DDP Setup Error
DDP setup errors prevent distributed training from initializing correctly.
DDP setup errors prevent distributed training from initializing correctly.
What this failure is
DDP Setup Error is a Distributed Training failure seen during ML training runs. DDP setup errors prevent distributed training from initializing correctly. Common tags: Ddp, Setup, Distributed, Multi Gpu.
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Why it happens (the mechanism)
MASTER_ADDR or MASTER_PORT not set correctly. Network interfaces not available between ranks. Ranks cannot reach each other on the network. World size mismatch between command and code. 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
- torchrun fails to initialize DDP
- Training cannot start with multiple GPUs
- DDP hangs at init
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| RuntimeError: Default process group has not been initialized | MASTER_ADDR or MASTER_PORT not set correctly |
| torch.distributed.init_process_group failed | Network interfaces not available between ranks |
| Address already in use error from DDP | Ranks cannot reach each other on the network |
Which systems are affected
- New DDP setups
- First time running distributed training
- Environment configuration issues
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: Default process group has not been initialized
- ✓Verified signal present: torch.distributed.init_process_group failed
- ✓Verified signal present: Address already in use error from DDP
- ✓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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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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Root cause
- MASTER_ADDR or MASTER_PORT not set correctly
- Network interfaces not available between ranks
- Ranks cannot reach each other on the network
- World size mismatch between command and code
The fix and how to prevent it
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