1/n Simplify spawn plugins: Simplify handling of multiprocessing queue

CHANGELOG.md

@@ -80,7 +80,11 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Moved `batch_to_device` method from `Accelerator` to `TrainingTypePlugin` ([#10649](https://github.com/PyTorchLightning/pytorch-lightning/pull/10649))
 
 
--
+- The `DDPSpawnPlugin` no longer overrides the `post_dispatch` plugin hook ([#10034](https://github.com/PyTorchLightning/pytorch-lightning/pull/10034))
+
+
+- The `LightningModule.{add_to_queue,get_from_queue}` hooks no longer get a `torch.multiprocessing.SimpleQueue` and instead receive a list based queue ([#10034](https://github.com/PyTorchLightning/pytorch-lightning/pull/10034))
+
 
 ### Deprecated
 
@@ -188,6 +192,12 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Removed argument `return_result` from the `DDPSpawnPlugin.spawn()` method ([#10867](https://github.com/PyTorchLightning/pytorch-lightning/pull/10867))
 
 
+- Removed the property `TrainingTypePlugin.results` and corresponding properties in subclasses ([#10034](https://github.com/PyTorchLightning/pytorch-lightning/pull/10034))
+
+
+- Removed the `mp_queue` attribute from `DDPSpawnPlugin` and `TPUSpawnPlugin` ([#10034](https://github.com/PyTorchLightning/pytorch-lightning/pull/10034))
+
+
 - Removed unnessesary `_move_optimizer_state` method overrides from `TPUSpawnPlugin` and `SingleTPUPlugin` ([#10849](https://github.com/PyTorchLightning/pytorch-lightning/pull/10849))
 
 

pytorch_lightning/core/lightning.py

@@ -1917,7 +1917,7 @@ def model_size(self) -> float:
             )
         return get_model_size_mb(self)
 
-    def add_to_queue(self, queue: torch.multiprocessing.SimpleQueue) -> None:
+    def add_to_queue(self, queue: pl.plugins.training_type.ddp_spawn._FakeQueue) -> None:
         """Appends the :attr:`trainer.callback_metrics` dictionary to the given queue. To avoid issues with memory
         sharing, we cast the data to numpy.
 
@@ -1931,7 +1931,7 @@ def add_to_queue(self, queue: torch.multiprocessing.SimpleQueue) -> None:
         if self.trainer and isinstance(self.trainer.training_type_plugin, pl.plugins.training_type.DDPSpawnPlugin):
             self.trainer.training_type_plugin.add_to_queue(self.trainer, queue)
 
-    def get_from_queue(self, queue: torch.multiprocessing.SimpleQueue) -> None:
+    def get_from_queue(self, queue: pl.plugins.training_type.ddp_spawn._FakeQueue) -> None:
         """Retrieve the :attr:`trainer.callback_metrics` dictionary from the given queue. To preserve consistency,
         we cast back the data to ``torch.Tensor``.
 

pytorch_lightning/plugins/training_type/ddp_spawn.py

@@ -14,8 +14,9 @@
 import logging
 import os
 import re
+from collections import UserList
 from multiprocessing.queues import SimpleQueue
-from typing import Any, Callable, Dict, List, Optional, Union
+from typing import Any, Callable, Dict, List, NamedTuple, Optional, Union
 
 import numpy as np
 import torch
@@ -45,7 +46,7 @@
 from pytorch_lightning.utilities.enums import _StrategyType
 from pytorch_lightning.utilities.model_helpers import is_overridden
 from pytorch_lightning.utilities.seed import reset_seed
-from pytorch_lightning.utilities.types import STEP_OUTPUT
+from pytorch_lightning.utilities.types import _PATH, STEP_OUTPUT
 
 if _TORCH_GREATER_EQUAL_1_8:
     from pytorch_lightning.utilities.distributed import register_ddp_comm_hook
@@ -80,7 +81,6 @@ def __init__(
         self.sync_batchnorm = False
         self._ddp_kwargs = kwargs
         self.num_processes = len(parallel_devices) if parallel_devices is not None else 0
-        self.mp_queue = None
         self._ddp_comm_state = ddp_comm_state
         self._ddp_comm_hook = ddp_comm_hook
         self._ddp_comm_wrapper = ddp_comm_wrapper
@@ -101,15 +101,6 @@ def num_nodes(self, num_nodes: int) -> None:
     def local_rank(self) -> int:
         return self._local_rank
 
-    def __getstate__(self):
-        """Makes this plugin pickleable without destroying the queue in the current process."""
-        state = self.__dict__.copy()
-        state["mp_queue"] = None
-        return state
-
-    def __setstate__(self, state):
-        self.__dict__ = state
-
     @property
     def root_device(self):
         return self.parallel_devices[self.local_rank]
@@ -125,9 +116,6 @@ def _is_single_process_single_device(self):
 
     def setup(self, trainer: "pl.Trainer") -> None:
         os.environ["MASTER_PORT"] = str(self.cluster_environment.main_port)
-        # pass in a state q
-        smp = mp.get_context("spawn")
-        self.mp_queue = smp.SimpleQueue()
         super().setup(trainer)
 
     def _setup_model(self, model: Module) -> DistributedDataParallel:
@@ -145,18 +133,24 @@ def set_world_ranks(self, process_idx: int = 0) -> None:
     def get_mp_spawn_kwargs(self, trainer: Optional["pl.Trainer"] = None) -> Dict[str, Any]:
         return {"nprocs": self.num_processes}
 
-    def start_training(self, trainer: "pl.Trainer") -> None:
-        self.spawn(self.new_process, trainer, self.mp_queue)
+    def start_training(self, trainer: "pl.Trainer") -> Any:
+        spawn_output: _SpawnOutput = self.spawn(self.new_process, trainer)
+        self.__recover_results_in_main_process(spawn_output, trainer)
         # reset optimizers, since main process is never used for training and thus does not have a valid optim state
         trainer.optimizers = []
+        return spawn_output.trainer_results
 
-    def start_evaluating(self, trainer: "pl.Trainer") -> None:
-        self.spawn(self.new_process, trainer, self.mp_queue)
+    def start_evaluating(self, trainer: "pl.Trainer") -> Any:
+        spawn_output: _SpawnOutput = self.spawn(self.new_process, trainer)
+        self.__recover_results_in_main_process(spawn_output, trainer)
+        return spawn_output.trainer_results
 
-    def start_predicting(self, trainer: "pl.Trainer") -> None:
-        self.spawn(self.new_process, trainer, self.mp_queue)
+    def start_predicting(self, trainer: "pl.Trainer") -> Any:
+        spawn_output: _SpawnOutput = self.spawn(self.new_process, trainer)
+        self.__recover_results_in_main_process(spawn_output, trainer)
+        return spawn_output.trainer_results
 
-    def spawn(self, function: Callable, *args: Any, **kwargs: Any) -> Optional[Any]:
+    def spawn(self, function: Callable, *args: Any, **kwargs: Any) -> Optional[Union[Any, "_SpawnOutput"]]:
         """Spawn processes that run the given function.
 
         Args:
@@ -191,9 +185,7 @@ def _worker_setup(self, process_idx: int):
             self.cluster_environment, self.torch_distributed_backend, self.global_rank, self.world_size
         )
 
-    def new_process(self, trainer: "pl.Trainer", mp_queue: SimpleQueue) -> None:
-        self.mp_queue = mp_queue
-
+    def new_process(self, trainer: "pl.Trainer") -> Optional["_SpawnOutput"]:
         # move the model to the correct device
         self.model_to_device()
 
@@ -208,28 +200,11 @@ def new_process(self, trainer: "pl.Trainer", mp_queue: SimpleQueue) -> None:
         self.barrier()
 
         results = trainer.run_stage()
-
-        # persist info in ddp_spawn
-        self.__transfer_distrib_spawn_state_on_fit_end(trainer, results)
+        outputs = self.__collect_rank_zero_results(trainer, results)
 
         # ensure that spawned processes go through teardown before joining
         trainer._call_teardown_hook()
-
-    def post_dispatch(self, trainer: "pl.Trainer"):
-        # restore main state with best weights
-        best_path = self.mp_queue.get()
-        last_path = self.mp_queue.get()
-        self._results = self.mp_queue.get()
-        # get the `callback_metrics` and set it to the trainer
-        # only in case the user does not override it.
-        # TODO: Remove the if in v1.7
-        if is_overridden("get_from_queue", self.lightning_module):
-            self.lightning_module.get_from_queue(self.mp_queue)
-        else:
-            self.get_from_queue(trainer, self.mp_queue)
-
-        # recover the weights of the processes trained in the children
-        self.__recover_child_process_weights(best_path, last_path)
+        return outputs
 
     def pre_configure_ddp(self):
         # if unset, default `find_unused_parameters` `True`
@@ -268,7 +243,7 @@ def determine_ddp_device_ids(self):
             return None
         return [self.root_device.index]
 
-    def __transfer_distrib_spawn_state_on_fit_end(self, trainer: "pl.Trainer", results: Any) -> None:
+    def __collect_rank_zero_results(self, trainer: "pl.Trainer", results: Any) -> Optional["_SpawnOutput"]:
         rank_zero_warn("cleaning up ddp environment...")
         checkpoint_callback = trainer.checkpoint_callback
         best_model_path = checkpoint_callback.best_model_path if checkpoint_callback else None
@@ -285,28 +260,37 @@ def __transfer_distrib_spawn_state_on_fit_end(self, trainer: "pl.Trainer", resul
             last_path = re.sub(".ckpt", ".tmp_end.ckpt", best_model_path)
             self.checkpoint_io.save_checkpoint(state_dict, last_path)
 
-        # todo, pass complete checkpoint as state dictionary
-        self.mp_queue.put(best_model_path)
-        self.mp_queue.put(last_path)
-        self.mp_queue.put(results)
         # adds the `callback_metrics` to the queue
-        # TODO: Remove the if in v1.7
+        extra = _FakeQueue()
         if is_overridden("add_to_queue", self.lightning_module):
-            self.lightning_module.add_to_queue(self.mp_queue)
+            # TODO: Remove the if in v1.7
+            self.lightning_module.add_to_queue(extra)
         else:
-            self.add_to_queue(trainer, self.mp_queue)
+            self.add_to_queue(trainer, extra)
 
-    def __recover_child_process_weights(self, best_path, last_path):
+        return _SpawnOutput(best_model_path, last_path, results, extra)
+
+    def __recover_results_in_main_process(self, spawn_output: "_SpawnOutput", trainer) -> None:
         # transfer back the best path to the trainer
         if self.lightning_module.trainer.checkpoint_callback:
-            self.lightning_module.trainer.checkpoint_callback.best_model_path = best_path
-        # todo, pass also best score
+            self.lightning_module.trainer.checkpoint_callback.best_model_path = spawn_output.best_model_path
 
+        # TODO: pass also best score
         # load last weights
-        if last_path is not None and self.lightning_module.trainer.state.fn == TrainerFn.FITTING:
-            ckpt = self.checkpoint_io.load_checkpoint(last_path, map_location=(lambda storage, loc: storage))
+        if spawn_output.last_path is not None and self.lightning_module.trainer.state.fn == TrainerFn.FITTING:
+            ckpt = self.checkpoint_io.load_checkpoint(
+                spawn_output.last_path, map_location=(lambda storage, loc: storage)
+            )
             self.lightning_module.load_state_dict(ckpt)
 
+        # get the `callback_metrics` and set it to the trainer
+        if is_overridden("get_from_queue", self.lightning_module):
+            # only in case the user does not override it.
+            # TODO: Remove the if in v1.7
+            self.lightning_module.get_from_queue(spawn_output.extra)
+        else:
+            self.get_from_queue(trainer, spawn_output.extra)
+
     def barrier(self, *args, **kwargs) -> None:
         if not distributed_available():
             return
@@ -372,23 +356,25 @@ def post_training_step(self):
         if not self.lightning_module.automatic_optimization:
             self.model.require_backward_grad_sync = True
 
-    def add_to_queue(self, trainer: "pl.Trainer", queue: torch.multiprocessing.SimpleQueue) -> None:
+    def add_to_queue(self, trainer: "pl.Trainer", queue: "_FakeQueue") -> None:
         """Appends the :attr:`trainer.callback_metrics` dictionary to the given queue. To avoid issues with memory
         sharing, we cast the data to numpy.
 
         Args:
+            trainer: reference to the Trainer.
             queue: the instance of the queue to append the data.
         """
         callback_metrics: dict = apply_to_collection(
             trainer.callback_metrics, torch.Tensor, lambda x: x.cpu().numpy()
         )  # send as numpy to avoid issues with memory sharing
         queue.put(callback_metrics)
 
-    def get_from_queue(self, trainer: "pl.Trainer", queue: torch.multiprocessing.SimpleQueue) -> None:
+    def get_from_queue(self, trainer: "pl.Trainer", queue: "_FakeQueue") -> None:
         """Retrieve the :attr:`trainer.callback_metrics` dictionary from the given queue. To preserve consistency,
         we cast back the data to ``torch.Tensor``.
 
         Args:
+            trainer: reference to the Trainer.
             queue: the instance of the queue from where to get the data.
         """
         # NOTE: `add_to_queue` needs to be called before
@@ -413,3 +399,23 @@ def teardown(self) -> None:
             self.lightning_module.cpu()
             # clean up memory
             torch.cuda.empty_cache()
+
+
+class _FakeQueue(UserList):
+    """Simulates a :class:`torch.multiprocessing.queue.SimpleQueue` interface using the Python list."""
+
+    def get(self) -> Any:
+        return self.pop(0)
+
+    def put(self, item: Any) -> None:
+        self.append(item)
+
+    def empty(self) -> bool:
+        return len(self) == 0
+
+
+class _SpawnOutput(NamedTuple):
+    best_model_path: Optional[_PATH]
+    last_path: Optional[_PATH]
+    trainer_results: Any
+    extra: _FakeQueue

pytorch_lightning/plugins/training_type/sharded_spawn.py

@@ -12,16 +12,15 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from contextlib import contextmanager
-from multiprocessing.queues import SimpleQueue
-from typing import Dict, Generator, List, Optional, Tuple
+from typing import Any, Dict, Generator, List, Optional, Tuple
 
 import torch
 from torch.nn import Module
 from torch.optim import Optimizer
 
 import pytorch_lightning as pl
 from pytorch_lightning.plugins.precision.sharded_native_amp import ShardedNativeMixedPrecisionPlugin
-from pytorch_lightning.plugins.training_type.ddp_spawn import DDPSpawnPlugin
+from pytorch_lightning.plugins.training_type.ddp_spawn import _FakeQueue, DDPSpawnPlugin
 from pytorch_lightning.trainer.states import TrainerFn
 from pytorch_lightning.utilities import _FAIRSCALE_AVAILABLE, rank_zero_only
 from pytorch_lightning.utilities.enums import _StrategyType
@@ -115,12 +114,12 @@ def pre_backward(self, closure_loss: torch.Tensor) -> None:
     def post_training_step(self):
         pass
 
-    def new_process(self, trainer: "pl.Trainer", mp_queue: SimpleQueue) -> None:
+    def new_process(self, trainer: "pl.Trainer") -> Optional[Tuple[Optional[str], Optional[str], Any, _FakeQueue]]:
         # Ensure that the scaler points to the correct process group
         # which is re-initialized in a new process
         if isinstance(self.precision_plugin, ShardedNativeMixedPrecisionPlugin):
             self._precision_plugin.scaler = ShardedGradScaler()
-        return super().new_process(trainer, mp_queue)
+        return super().new_process(trainer)
 
     @classmethod
     def register_plugins(cls, plugin_registry: Dict) -> None: