Fix TP-only parallelism config: always include dp_shard in device mesh#3986
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songhappy wants to merge 1 commit intohuggingface:mainfrom
Open
Fix TP-only parallelism config: always include dp_shard in device mesh#3986songhappy wants to merge 1 commit intohuggingface:mainfrom
songhappy wants to merge 1 commit intohuggingface:mainfrom
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What does this PR do?
Fixes a crash when using TP-only parallelism (e.g., [tp_size=8, dp_shard_size=1] with error of:
KeyError: "Invalid mesh_dim_names ('dp_shard_cp',) specified. Valid mesh_dim_names are ['tp']."Root cause: _get_mesh() only includes dimensions with size > 1 in the mesh, so dp_shard is excluded when dp_shard_size=1. However, build_device_mesh() unconditionally tries to create a dp_shard_cp submesh from dp_shard + cp, which fails because neither dimension exists in the mesh.
Fix (two changes):
_get_mesh(): Always include dp_shard in the mesh dimensions (even when size == 1) so that FSDP2 has a valid submesh. A size-1 shard dimension is a no-op but keeps the composable API consistent.
build_device_mesh(): Build the dp_shard_cp submesh from dimensions actually present in the mesh instead of relying on dp_shard_cp_dim_names which assumes both dimensions exist.
Impact:
No effect on existing FSDP-only or FSDP+TP configs (dp_shard was already present). Only fixes the TP-only case where dp_shard was missing.