Is this a hardware or a software issue?

Network 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 network 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 NCCL 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 #RoCEv2 Communication Timeout. 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 RoCEv2 Communication Timeout is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

Is there a fingerprint I can monitor?

Yes. Training hangs completely during distributed collective operations (e.g., all_reduce). 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 Vendor Err 129 (RDMA Timeout)

Q: 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 vendor error 129 represents an infiniband transport retry counter exceeded error. in a rocev2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. rocev2 requires a lossless fabric (via priority flow control / pfc) or properly configured congestion control (dcqcn). when packets drop, the nic retries sending until the retry limit is hit, yielding error 129.. Validating against the symptom list in this entry is the fastest way to confirm the diagnosis.

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 vendor error 129 represents an infiniband transport retry counter exceeded error. in a rocev2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. rocev2 requires a lossless fabric (via priority flow control / pfc) or properly configured congestion control (dcqcn). when packets drop, the nic retries sending until the retry limit is hit, yielding error 129. 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.

Vendor Error 129 represents an InfiniBand transport retry counter exceeded error. In a RoCEv2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. RoCEv2 requires a lossless fabric (via Priority Flow Control / PFC) or properly configured congestion control (DCQCN). When packets drop, the NIC retries sending until the retry limit is hit, yielding error 129.

Quick answer

Vendor Error 129 represents an InfiniBand transport retry counter exceeded error.

Symptom: NCCL WARN Call to ibv_poll_cq() failed with error 12
Root cause: Vendor Error 129 represents an InfiniBand transport retry counter exceeded error. In a RoCEv2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. RoCEv2 requires a lossless fabric (via Priority Flow Control / PFC) or properly configured congestion control (DCQCN).
Recommended fix: NCCL_IB_TC=128
How Denpex helps: Denpex matches NCCL Vendor Err 129 (RDMA Timeout) 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.

Network#RoCEv2 Communication Timeout

What this failure is

NCCL Vendor Err 129 (RDMA Timeout) is a Network failure seen during ML training runs. Vendor Error 129 represents an InfiniBand transport retry counter exceeded error. In a RoCEv2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. RoCEv2 requires a lossless fabric (via Priority Flow Control / PFC) or properly configured congestion control (DCQCN). When packets drop, the NIC retries sending until the retry limit is hit, yielding error 129. Common tags: RoCEv2 Communication Timeout.

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Why it happens (the mechanism)

The error looks like a hardware failure (vendor err) or a simple timeout, leading engineers to suspect a faulty NIC or cable. In reality, the physical hardware is fine, but the network switch configuration for QoS/PFC is misaligned with the NIC's traffic class.

What you'll observe

NCCL WARN Call to ibv_poll_cq() failed with error 12
NCCL WARN NET/IB : Got completion with error 12, opcode 0, len 0, vendor err 129
Watchdog caught collective operation timeout

Common symptoms and what they mean

Symptom	Why it happens
Training hangs completely during distributed collective operations (e.g., all_reduce).	Vendor Error 129 represents an InfiniBand transport retry counter exceeded error. In a RoCEv2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. RoCEv2 requires a lossless fabric (via Priority Flow Control / PFC) or properly configured congestion control (DCQCN). When packets drop, the NIC retries sending until the retry limit is hit, yielding error 129.
Eventually times out and crashes the training job.	Vendor Error 129 represents an InfiniBand transport retry counter exceeded error. In a RoCEv2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. RoCEv2 requires a lossless fabric (via Priority Flow Control / PFC) or properly configured congestion control (DCQCN). When packets drop, the NIC retries sending until the retry limit is hit, yielding error 129.
Occurs intermittently under heavy network load, rather than at startup.	Vendor Error 129 represents an InfiniBand transport retry counter exceeded error. In a RoCEv2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. RoCEv2 requires a lossless fabric (via Priority Flow Control / PFC) or properly configured congestion control (DCQCN). When packets drop, the NIC retries sending until the retry limit is hit, yielding error 129.

Which systems are affected

NCCL
PyTorch
Mellanox ConnectX
RoCEv2

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.

✓Check NIC error counters: `ethtool -S <interface> | grep rx_dropped`
✓Run `NCCL_DEBUG=INFO NCCL_DEBUG_SUBSYS=NET` to confirm the error originates in the IB transport layer.
✓Verify switch PFC and ECN (Explicit Congestion Notification) settings.

Searchable error signature

search key

NCCL WARN Call to ibv_poll_cq() failed with error 12
NCCL WARN NET/IB : Got completion with error 12, opcode 0, len 0, vendor err 129
Watchdog caught collective operation timeout

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

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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

Vendor Error 129 represents an InfiniBand transport retry counter exceeded error. In a RoCEv2 environment, this typically happens because the network is lossy and packets are being dropped by the switch due to congestion. RoCEv2 requires a lossless fabric (via Priority Flow Control / PFC) or properly configured congestion control (DCQCN). When packets drop, the NIC retries sending until the retry limit is hit, yielding error 129.

The fix and how to prevent it

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References

↗NCCL Vendor Err 129 Troubleshooting

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