Disk-level delta weight sync#2089
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This was referenced Jun 16, 2026
Ship only the changed bytes between weight syncs as a canonical HF delta checkpoint; rollout hosts apply it into a host-local checkpoint and reload via the vanilla update_weights_from_disk path. Replaces the NCCL delta transport from THUDM#1806 with a disk-only path that needs no engine-side delta support.
sync_local_checkpoint (was sync_weights) materializes the base lazily via the idempotent init_local_checkpoint instead of a background thread; record per-sync update time in update_weight_metrics; state the pre-read/pre-push hooks' purpose (non-POSIX filesystem coherence).
The actor's update_weights is already @timer-wrapped (perf/update_weights_time), so the per-sync total/publish/reload breakdown was duplicate instrumentation. Keep only the delta-specific metrics (density, wire bytes).
The delta scaffold reworked the update-weight args: delta requires --update-weight-transport=disk (was nccl-or-disk), needs --update-weight-local-checkpoint-dir, and the --update-weight-delta-dir compatibility alias is gone (the directory belongs to the transport, not the encoding). Drop the alias resolve/backfill/conflict tests, point the transport and colocate tests at the disk path, and cover the local-checkpoint requirement.
With the delta-dir alias gone, _resolve_update_weight_disk_dir no longer normalizes anything — it's a single transport-level check, so fold it into _validate_update_weight_args.
slime_validate_args validates everything else inline; the extracted _validate_update_weight_args was the lone exception. Fold it in and test it the same way as the other slime_validate_args checks (make_slime_validate_args).
Materialize the host-local checkpoint in a daemon thread at engine init so the one-time base copy overlaps sglang launch and the first rollout (which serves from init-loaded weights) instead of blocking the first delta reload. The first sync_local_checkpoint's init_local_checkpoint is idempotent and flock-guarded, so it either finds the copy done or blocks on the same lock — no join needed.
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Ship only the changed bytes between weight syncs instead of a full checkpoint, for non-colocated training/inference across clusters. The trainer publishes a per-tensor delta as a canonical HF checkpoint directory; each rollout host applies it in place and reloads via the ordinary
update_weights_from_diskpath — no delta-specific engine code. Replaces the NCCL delta transport from #1806.The delta is computed on raw tensor bytes and the engine just reloads a standard checkpoint, so the rollout side stays decoupled from the trainer and is free to use:
Future work
overwriteencoding instead of reloading the full checkpoint (larger sglang refactor).