Fix gradient accumulation under FSDP2: defer cross-replica all-reduce - #162
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Fix gradient accumulation under FSDP2: defer cross-replica all-reduce#162Sullivan07043 wants to merge 1 commit into
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ddp_sync_grad only handled DistributedDataParallel. Under FSDP2 (fully_shard) it was a silent no-op, so every micro-step of an accumulation window ran the full cross-node gradient reduction and grad_accum_iter saved no communication (measured: accum=4 step = 4x the accum=1 step on 2x8 A100 over Ethernet). Defer only the cross-replica all-reduce via FSDPModule.set_requires_all_reduce on non-boundary micro-steps. The intra-node reduce-scatter still runs every micro-step, so gradients stay sharded and per-rank memory does not grow. Validated on 2x 8xA100-80GB (HSDP shard=8 replicate=2): accum=4 step 54.5 s vs 88.8 s unamortized on a fleet whose accum=1 step is 22.2 s.
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Fixes #161 (
ddp_sync_gradis a silent no-op under FSDP2, sotrainer.grad_accum_itersaves no communication).Change. Add an FSDP2 branch to
ddp_sync_grad: on non-boundary micro-steps, defer only the cross-replica all-reduce viaFSDPModule.set_requires_all_reduce(enabled, recurse=False)on everyFSDPModule. Restore it in thefinallyblock. The intra-node reduce-scatter still runs each micro-step, so gradients stay sharded and per-rank memory does not grow. DDP behavior is unchanged.Validation (2x 8xA100-80GB, Ethernet, HSDP shard=8 replicate=2, batch 64/rank, fleet whose accum=1 step is 22.2 s):
Decomposition from the two measured points: compute 10.8 s + sync 11.4 s per micro-batch. Throughput 1.63x at accum=4. Loss stayed in the expected band.
One commit, 18 added lines, no API change.