Disable memory sharing on model parameters in ddp-spawn (#18238)

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

(cherry picked from commit a0ca2c8)

src/lightning/fabric/CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
+## [UnReleased] - 2023-08-DD
+
+### Fixed
+
+- Fixed model parameters getting shared between processes when running with `strategy="ddp_spawn"` and `accelerator="cpu"`; this has a necessary memory impact, as parameters are replicated for each process now ([#18238](https://github.com/Lightning-AI/lightning/pull/18238))
+
 
 ## [2.0.7] - 2023-08-14
 

src/lightning/fabric/strategies/launchers/multiprocessing.py

@@ -11,6 +11,7 @@
 # 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 itertools
 import os
 from dataclasses import dataclass
 from multiprocessing.queues import SimpleQueue
@@ -19,7 +20,10 @@
 import torch
 import torch.backends.cudnn
 import torch.multiprocessing as mp
+from lightning_utilities import apply_to_collection
+from torch.nn import Module
 
+from lightning.fabric.accelerators.cpu import CPUAccelerator
 from lightning.fabric.strategies.launchers.launcher import _Launcher
 from lightning.fabric.utilities.apply_func import move_data_to_device
 from lightning.fabric.utilities.imports import _IS_INTERACTIVE
@@ -122,6 +126,10 @@ def _wrapping_function(
     ) -> None:
         if global_states:
             global_states.restore()
+
+        if self._start_method == "spawn" and isinstance(self._strategy.accelerator, CPUAccelerator):
+            args, kwargs = _disable_module_memory_sharing((args, kwargs))
+
         os.environ["LOCAL_RANK"] = str(process_idx)
         results = function(*args, **kwargs)
 
@@ -190,3 +198,21 @@ def _check_bad_cuda_fork() -> None:
     if _IS_INTERACTIVE:
         message += " You will have to restart the Python kernel."
     raise RuntimeError(message)
+
+
+def _disable_module_memory_sharing(data: Any) -> Any:
+    """Disables memory sharing on parameters and buffers of `nn.Module`s contained in the given collection.
+
+    Note: This is only required when running on CPU.
+    """
+    # PyTorch enables memory sharing automatically on all tensors that are passed through `mp.spawn`.
+    # For model weights and buffers, this is undesired and can lead to race conditions between processes.
+    # Hence, we copy the tensors in the entire module to ensure it doesn't share memory with other processes.
+
+    @torch.no_grad()
+    def unshare(module: Module) -> Module:
+        for tensor in itertools.chain(module.parameters(), module.buffers()):
+            tensor.data = tensor.data.clone()
+        return module
+
+    return apply_to_collection(data, function=unshare, dtype=Module)

src/lightning/pytorch/CHANGELOG.md

@@ -38,6 +38,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed redundant `iter()` call to dataloader when checking dataloading configuration ([#18415](https://github.com/Lightning-AI/lightning/pull/18415))
 
+- Fixed model parameters getting shared between processes when running with `strategy="ddp_spawn"` and `accelerator="cpu"`; this has a necessary memory impact, as parameters are replicated for each process now ([#18238](https://github.com/Lightning-AI/lightning/pull/18238))
+
 
 ## [2.0.5] - 2023-07-07
 

src/lightning/pytorch/strategies/launchers/multiprocessing.py

@@ -27,10 +27,11 @@
 from torch import Tensor
 
 import lightning.pytorch as pl
-from lightning.fabric.strategies.launchers.multiprocessing import _check_bad_cuda_fork
+from lightning.fabric.strategies.launchers.multiprocessing import _check_bad_cuda_fork, _disable_module_memory_sharing
 from lightning.fabric.utilities import move_data_to_device
 from lightning.fabric.utilities.seed import _collect_rng_states, _set_rng_states
 from lightning.fabric.utilities.types import _PATH
+from lightning.pytorch.accelerators import CPUAccelerator
 from lightning.pytorch.strategies.launchers.launcher import _Launcher
 from lightning.pytorch.trainer.connectors.signal_connector import _SIGNUM
 from lightning.pytorch.trainer.states import TrainerFn, TrainerState
@@ -145,6 +146,9 @@ def _wrapping_function(
     ) -> None:
         if global_states:
             global_states.restore()
+        if self._start_method == "spawn" and isinstance(self._strategy.accelerator, CPUAccelerator):
+            args, kwargs = _disable_module_memory_sharing((args, kwargs))
+
         os.environ["LOCAL_RANK"] = str(process_idx)
         results = function(*args, **kwargs)
 

tests/tests_fabric/strategies/launchers/test_multiprocessing.py

@@ -23,20 +23,20 @@
 
 @RunIf(skip_windows=True)
 @pytest.mark.parametrize("start_method", ["fork", "forkserver"])
-def test_multiprocessing_launcher_interactive_compatible(start_method):
+def test_interactive_compatible(start_method):
     launcher = _MultiProcessingLauncher(strategy=Mock(), start_method=start_method)
     assert launcher.is_interactive_compatible == (start_method == "fork")
 
 
 @mock.patch("lightning.fabric.strategies.launchers.multiprocessing.mp.get_all_start_methods", return_value=[])
-def test_multiprocessing_launcher_forking_on_unsupported_platform(_):
+def test_forking_on_unsupported_platform(_):
     with pytest.raises(ValueError, match="The start method 'fork' is not available on this platform"):
         _MultiProcessingLauncher(strategy=Mock(), start_method="fork")
 
 
 @pytest.mark.parametrize("start_method", ["spawn", pytest.param("fork", marks=RunIf(standalone=True))])
 @mock.patch("lightning.fabric.strategies.launchers.multiprocessing.mp")
-def test_multiprocessing_launcher_start_method(mp_mock, start_method):
+def test_start_method(mp_mock, start_method):
     mp_mock.get_all_start_methods.return_value = [start_method]
     launcher = _MultiProcessingLauncher(strategy=Mock(), start_method=start_method)
     launcher.launch(function=Mock())
@@ -51,7 +51,7 @@ def test_multiprocessing_launcher_start_method(mp_mock, start_method):
 
 @pytest.mark.parametrize("start_method", ["spawn", pytest.param("fork", marks=RunIf(standalone=True))])
 @mock.patch("lightning.fabric.strategies.launchers.multiprocessing.mp")
-def test_multiprocessing_launcher_restore_globals(mp_mock, start_method):
+def test_restore_globals(mp_mock, start_method):
     """Test that we pass the global state snapshot to the worker function only if we are starting with 'spawn'."""
     mp_mock.get_all_start_methods.return_value = [start_method]
     launcher = _MultiProcessingLauncher(strategy=Mock(), start_method=start_method)
@@ -89,7 +89,7 @@ def test_global_state_snapshot():
 @pytest.mark.parametrize("start_method", ["fork", "forkserver"])
 @mock.patch("torch.cuda.is_initialized", return_value=True)
 @mock.patch("lightning.fabric.strategies.launchers.multiprocessing.mp")
-def test_multiprocessing_launcher_check_for_bad_cuda_fork(mp_mock, _, start_method):
+def test_check_for_bad_cuda_fork(mp_mock, _, start_method):
     mp_mock.get_all_start_methods.return_value = [start_method]
     launcher = _MultiProcessingLauncher(strategy=Mock(), start_method=start_method)
     with pytest.raises(RuntimeError, match="Lightning can't create new processes if CUDA is already initialized"):

tests/tests_fabric/strategies/launchers/test_multiprocessing_integration.py

@@ -0,0 +1,55 @@
+# 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.
+import pytest
+import torch
+import torch.nn as nn
+
+from lightning.fabric import Fabric
+from tests_fabric.helpers.runif import RunIf
+
+
+class SimpleModel(nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.layer = nn.Linear(2, 2)
+        self.tied_layer = nn.Linear(2, 2)
+        self.tied_layer.weight = self.layer.weight
+        self.register_buffer("buffer", torch.ones(3))
+
+
+@pytest.mark.parametrize("strategy", ["ddp_spawn", pytest.param("ddp_fork", marks=RunIf(skip_windows=True))])
+def test_memory_sharing_disabled(strategy):
+    """Test that the multiprocessing launcher disables memory sharing on model parameters and buffers to avoid race
+    conditions on model updates."""
+    tensor = torch.rand(4)
+    model = SimpleModel()
+    assert not tensor.is_shared()
+    assert not model.layer.weight.is_shared()
+    assert model.layer.weight.data_ptr() == model.tied_layer.weight.data_ptr()
+
+    fabric = Fabric(accelerator="cpu", devices=2, strategy=strategy)
+    fabric.launch(_test_memory_sharing_disabled, tensor, model)
+
+
+def _test_memory_sharing_disabled(fabric, tensor, model):
+    is_spawn = fabric.strategy.launcher._start_method == "spawn"
+    assert not is_spawn or tensor.is_shared()
+    assert not model.layer.weight.is_shared()
+    assert not model.tied_layer.weight.is_shared()
+    assert not model.buffer.is_shared()
+
+    # weights remain tied
+    assert model.layer.weight.data_ptr() == model.tied_layer.weight.data_ptr()
+    assert torch.equal(model.layer.weight.data, model.tied_layer.weight.data)
+    fabric.barrier()

tests/tests_pytorch/strategies/launchers/test_multiprocessing.py

@@ -175,3 +175,31 @@ def test_kill():
     with patch("os.kill") as kill_patch:
         launcher.kill(15)
     assert kill_patch.mock_calls == [call(proc0.pid, 15), call(proc1.pid, 15)]
+
+
+class SimpleModel(BoringModel):
+    def __init__(self):
+        super().__init__()
+        self.tied_layer = torch.nn.Linear(32, 2)
+        self.tied_layer.weight = self.layer.weight
+        self.register_buffer("buffer", torch.ones(3))
+
+    def on_fit_start(self) -> None:
+        assert not self.layer.weight.is_shared()
+        assert not self.tied_layer.weight.is_shared()
+        assert not self.buffer.is_shared()
+
+        # weights remain tied
+        assert self.layer.weight.data_ptr() == self.tied_layer.weight.data_ptr()
+        assert torch.equal(self.layer.weight.data, self.tied_layer.weight.data)
+
+
+def test_memory_sharing_disabled():
+    """Test that the multiprocessing launcher disables memory sharing on model parameters and buffers to avoid race
+    conditions on model updates."""
+    model = SimpleModel()
+    assert not model.layer.weight.is_shared()
+    assert model.layer.weight.data_ptr() == model.tied_layer.weight.data_ptr()
+
+    trainer = Trainer(accelerator="cpu", devices=2, strategy="ddp_spawn", max_steps=0)
+    trainer.fit(model)