# FSDP2

PyTorch 2.15 makes the rewritten NCCL backend, previewed as opt-in nccl2 in 2.14, the default for every nccl process group, keeping TORCH_DIST_USE_NCCL2=0 and the nccl-legacy name as rollback paths. At cluster scale a single stalled GPU could halt a whole training run and the cause was often reproduced by hand; a backend-agnostic flight recorder now writes a post-mortem to disk when a collective hangs, and a timeout on any rank triggers dumps from every rank.

PyTorch 2.15 makes the rewritten NCCL backend, previewed as opt-in nccl2 in 2.14, the default for every nccl process group, keeping TORCH_DIST_USE_NCCL2=0 and the nccl-legacy name as rollback paths. At cluster scale a single stalled GPU could halt a whole training run and the cause was often reproduced by hand; a backend-agnostic flight recorder now writes a post-mortem to disk when a collective hangs, and a timeout on any rank triggers dumps from every rank.