NCCL CUDA Failure
NCCL CUDA failures occur when the underlying CUDA runtime has issues, causing all NCCL operations to fail.
NCCL CUDA failures occur when the underlying CUDA runtime has issues, causing all NCCL operations to fail.
What this failure is
NCCL CUDA Failure is a Communication failure seen during ML training runs. NCCL CUDA failures occur when the underlying CUDA runtime has issues, causing all NCCL operations to fail. Common tags: Nccl, Cuda, Error, Communication.
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Why it happens (the mechanism)
GPU has CUDA error. GPU memory corruption. Driver issues. CUDA toolkit incompatibility with NCCL. 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
- NCCL operations fail with CUDA error
- Training crashes with CUDA error in NCCL context
- NCCL collective returns error code
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL WARN CUDA error | GPU has CUDA error |
| RuntimeError: NCCL error in: | GPU memory corruption |
| Cuda failure in collective | Driver issues |
Which systems are affected
- Multi-node training with NCCL
- Any NCCL-based distributed training
- CUDA-aware workloads
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 CUDA error
- ✓Verified signal present: RuntimeError: NCCL error in:
- ✓Verified signal present: Cuda failure in collective
- ✓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
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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
- GPU has CUDA error
- GPU memory corruption
- Driver issues
- CUDA toolkit incompatibility with NCCL
The fix and how to prevent it
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