[DeviceMesh] Implement a device mesh concatenate api for submesh and SPMD use case #163358
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This was referenced Sep 19, 2025
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🔗 Helpful Links🧪 See artifacts and rendered test results at hud.pytorch.org/pr/163358
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…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
ezyang
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Sep 21, 2025
ezyang
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Sep 21, 2025
ezyang
reviewed
Sep 21, 2025
torch/distributed/device_mesh.py
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| for mesh_nd in pg_ranks_by_dim: |
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?!?! Why do you need to do it for every mesh_nd? Is this because you're triggering comms to initialize PGs?
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so long story short, we need all ranks to call new_group which is hidden very deep in the stack to initialize PGs. Otherwise the code will hang.
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…or submesh" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
fduwjj
changed the title
…ubmesh and SPMD use case" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…ubmesh and SPMD use case" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…ubmesh and SPMD use case" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
…ubmesh and SPMD use case" Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users. One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation. cc H-Huang awgu wanchaol fegin wz337 wconstab d4l3k pragupta ezyang msaroufim dcci [ghstack-poisoned]
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Stack from ghstack (oldest at bottom):
Today FSDP needs to slicing out spmd mesh from root mesh here: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/_fully_shard/_fsdp_param.py#L301. But essentially, users want is a concatenate of some submesh into a big mesh and used as a spmd mesh. This PR is tentatively trying to implement this API for users.
One thing to note is that, all sub-mesh needs to slicing/flatten or unflatten from same root mesh otherwise the indices make no sense when it comes to mesh indexing and device allocation.
cc @H-Huang @awgu @wanchaol @fegin @wz337 @wconstab @d4l3k @pragupta @ezyang @msaroufim @dcci