NCCL 'Could not find NET with id 0' (NIC Fusion)
Intermittent NCCL internal errors during init on partial-node allocations, caused by the NIC-fusion feature remapping NET IDs so the hardcoded fallback NET/0 no longer exists.
Intermittent NCCL internal errors during init on partial-node allocations, caused by the NIC-fusion feature remapping NET IDs so the hardcoded fallback NET/0 no longer exists.
What this failure is
NCCL 'Could not find NET with id 0' (NIC Fusion) is a Communication failure seen during ML training runs. Intermittent NCCL internal errors during init on partial-node allocations, caused by the NIC-fusion feature remapping NET IDs so the hardcoded fallback NET/0 no longer exists. Common tags: Nccl, Nic Fusion, Net Id, Kubernetes.
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Why it happens (the mechanism)
NIC fusion (2.26.2) renames/remaps NET IDs. When graph path-finding fails it falls back to hardcoded NET id 0. But fusion may have removed NET/0, so the lookup fails. 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
- Intermittent (~25%) failures at torch.distributed.barrier()/init
- Fails on partial-node Kubernetes GPU allocations
- Started on NCCL 2.26.2
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| ncclInternalError: Internal check failed. Last error: Could not find NET with id 0 | NIC fusion (2.26.2) renames/remaps NET IDs |
| Could not find a path for pattern 1, falling back to simple order | When graph path-finding fails it falls back to hardcoded NET id 0 |
| Intermittent init failures on partial-node jobs | But fusion may have removed NET/0, so the lookup fails |
Which systems are affected
- NCCL 2.26.2 with NIC fusion
- Kubernetes partial-node GPU allocations
- HGX A100 / InfiniBand nodes
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: ncclInternalError: Internal check failed. Last error: Could not find NET with id 0
- ✓Verified signal present: Could not find a path for pattern 1, falling back to simple order
- ✓Verified signal present: Intermittent init failures on partial-node jobs
- ✓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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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
- NIC fusion (2.26.2) renames/remaps NET IDs
- When graph path-finding fails it falls back to hardcoded NET id 0
- But fusion may have removed NET/0, so the lookup fails
The fix and how to prevent it
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