NCCL IB HCA Mismatch
NCCL InfiniBand HCA (Host Channel Adapter) configuration mismatches cause slow or failed inter-node communication.
NCCL InfiniBand HCA (Host Channel Adapter) configuration mismatches cause slow or failed inter-node communication.
What this failure is
NCCL IB HCA Mismatch is a Communication failure seen during ML training runs. NCCL InfiniBand HCA (Host Channel Adapter) configuration mismatches cause slow or failed inter-node communication. Common tags: Nccl, Infiniband, Hca, Communication.
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Why it happens (the mechanism)
Wrong IB device selected by NCCL. IB subnet manager not configured. RoCE vs IB mode mismatch. Multiple IB HCAs with different speeds. 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 allreduce is slow across nodes
- NCCL uses wrong IB device
- Inter-node bandwidth is low
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL WARN NET/IB: no usable sockets | Wrong IB device selected by NCCL |
| NCCL falls back to TCP/IP | IB subnet manager not configured |
| Inter-node bandwidth much lower than intra-node | RoCE vs IB mode mismatch |
Which systems are affected
- Multi-node training with InfiniBand
- Training on GPU clusters with multiple IB adapters
- High-performance distributed training
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 NET/IB: no usable sockets
- ✓Verified signal present: NCCL falls back to TCP/IP
- ✓Verified signal present: Inter-node bandwidth much lower than intra-node
- ✓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
- Wrong IB device selected by NCCL
- IB subnet manager not configured
- RoCE vs IB mode mismatch
- Multiple IB HCAs with different speeds
The fix and how to prevent it
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Don't just read the fix, diagnose your run
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