NCCL Broadcast Hang
NCCL broadcast operations can hang when one rank fails to participate in the collective.
NCCL broadcast operations can hang when one rank fails to participate in the collective.
What this failure is
NCCL Broadcast Hang is a Distributed Training failure seen during ML training runs. NCCL broadcast operations can hang when one rank fails to participate in the collective. Common tags: Nccl, Broadcast, Hang, Distributed.
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Why it happens (the mechanism)
One rank stuck or slow. Network partition between nodes. Rank not initialized for broadcast. Broadcast source rank has issue. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- Training hangs at broadcast operation
- One rank doesn't respond to broadcast
- Distributed init fails at broadcast step
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL WARN broadcast timeout | One rank stuck or slow |
| NCCL hangs at all ranks except one | Network partition between nodes |
| torch.distributed.broadcast() hangs forever | Rank not initialized for broadcast |
Which systems are affected
- Distributed training initialization
- DDP setup with custom broadcast
- Multi-node training with broadcast operations
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: NCCL WARN broadcast timeout
- ✓Verified signal present: NCCL hangs at all ranks except one
- ✓Verified signal present: torch.distributed.broadcast() hangs forever
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
The root cause is on this page and stays free. A free account adds the exact remediation steps, saved history, and the fix on every entry in the encyclopedia.
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Diagnose this failure in VS Code
Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.
Install the free VS Code extensionNCCL errors in context
NCCL is where a distributed job reports failure, which is not the same as where it failed. The hub lists every common NCCL error next to what it actually indicates, and the environment variables that tell them apart.
Compare every nccl error side by sideRelated failures to investigate next
Root cause
- One rank stuck or slow
- Network partition between nodes
- Rank not initialized for broadcast
- Broadcast source rank has issue
The fix and how to prevent it
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Don't just read the fix, diagnose your run
The encyclopedia tells you what went wrong. Denpex tells you what went wrong in YOUR training run. With your logs, your config, and your stack.