DDP OOM During AllReduce
DDP OOM during allreduce occurs when NCCL communication buffers exceed available GPU memory.
DDP OOM during allreduce occurs when NCCL communication buffers exceed available GPU memory.
What this failure is
DDP OOM During AllReduce is a Distributed Training failure seen during ML training runs. DDP OOM during allreduce occurs when NCCL communication buffers exceed available GPU memory. Common tags: Ddp, Oom, Allreduce, Memory.
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Why it happens (the mechanism)
NCCL_BUFFSIZE too large for available memory. Gradient bucket size exceeds memory. NCCL communication buffers consume significant memory. 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 error during gradient synchronization
- DDP fails with CUDA OOM during backward pass
- Memory usage spikes during allreduce
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| OOM during NCCL allreduce | NCCL_BUFFSIZE too large for available memory |
| DDP gradient bucket exceeds memory | Gradient bucket size exceeds memory |
| NCCL buffer allocation fails | NCCL communication buffers consume significant memory |
Which systems are affected
- DDP training with large gradient buckets
- Models with many parameters
- Limited GPU memory scenarios
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 during NCCL allreduce
- ✓Verified signal present: DDP gradient bucket exceeds memory
- ✓Verified signal present: NCCL buffer allocation fails
- ✓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
- NCCL_BUFFSIZE too large for available memory
- Gradient bucket size exceeds memory
- NCCL communication buffers consume significant memory
The fix and how to prevent it
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