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 a brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable infiniband (rc) connection between nodes. nccl encounters an unrecoverable rdma error and raises it, triggering the pytorch watchdog to cleanly tear down the job.. Validating against the symptom list in this entry is the fastest way to confirm the diagnosis.

Is this a hardware or a software issue?

Hardware 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 hardware 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 PyTorch 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 #Network Partition. 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 Network Partition is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

Is there a fingerprint I can monitor?

Yes. Training fails suddenly mid-epoch with a watchdog caught unhandled error. 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.

InfiniBand Link Flap Triggering NCCL Async Handshake Failure

Q: 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 a brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable infiniband (rc) connection between nodes. nccl encounters an unrecoverable rdma error and raises it, triggering the pytorch watchdog to cleanly tear down the job. 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.

A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes. NCCL encounters an unrecoverable RDMA error and raises it, triggering the PyTorch watchdog to cleanly tear down the job.

Quick answer

A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes.

Symptom: NCCL WARN Call to ibv_poll_cq failed
Root cause: A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes. NCCL encounters an unrecoverable RDMA error and raises it, triggering the PyTorch watchdog to cleanly tear down the job.
Recommended fix: Identify and replace failing IB hardware iblinkinfo Replacing faulty cables or optical transceivers resolves the physical layer instability causing the RDMA disconnects.
How Denpex helps: Denpex matches InfiniBand Link Flap Triggering NCCL Async Handshake Failure across every rank in a distributed run and reports which rank failed first, so you act on the initiating node instead of the loudest one.

Hardware#Network Partition

What this failure is

InfiniBand Link Flap Triggering NCCL Async Handshake Failure is a Hardware failure seen during ML training runs. A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes. NCCL encounters an unrecoverable RDMA error and raises it, triggering the PyTorch watchdog to cleanly tear down the job. Common tags: Network Partition.

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Why it happens (the mechanism)

The initial error caught by users is the PyTorch watchdog error, which masks the underlying hardware InfiniBand transport failure in the system logs.

What you'll observe

Watchdog caught unhandled error
NCCL WARN Call to ibv_poll_cq failed
ib_cq_poll failed: transport retry counter exceeded

Common symptoms and what they mean

Symptom	Why it happens
Training fails suddenly mid-epoch with a watchdog caught unhandled error.	A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes. NCCL encounters an unrecoverable RDMA error and raises it, triggering the PyTorch watchdog to cleanly tear down the job.
Dmesg on one of the nodes shows InfiniBand link state changing to DOWN then UP.	A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes. NCCL encounters an unrecoverable RDMA error and raises it, triggering the PyTorch watchdog to cleanly tear down the job.
NCCL logs indicate polling failure on completion queue (CQ).	A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes. NCCL encounters an unrecoverable RDMA error and raises it, triggering the PyTorch watchdog to cleanly tear down the job.

Which systems are affected

PyTorch
NCCL
InfiniBand

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.

✓Run `dmesg -T | grep -i ib` on the failing nodes.
✓Check InfiniBand counters: `ibstat` or `ibv_devinfo`.
✓Check switch logs for port flapping.

Searchable error signature

search key

NCCL WARN Call to ibv_poll_cq failed
Watchdog caught unhandled error
Dmesg on one of the nodes shows InfiniBand link state changing to DOWN then UP.

Use this text as a lookup key in logs and upstream issue trackers. It is not presented as a captured customer log. Confirm the cause from your own preceding events, versions, configuration and the cited references.

The fix and the prevention pattern

The root cause is on this page and stays free. A free account adds the exact remediation steps, saved history, and the fix on every entry in the encyclopedia.

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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

Root cause

A brief hardware network event (link flap, switch reset, optical transceiver issue) breaks the reliable InfiniBand (RC) connection between nodes. NCCL encounters an unrecoverable RDMA error and raises it, triggering the PyTorch watchdog to cleanly tear down the job.

The fix and how to prevent it

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References

↗NVIDIA Forums: ib_cq_poll failed: transport retry counter exceeded

Don't just read the fix, diagnose your run

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