Is this a hardware or a software issue?

Memory 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 memory 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 FlashAttention 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 #Kernel Out of Bounds. 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 Kernel Out of Bounds is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

Is there a fingerprint I can monitor?

Yes. Training large language models with variable sequence lengths crashes unexpectedly. 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.

FlashAttention Max Sequence Length Illegal Access

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 when using variable length sequences in flashattention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated kv-cache or attention mask bounds.. 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 when using variable length sequences in flashattention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated kv-cache or attention mask bounds. 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.

When using variable length sequences in FlashAttention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated KV-cache or attention mask bounds.

Quick answer

When using variable length sequences in FlashAttention (e.

Symptom: RuntimeError: CUDA error: an illegal memory access was encountered
Root cause: When using variable length sequences in FlashAttention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated KV-cache or attention mask bounds.
Recommended fix: Dynamically compute max sequence length max_seqlen = int((cu_seqlens[1:] - cu_seqlens[:-1]).max()) Ensures the kernel is instantiated with the correct bounds for the current batch's actual sequence length.
How Denpex helps: Denpex matches FlashAttention Max Sequence Length Illegal Access 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.

Memory#Kernel Out of Bounds

What this failure is

FlashAttention Max Sequence Length Illegal Access is a Memory failure seen during ML training runs. When using variable length sequences in FlashAttention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated KV-cache or attention mask bounds. Common tags: Kernel Out Of Bounds.

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Why it happens (the mechanism)

FlashAttention uses custom fused CUDA kernels that often bypass PyTorch's built-in bounds checking for performance. The resulting illegal memory access is silent until the context is corrupted.

What you'll observe

RuntimeError: CUDA error: an illegal memory access was encountered
flash_bwd_kernel
flash_fwd_kernel

Common symptoms and what they mean

Symptom	Why it happens
Training large language models with variable sequence lengths crashes unexpectedly.	When using variable length sequences in FlashAttention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated KV-cache or attention mask bounds.
Only crashes on specific batches with exceptionally long sequences.	When using variable length sequences in FlashAttention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated KV-cache or attention mask bounds.
Error happens deep inside the `flash_attn_func`.	When using variable length sequences in FlashAttention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated KV-cache or attention mask bounds.

Which systems are affected

FlashAttention
PyTorch
CUDA

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.

✓Print the max difference in `cu_seqlens` for every batch before passing to FlashAttention.
✓Compare actual max sequence length to the statically allocated `max_seqlen` config.

Searchable error signature

search key

RuntimeError: CUDA error: an illegal memory access was encountered

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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Root cause

When using variable length sequences in FlashAttention (e.g., passing `cu_seqlens`), if the actual sequence length in the data exceeds the `max_seqlen_k` or `max_seqlen_q` parameter passed to the kernel, the highly optimized block-wise memory loader reads outside the allocated KV-cache or attention mask bounds.

The fix and how to prevent it

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References

↗FlashAttention CUDA illegal memory access with variable seqlen

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