NCCL Random Segfault from Out-of-Order NIC Names
Inconsistent NIC enumeration (e.g. mlx5_3 before mlx5_0) on one node breaks NCCL topology detection and causes random multi-node segfaults; fixed in NCCL 2.23.4.
Inconsistent NIC enumeration (e.
What this failure is
NCCL Random Segfault from Out-of-Order NIC Names is a Communication failure seen during ML training runs. Inconsistent NIC enumeration (e.g. mlx5_3 before mlx5_0) on one node breaks NCCL topology detection and causes random multi-node segfaults; fixed in NCCL 2.23.4. Common tags: Nccl, Infiniband, Nic Ordering, Segfault.
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Why it happens (the mechanism)
One node enumerates NICs out of order. NCCL topology detection assumes ordered NIC names. Out-of-bounds access during NIC setup on the misordered node. 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
- Random segfaults during nccl-tests across 4+ H100 nodes
- Inconsistent. Sometimes works, sometimes crashes
- Single-node runs are fine
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Random segfault during multi-node nccl-tests | One node enumerates NICs out of order |
| ibstat shows out-of-order NIC names (mlx5_3 before mlx5_0) | NCCL topology detection assumes ordered NIC names |
| Out-of-bounds access during NIC setup | Out-of-bounds access during NIC setup on the misordered node |
Which systems are affected
- NCCL 2.19+
- H100 nodes with ConnectX-6 HCAs
- Multi-node only
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: Random segfault during multi-node nccl-tests
- ✓Verified signal present: ibstat shows out-of-order NIC names (mlx5_3 before mlx5_0)
- ✓Verified signal present: Out-of-bounds access during NIC setup
- ✓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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Root cause
- One node enumerates NICs out of order
- NCCL topology detection assumes ordered NIC names
- Out-of-bounds access during NIC setup on the misordered node
The fix and how to prevent it
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