Activation Memory Spike
Activation memory spikes during specific operations cause transient OOM even when steady-state memory fits.
Activation memory spikes during specific operations cause transient OOM even when steady-state memory fits.
What this failure is
Activation Memory Spike is a Memory failure seen during ML training runs. Activation memory spikes during specific operations cause transient OOM even when steady-state memory fits. Common tags: Activation Memory, Attention, Sequence Length, Peak Memory.
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Why it happens (the mechanism)
Attention has quadratic memory in sequence length. Specific batch has unusually long sequences. Activation memory varies with input shape. No memory headroom for peaks. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- OOM on specific batches or sequence lengths
- Memory spikes during attention or large matmul
- OOM is non-deterministic
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| OOM on batches with long sequences | Attention has quadratic memory in sequence length |
| Memory spikes correlate with specific input shapes | Specific batch has unusually long sequences |
| OOM is harder to reproduce | Activation memory varies with input shape |
Which systems are affected
- Transformer training with variable-length sequences
- Models with attention layers
- Training with dynamic batch sizes
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.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: OOM on batches with long sequences
- ✓Verified signal present: Memory spikes correlate with specific input shapes
- ✓Verified signal present: OOM is harder to reproduce
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Root cause
- Attention has quadratic memory in sequence length
- Specific batch has unusually long sequences
- Activation memory varies with input shape
- No memory headroom for peaks
The fix and how to prevent it
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