new dist_cp save planner to fix issue that each rank needs to download all checkpoint files #3271
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…ad all checkpoint files (#3271) * a * a * a * a * lint * lint --------- Co-authored-by: Mihir Patel <mihir.v.patel7@gmail.com>
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what's the issue
PyTorch Issue: pytorch/pytorch#125740
When pytorch 2.3 saves checkpoint files, if there are duplicated tensors, it will do dedup. In the dedup, it tries to balance the storage needed for each rank, so the duplicated tensors are saved on different ranks. The consequest is that, when loading checkpoint, each rank needs to read files saved by different ranks even without resharding. Ideally, each rank_k only needs to download rank_k and rank_0 files.
what's the fix
we save the duplicated tensors to the smallest rank only. This is how torch2.1 and torch2.2 do the dedup.
test
Tested on 512 GPUs. Save and load both work.