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The Wire · Showcase

JAX FIXES CRITICAL SHARD_MAP GRADIENT BUG AFTER PARTIAL ROLLBACK

By RepoJournal · Filed · About Google

JAX reverted a commit that introduced incorrect gradient computation in distributed operations, then shipped the proper fix with safeguards.

The team rolled back ac12cbaa1f [1] after discovering it was inserting unnecessary collective operations during backward passes in shard_map. The root cause: wrong pspec calculations when check_vma=False [2]. Engineers have now landed the corrected version [3] that properly uses the cotangent spec to determine which mesh axes need synchronization. New test coverage [4] ensures unreduced forward inputs trigger correct psum insertion on the backward pass. This fixes a class of silent correctness bugs in distributed training where gradients would compute but with unnecessary communication overhead.

Action items

References

  1. [1] Reverts ac12cbaa1f877ef38a128000b8c561f136fc7f01 ↗ google/jax
  2. [2] Use the cotangent spec when calculating which mesh axes to do psum over if check_vma=False in shard_map transpose. google/jax
  3. [3] Use the cotangent spec when calculating which mesh axes to do psum over if check_vma=False in shard_map transpose. ↗ google/jax
  4. [4] Add a test to make sure that if input is unreduced on fwd (with check_vma=False on shard_map), shard_map inserts a psum on bwd similar to invarying. ↗ google/jax

FAQ

What changed in Google on May 25, 2026?
JAX reverted a commit that introduced incorrect gradient computation in distributed operations, then shipped the proper fix with safeguards.
What should Google teams do about it?
Pull latest JAX and run distributed training tests to verify gradient correctness • Check if your shard_map usage with check_vma=False was affected by the reverted commit
Which Google repositories shipped on May 25, 2026?
google/jax

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