Fix FSDP device mesh initialization on CPU#21816
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Initialize device mesh with the strategy's root device type instead of hardcoded "cuda", allowing FSDP to run on CPU-only environments.
Yonghui-Lee
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ethanwharris,
justusschock and
tchaton
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July 9, 2026 04:08
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Pull request overview
This PR fixes FSDP DeviceMesh initialization to support CPU-only environments by using the strategy’s root_device.type instead of a hardcoded "cuda", and adds regression tests to validate CPU device-mesh initialization in both Lightning and Fabric.
Changes:
- Initialize
DeviceMeshusingself.root_device.typein bothlightning.pytorchandlightning.fabricFSDP strategies. - Add CPU regression tests ensuring
init_device_meshis called with"cpu"when running with CPU parallel devices. - Add missing
RunIfimport in Fabric FSDP tests.
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| src/lightning/pytorch/strategies/fsdp.py | Use root_device.type for init_device_mesh instead of hardcoded "cuda" |
| src/lightning/fabric/strategies/fsdp.py | Use root_device.type for init_device_mesh instead of hardcoded "cuda" |
| tests/tests_pytorch/strategies/test_fsdp.py | Add regression test for CPU device-mesh initialization |
| tests/tests_fabric/strategies/test_fsdp.py | Add regression test (and RunIf import) for CPU device-mesh initialization |
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| @RunIf(min_torch="2.2") | ||
| @mock.patch("torch.distributed.device_mesh.init_device_mesh") | ||
| @mock.patch("torch.distributed.init_process_group") | ||
| def test_device_mesh_initialization_cpu(init_process_group_mock, init_device_mesh_mock): |
| @RunIf(min_torch="2.2") | ||
| @mock.patch("torch.distributed.device_mesh.init_device_mesh") | ||
| @mock.patch("torch.distributed.init_process_group") | ||
| def test_device_mesh_initialization_cpu(init_process_group_mock, init_device_mesh_mock): |
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What does this PR do?
Fixes #21814
Initialize device mesh with the strategy's root device type instead of hardcoded "cuda", allowing FSDP to run on CPU-only environments.
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