    @override
    def root_device(self) -> torch.device:
        """Return the root device."""

    @property
    def global_rank(self) -> int:
        return self.cluster_environment.global_rank() if self.cluster_environment is not None else 0

    @property
    def local_rank(self) -> int:
        return self.cluster_environment.local_rank() if self.cluster_environment is not None else 0

    @property
    def node_rank(self) -> int:
        return self.cluster_environment.node_rank() if self.cluster_environment is not None else 0

    @property
    def world_size(self) -> int:
        return self.cluster_environment.world_size() if self.cluster_environment is not None else 1

    @property
    @override
    def is_global_zero(self) -> bool:
        return self.global_rank == 0

    @property
    def parallel_devices(self) -> Optional[list[torch.device]]:
        return self._parallel_devices

    @parallel_devices.setter
    def parallel_devices(self, parallel_devices: Optional[list[torch.device]]) -> None:
        self._parallel_devices = parallel_devices

    @property
    def distributed_sampler_kwargs(self) -> dict[str, Any]:
        return {
            "num_replicas": len(self.parallel_devices) if self.parallel_devices is not None else 0,
            "rank": self.global_rank,
        }

    @override
    def all_gather(self, tensor: Tensor, group: Optional[Any] = None, sync_grads: bool = False) -> Tensor:
        """Perform a all_gather on all processes."""
        return _all_gather_ddp_if_available(tensor, group=group, sync_grads=sync_grads)

    @override
    def reduce_boolean_decision(self, decision: bool, all: bool = True) -> bool:
        """Reduces a boolean decision over distributed processes. By default is analagous to ``all`` from the standard
        library, returning ``True`` only if all input decisions evaluate to ``True``. If ``all`` is set to ``False``,
        it behaves like ``any`` instead.

        Args:
            decision: A single input decision.
            all: Whether to logically emulate ``all`` or ``any``. Defaults to True.

        Returns:
            bool: The reduced boolean decision.

        """
        decision = torch.tensor(int(decision), device=self.root_device)
        decision = self.reduce(
            decision,
            reduce_op=ReduceOp.SUM,  # type: ignore[arg-type]
        )
        decision = bool(decision == self.world_size) if all else bool(decision)
        return decision

    @contextmanager
    def block_backward_sync(self) -> Generator:
        """Blocks ddp sync gradients behaviour on backwards pass.

        This is useful for skipping sync when accumulating gradients, reducing communication overhead
        Returns: context manager with sync behaviour off

        """
        if isinstance(self.model, pl.utilities.types.DistributedDataParallel):
            with self.model.no_sync():
                yield None
        else:
            yield None

    @override
    def teardown(self) -> None:
        assert self.cluster_environment is not None
        self.cluster_environment.teardown()
        super().teardown()