For distributed training and cluster infrastructure engineers

Distinguish an InfiniBand mlx5 fabric flap from an application rank stall on Lambda.

Correlate Mellanox RDMA transport retry counters, NVSwitch health telemetry, and rank-level Python tracebacks to isolate which 8x H100 node triggered the collective abort.

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Replay a resolved Lambda 8x H100 cluster incident

Upload your log file. Review the selected evidence and masking, then run diagnosis. Keep the original log for any follow-up questions.

Before uploading, review cloud data handling and local options.

training_logs.txt

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Upload your log file. Large files are processed in your browser. Only selected evidence is submitted when you run diagnosis.

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Up to 50 diagnoses a day for 30 days. Verify your work email to activate an eligible company evaluation, then replay a resolved incident and compare the evidence and next steps with your team's confirmed cause.

What a useful evaluation should establish

Verify whether the triage report isolates the specific degraded HCA port and node instead of falsely flagging healthy victim ranks waiting at the collective barrier.

Use Denpex alongside your existing telemetry tools to investigate submitted incident evidence. With monitoring configured on a supported plan, detected failures can trigger alerts and an automatic diagnosis from the captured evidence.

Inside Denpex

See the evidence, not just the claim

Follow the causal chain, inspect the quoted evidence and prepare the next handoff. These actual product captures use founder-account examples and the VS Code extension, with the source identified under each image. Open a capture to inspect it at full size.

Denpex founder-account example: a causal graph connects an Xid 48 ECC event to a rank exit and downstream collective failures, with quoted evidence for each stage. — Live founder-account capture from the bundled NCCL/ECC example. Follow the hardware event, rank exit and downstream symptoms with a supporting log quote at each stage.Inspect original image

Denpex hardware handoff summary asks the operator to capture diagnostic evidence and verify RMA eligibility with the vendor. — Actual hardware handoff summary from the same founder-account example. Capture diagnostic evidence before an approved reset, then confirm RMA eligibility with the vendor.Inspect original image

Denpex historical example says a collective timeout does not establish the initiating rank, cause or failure domain and requests distinguishing evidence before a remedy. — Founder-account example restored from history. The original timeout remains unattributed, and the UI keeps the missing-evidence assessment visible.Inspect original image

Denpex VS Code extension displaying a bundled OOM-before-NCCL example, matched log evidence, verification guidance and an investigation handoff. — Actual VS Code extension capture of the bundled OOM-before-NCCL example. Review matched log evidence, verification guidance and the investigation handoff without leaving the editor.Inspect original image

Failure Signature Triage

Lambda - Observed Runtime Evidence

Read Lambda documentation

Incident pattern summary

A 4-node 8x H100 training job on Lambda aborts mid-epoch with Watchdog collective timeout across 32 ranks. Inspecting raw logs reveals Rank 19 logged an IB Verbs transport retry counter exceeded error 45 seconds before the PyTorch watchdog fired.

Terminal traceback excerpt (Lambda)

lambda-node-03:4120:4298 [3] misc/ibvwrap.cc:284 NCCL WARN Call to ibv_poll_cq returned error 12 (Transport retry counter exceeded)
lambda-node-03:4120:4298 [3] NCCL INFO comm 0x7f8b94001090 rank 19 nranks 32 cudaDev 3 nvmlDev 3 - Abort COMPLETE
[E ProcessGroupNCCL.cpp:563] [Rank 19] Some NCCL operations have failed or timed out.

Recommended verification flags

Set these environment flags in your training container or launch script to capture high-resolution collective logs:

NCCL_DEBUG=INFONCCL_DEBUG_SUBSYS=INIT,NETNCCL_IB_TIMEOUT=22NCCL_IB_RETRY_CNT=7

Diagnostic Scope Boundary

Denpex cannot query physical Quantum-2 switch fabric counters or reseat optical transceivers on Lambda infrastructure. The evaluation isolates the suspect node and HCA port for faster Lambda support escalation.