DDP Port Conflict
DDP port conflicts prevent distributed training from starting when the master port is already in use.
DDP port conflicts prevent distributed training from starting when the master port is already in use.
What this failure is
DDP Port Conflict is a Communication failure seen during ML training runs. DDP port conflicts prevent distributed training from starting when the master port is already in use. Common tags: Ddp, Port, Conflict, Distributed.
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Why it happens (the mechanism)
Default port 29500 already in use. MASTER_PORT collision. Multiple jobs trying to use same port. Port not released after job end. 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 training fails to start
- Address already in use error
- Distributed init hangs
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| RuntimeError: Address already in use | Default port 29500 already in use |
| torch.distributed.DistBackendError | MASTER_PORT collision |
| bind: address already in use | Multiple jobs trying to use same port |
Which systems are affected
- Multi-node DDP training
- DDP with multiple jobs on same node
- DDP debugging
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: Address already in use
- ✓Verified signal present: torch.distributed.DistBackendError
- ✓Verified signal present: bind: address already in use
- ✓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
- Default port 29500 already in use
- MASTER_PORT collision
- Multiple jobs trying to use same port
- Port not released after job end
The fix and how to prevent it
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