FSDP Mixed Precision Error
FSDP mixed precision errors arise when parameter precision settings conflict between FSDP wrapping and autocast.
FSDP mixed precision errors arise when parameter precision settings conflict between FSDP wrapping and autocast.
What this failure is
FSDP Mixed Precision Error is a Distributed Training failure seen during ML training runs. FSDP mixed precision errors arise when parameter precision settings conflict between FSDP wrapping and autocast. Common tags: Fsdp, Mixed Precision, Amp, Bf16.
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Why it happens (the mechanism)
FSDP re-shards parameters at different precision than forward pass. Autocast context interacts incorrectly with FSDP parameter access. Gradient accumulation with FSDP causes precision drift. 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
- Loss becomes NaN after FSDP wrapping
- Mixed precision training diverges with FSDP
- FSDP and AMP interaction produces numerical issues
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Loss NaN with FSDP + AMP but not with either alone | FSDP re-shards parameters at different precision than forward pass |
| FSDP mixed precision config mismatch between layers | Autocast context interacts incorrectly with FSDP parameter access |
| Gradient scaler overflow with FSDP enabled | Gradient accumulation with FSDP causes precision drift |
Which systems are affected
- FSDP with torch.cuda.amp
- FSDP with bf16 mode
- Models with both FP32 and FP16 operations
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: Loss NaN with FSDP + AMP but not with either alone
- ✓Verified signal present: FSDP mixed precision config mismatch between layers
- ✓Verified signal present: Gradient scaler overflow with FSDP enabled
- ✓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
- FSDP re-shards parameters at different precision than forward pass
- Autocast context interacts incorrectly with FSDP parameter access
- Gradient accumulation with FSDP causes precision drift
The fix and how to prevent it
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