DDP Setup Error
DDP setup errors occur when distributed data parallel training is not properly configured, preventing multi-GPU training.
DDP setup errors occur when distributed data parallel training is not properly configured, preventing multi-GPU training.
What this failure is
DDP Setup Error is a Communication failure seen during ML training runs. DDP setup errors occur when distributed data parallel training is not properly configured, preventing multi-GPU training. Common tags: Ddp, Setup, Distributed, Torchrun.
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Why it happens (the mechanism)
Init_process_group not called. Backend not specified. MASTER_ADDR/MASTER_PORT not set. Rank and world_size not set. Torchrun not used correctly. 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
- DDP fails to start
- torch.distributed.init_process_group fails
- DDP hangs at first all-reduce
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| RuntimeError: Distributed package doesn't have NCCL built | init_process_group not called |
| Default process group is not initialized | backend not specified |
| Address already in use | MASTER_ADDR/MASTER_PORT not set |
| Timeout reached | rank and world_size not set |
Which systems are affected
- Multi-GPU training with DDP
- First-time DDP setup
- Migrating from DP to DDP
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: RuntimeError: Distributed package doesn't have NCCL built
- ✓Verified signal present: Default process group is not initialized
- ✓Verified signal present: Address already in use
- ✓Verified signal present: Timeout reached
- ✓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
- init_process_group not called
- backend not specified
- MASTER_ADDR/MASTER_PORT not set
- rank and world_size not set
- torchrun not used correctly
The fix and how to prevent it
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