Is this a hardware or a software issue?

Model 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 model 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 FSDP 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 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 NCCL 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 specifically during the backward pass or optimizer step when using Fully Sharded Data Parallel (FSDP). 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.

FSDP Mismatched Tensor Shapes Triggering NCCL Hang

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 in fsdp, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the allgather operation for gradients or model parameters. nccl expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count.. 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 in fsdp, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the allgather operation for gradients or model parameters. nccl expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count. 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.

In FSDP, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the AllGather operation for gradients or model parameters. NCCL expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count.

Quick answer

In FSDP, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the AllGather operation for gradients or model parameters.

Symptom: RuntimeError: NCCL error in: /pytorch/torch/lib/c10d/ProcessGroupNCCL.cpp
Root cause: In FSDP, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the AllGather operation for gradients or model parameters. NCCL expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count.
Recommended fix: Pad inputs to uniform lengths or use Join context from torch.distributed.algorithms.join import Join Ensuring all ranks process the exact same tensor shapes prevents NCCL from hanging due to size mismatches.
How Denpex helps: Denpex investigates FSDP Mismatched Tensor Shapes Triggering NCCL Hang 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.

Model#NCCL Timeout

What this failure is

FSDP Mismatched Tensor Shapes Triggering NCCL Hang is a Model failure seen during ML training runs. In FSDP, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the AllGather operation for gradients or model parameters. NCCL expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count. Common tags: NCCL Timeout.

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Why it happens (the mechanism)

The timeout happens deep in NCCL during an implicitly triggered FSDP collective, not an explicit user call. The user is unaware an AllGather was even happening.

What you'll observe

Watchdog caught collective operation timeout: WorkNCCL(SeqNum=..., OpType=ALLGATHER
RuntimeError: NCCL error in: /pytorch/torch/lib/c10d/ProcessGroupNCCL.cpp

Common symptoms and what they mean

Symptom	Why it happens
Training hangs specifically during the backward pass or optimizer step when using Fully Sharded Data Parallel (FSDP).	In FSDP, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the AllGather operation for gradients or model parameters. NCCL expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count.
The hang occurs reliably at the same point (e.g., first iteration of an epoch where variable sequence length is introduced).	In FSDP, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the AllGather operation for gradients or model parameters. NCCL expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count.

Which systems are affected

PyTorch FSDP
NCCL

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.

✓Log batch sizes and input shapes on each rank right before the forward pass.
✓Enable `TORCH_DISTRIBUTED_DEBUG=DETAIL` to catch shape mismatches before they hit NCCL.

Root cause

In FSDP, dynamic input shapes or uneven batch sizes across ranks can lead to mismatched tensor sizes during the AllGather operation for gradients or model parameters. NCCL expects the collective operation to use exact matching sizes across all ranks; otherwise, it hangs waiting for the expected byte count.

The fix and how to prevent it

Searchable error signature

search key

RuntimeError: NCCL error in: /pytorch/torch/lib/c10d/ProcessGroupNCCL.cpp
Watchdog caught collective operation timeout: WorkNCCL(SeqNum=..., OpType=ALLGATHER

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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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

References

↗PyTorch FSDP documentation on uneven inputs

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