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 the attention scores are calculated as `q @ k.t`. before scaling by `1/sqrt(d_k)`, these dot products can be very large. if calculated purely in fp16, the dot product can exceed 65,504, causing an overflow to `inf`. softmax on `inf` values produces `nan`.. Validating against the symptom list in this entry is the fastest way to confirm the diagnosis.

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 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 #Numerical Instability. 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 Numerical Instability is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

Is there a fingerprint I can monitor?

Yes. During early training steps, the model outputs completely NaN logits. 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.

Attention Softmax FP16 Overflow

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 the attention scores are calculated as `q @ k.t`. before scaling by `1/sqrt(d_k)`, these dot products can be very large. if calculated purely in fp16, the dot product can exceed 65,504, causing an overflow to `inf`. softmax on `inf` values produces `nan`. 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.

The attention scores are calculated as `Q @ K.T`. Before scaling by `1/sqrt(d_k)`, these dot products can be very large. If calculated purely in FP16, the dot product can exceed 65,504, causing an overflow to `inf`. Softmax on `inf` values produces `NaN`.

Quick answer

The attention scores are calculated as `Q @ K.

Root cause: The attention scores are calculated as `Q @ K.T`. Before scaling by `1/sqrt(d_k)`, these dot products can be very large.
Recommended fix: Use Scaled Dot Product Attention attn_output = torch.nn.functional.scaled_dot_product_attention(q, k, v) PyTorch's SDPA implementation natively handles the scaling efficiently and uses precision safeguards (like casting to FP32 or using FlashAttention) to prevent overflow.
How Denpex helps: Denpex investigates Attention Softmax FP16 Overflow 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#Numerical Instability

What this failure is

Attention Softmax FP16 Overflow is a Model failure seen during ML training runs. The attention scores are calculated as `Q @ K.T`. Before scaling by `1/sqrt(d_k)`, these dot products can be very large. If calculated purely in FP16, the dot product can exceed 65,504, causing an overflow to `inf`. Softmax on `inf` values produces `NaN`. Common tags: Numerical Instability.

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Why it happens (the mechanism)

Because the `1/sqrt(d_k)` scaling happens immediately after the matmul in code, developers assume the values are safely scaled down. However, the overflow happens inside the matmul itself before the division can rescue it.

What you'll observe

NaNs propagating from Attention blocks
Attention probabilities becoming completely zero or uniform NaNs

Common symptoms and what they mean

Symptom	Why it happens
During early training steps, the model outputs completely NaN logits.	The attention scores are calculated as `Q @ K.T`. Before scaling by `1/sqrt(d_k)`, these dot products can be very large. If calculated purely in FP16, the dot product can exceed 65,504, causing an overflow to `inf`. Softmax on `inf` values produces `NaN`.
Occurs predominantly when the sequence length or embedding dimension is very large.	The attention scores are calculated as `Q @ K.T`. Before scaling by `1/sqrt(d_k)`, these dot products can be very large. If calculated purely in FP16, the dot product can exceed 65,504, causing an overflow to `inf`. Softmax on `inf` values produces `NaN`.

Which systems are affected

PyTorch
Transformers
Mixed Precision

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.

✓Isolate the attention block and pass a random tensor.
✓Check `torch.isinf(attn_weights).any()` right after the `Q @ K.T` step.

Root cause

The attention scores are calculated as `Q @ K.T`. Before scaling by `1/sqrt(d_k)`, these dot products can be very large. If calculated purely in FP16, the dot product can exceed 65,504, causing an overflow to `inf`. Softmax on `inf` values produces `NaN`.

The fix and how to prevent it

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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References

↗NaNs in manual attention due to FP16

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