NCCL Timeout Cascade from Single Rank Failure
A single rank failure cascades into mass NCCL timeouts across most or all ranks as they wait at a collective barrier. The cascade signature is one rank failing first, followed by a wave of timeouts on all other ranks.
A single rank failure cascades into mass NCCL timeouts across most or all ranks as they wait at a collective barrier.
What this failure is
NCCL Timeout Cascade from Single Rank Failure is a Distributed Communication failure seen during ML training runs. A single rank failure cascades into mass NCCL timeouts across most or all ranks as they wait at a collective barrier. The cascade signature is one rank failing first, followed by a wave of timeouts on all other ranks. Common tags: Nccl, Cascade, Timeout, Distributed.
Is this what broke your run? Paste your log.
You're reading about NCCL Timeout Cascade from Single Rank Failure. Paste your own crash log or traceback below and get the real root cause for YOUR run, not this generic entry. No account, no card. Logs are masked at ingress and never saved to account history.
Want 14 days on the Scale plan?
Request an evaluation code. A verified workplace organization activates up to 50 diagnoses a day, alerts, history, and follow-up questions. No credit card or automatic subscription.
Why it happens (the mechanism)
A single root cause rank encountered a genuine failure (GPU OOM, Xid 31/79, host OOM killer, network link down). All other ranks are healthy. They only timed out because they were waiting for the failed rank at the collective barrier. The cascade is not a network issue. It is a mathematical certainty: all ranks must participate in every collective. 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
- Single GPU/rank fails (OOM, Xid, host crash)
- All other ranks wait at the next collective barrier for the failed rank
- NCCL watchdog timeout fires on every waiting rank simultaneously
- Logs show NCCL timeout detected on X/Y ranks where Y is most of the cluster
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL timeout detected on >50% of ranks in the job | A single root cause rank encountered a genuine failure (GPU OOM, Xid 31/79, host OOM killer, network link down) |
| One rank consistently has the earliest error timestamp | All other ranks are healthy. They only timed out because they were waiting for the failed rank at the collective barrier |
| All downstream errors are identical NCCL timeout messages | The cascade is not a network issue. It is a mathematical certainty: all ranks must participate in every collective |
| No independent errors on the downstream ranks. They only timed out waiting | A single root cause rank encountered a genuine failure (GPU OOM, Xid 31/79, host OOM killer, network link down) |
Which systems are affected
- Large-scale distributed training (64+ GPUs)
- Any NCCL-based training where a single rank failure blocks all collectives
- Jobs without elastic training or automatic rank replacement
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 timeout detected on >50% of ranks in the job
- ✓Verified signal present: One rank consistently has the earliest error timestamp
- ✓Verified signal present: All downstream errors are identical NCCL timeout messages
- ✓Verified signal present: No independent errors on the downstream ranks. They only timed out waiting
- ✓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.
Sign up free. Unlock the full analysisNo credit card. Daily allowance follows verified trust tier. Instant access.
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
- A single root cause rank encountered a genuine failure (GPU OOM, Xid 31/79, host OOM killer, network link down)
- All other ranks are healthy. They only timed out because they were waiting for the failed rank at the collective barrier
- The cascade is not a network issue. It is a mathematical certainty: all ranks must participate in every collective
The fix and how to prevent it
Evaluate Denpex on your own logs
Request a Scale evaluation code. A verified workplace organization activates 14 days with up to 50 diagnoses a day. Every account keeps its current diagnosis allowance and gets a verification path. No card or automatic subscription.
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.