What does this failure look like in production?

In production this failure typically appears during the first tens-to-hundreds of training steps: loss curves become non-monotonic, validation metrics plateau above the baseline, and downstream checkpoints either diverge or fail to improve. Engineers frequently misread it as data drift when the underlying mechanism is nic selection algorithm mishandles an odd nic count (3). Validating against the symptom list in this entry is the fastest way to confirm the diagnosis.

Is this a hardware or a software issue?

Communication failures can look like either layer depending on the symptom. The discriminator is the resolution of the symptom to the recommended fix: if applying the steps under "How to fix it" eliminates the symptom, this is a software/configuration class issue regardless of the hardware it surfaced on. Repeat the fix on a fresh node to confirm.

How long does the fix typically take?

Most resolutions from this entry take 10 to 30 minutes of engineer time once the diagnosis is confirmed. Long fixes come from skipping the diagnosis step and chasing apparent symptoms (validation drift, memory growth, slow throughput) instead of the actual root cause listed above. Apply the fix in a small experiment first, then roll it forward.

Does this affect inference or only training?

Primarily this is a communication failure observed during training, but the same misconfiguration can leak into downstream evaluation and serving. Treat the prevention checklist as a guardrail: any pipeline that touches a trained checkpoint should re-validate the failure mode before promoting the model.

What's the cheapest way to confirm this is the problem?

Cheapest signal: reproduce the symptom on a 1-GPU toy run with the same configuration. If the toy run exhibits the same behavior, the cause is the configuration rather than the cluster. If only the multi-GPU run is affected, the failure is a distributed interaction, so Nodes with exactly 3 InfiniBand HCAs is the most likely layer to inspect.

Can the Denpex platform detect this automatically?

Yes. Denpex correlates the symptoms in this entry against your training logs in real time and surfaces the failure taxonomy, root cause, and exact remediation command on the console. Three full diagnoses are included free during signup, and the encyclopedia entry you are reading is generated from the same signal library that powers the platform.

What related failures should I check next?

Closely related failures cluster around #nccl, #infiniband, #odd-nic. After applying this fix, validate against the related entries below to confirm no overlapping cause is masking as this one. Engineers routinely discover that the dominant failure mode is actually a second, sibling failure compounded on top.

Will this recur after the fix?

Applied once with the prevention checklist, this failure does not recur under normal operation. The recurring variant most engineers encounter is the re-emergence of the same symptom on a different model or stack after nccl is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

How does this scale from 1 GPU to 1000 GPUs?

At single-GPU scale the symptom usually looks like slow convergence or a single anomalous loss value. At 64+ GPU scale, distributed interactions amplify nic selection algorithm mishandles an odd nic count (3) through collectives, and the failure can manifest as a full job abort. Always reproduce the fix at the smallest cluster configuration that still exhibits the symptom before promoting to production scale.

Is there a fingerprint I can monitor?

Yes. NCCL hang during bootstrap with 3 HCAs detected is the highest-signal fingerprint. Wire a metric or alert on that signal (or its absence after the fix) so you can detect the recurrence automatically. The Denpex platform encodes this exact fingerprint and surfaces it directly in the dashboard.

What is the official documentation that covers this?

Reference documentation is listed in the References section at the bottom of this entry. The most authoritative URLs are the upstream framework docs and the issue trackers cited; the Denpex entry synthesizes both into the executive, engineer, and on-call perspectives so each audience can act without reading every linked source.

Does this entry include working code?

Yes. The "Code Examples" section contains copy-paste-ready snippets that target the most common stacks (PyTorch, Hugging Face Trainer, DeepSpeed, Megatron-LM, vLLM/Transformers). Each snippet is annotated with why it works so you can adapt the pattern to your own framework without losing fidelity.

NCCL Hangs with Exactly Three InfiniBand NICs

NCCL's NIC selection/path-finding can deadlock when a node has an odd number of HCAs (specifically three), while 1, 2, or 4+ work fine.

Quick answer

NCCL's NIC selection/path-finding can deadlock when a node has an odd number of HCAs (specifically three), while 1, 2, or 4+ work fine.

Root cause: NIC selection algorithm mishandles an odd NIC count (3). Ring/tree topology path-finding loops or deadlocks. Bootstrap never completes.
Recommended fix: NCCL_IB_HCA=mlx5_0
How Denpex helps: Denpex investigates NCCL Hangs with Exactly Three InfiniBand NICs using the evidence you provide or your connected workload collects. Earlier rank, host or application evidence is needed to distinguish an initiating failure from a downstream report.

Communication#nccl#infiniband#odd-nic#hang#topology

What this failure is

NCCL Hangs with Exactly Three InfiniBand NICs is a Communication failure seen during ML training runs. NCCL's NIC selection/path-finding can deadlock when a node has an odd number of HCAs (specifically three), while 1, 2, or 4+ work fine. Common tags: Nccl, Infiniband, Odd Nic, Hang.

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Why it happens (the mechanism)

NIC selection algorithm mishandles an odd NIC count (3). Ring/tree topology path-finding loops or deadlocks. Bootstrap never completes. 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 hangs at init with exactly 3 IB HCAs
1, 2, or 4+ NICs work
Path-finding gets stuck

Common symptoms and what they mean

Symptom	Why it happens
NCCL hang during bootstrap with 3 HCAs detected	NIC selection algorithm mishandles an odd NIC count (3)
Ring/tree topology computation stalls	Ring/tree topology path-finding loops or deadlocks
No progress, eventual watchdog timeout	Bootstrap never completes

Which systems are affected

Nodes with exactly 3 InfiniBand HCAs
Older NCCL with odd-NIC handling bug
Multi-node IB

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 hang during bootstrap with 3 HCAs detected
✓Verified signal present: Ring/tree topology computation stalls
✓Verified signal present: No progress, eventual watchdog timeout
✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.

Root cause

NIC selection algorithm mishandles an odd NIC count (3)
Ring/tree topology path-finding loops or deadlocks
Bootstrap never completes

The fix and how to prevent it

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.

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NCCL 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 side