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[tinker] Fix multi-engine LoRA broadcast barrier hang (follow-on to merged #1720) #1728

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[tinker] Fix multi-engine LoRA broadcast barrier hang (follow-on to merged #1720) #1728

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14 changes: 11 additions & 3 deletions skyrl/backends/skyrl_train/workers/megatron/megatron_worker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -999,7 +999,17 @@ async def _save_lora_adapters_and_sync(
with open(os.path.join(lora_sync_path, "adapter_config.json"), "w", encoding="utf-8") as f:
json.dump(adapter_config, f, ensure_ascii=False, indent=4)

# Send LoRA disk loading request to inference engine.
# Sync after rank-0 disk write so all ranks see consistent state on
# Weka. Critically, this barrier is BEFORE the rank-0 HTTP fan-out to
# inference engines — fan-out to 4 engines on multi-engine configs can
# take >gloo-timeout (~10s) which would hang ranks 1-3 at the barrier.
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@SumanthRH SumanthRH May 29, 2026

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This is too low for GLOO timeout?

For Megatron, we init process group here:

https://github.com/hershg/SkyRL/blob/976bfe3a45f6f1e3db3b5e84b5a3d5d485d0eb67/skyrl/backends/skyrl_train/workers/megatron/megatron_worker.py#L512-L521

The default timeout value is SKYRL_WORKER_NCCL_TIMEOUT_IN_S which is 600s.

Are you sure this fix is needed? Have you overridden SKYRL_WORKER_NCCL_TIMEOUT_IN_S in some way?

torch.distributed.barrier()

# Rank-0 HTTP fan-out happens AFTER the barrier, so ranks 1-3 don't
# block on its duration. The caller's outer barrier
# (broadcast_to_inference_engines, line ~1029) still gates the next
# training phase on the fan-out completing.
if torch.distributed.get_rank() == 0:
from skyrl.backends.skyrl_train.inference_servers.remote_inference_client import (
RemoteInferenceClient,
)
Expand All @@ -1010,8 +1020,6 @@ async def _save_lora_adapters_and_sync(
lora_request = LoraLoadRequest(lora_path=lora_sync_path, lora_name=lora_name)
await inference_engine_client.update_named_weights(lora_request)

torch.distributed.barrier()

async def broadcast_to_inference_engines(
self,
inference_engine_client: "InferenceEngineInterface",
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