DDP Port Conflict
DDP port conflicts occur when the chosen master port is already in use by another process.
DDP port conflicts occur when the chosen master port is already in use by another process.
What this failure is
DDP Port Conflict is a Distributed Training failure seen during ML training runs. DDP port conflicts occur when the chosen master port is already in use by another process. Common tags: Ddp, Port, Conflict, Distributed.
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Why it happens (the mechanism)
Another process (often previous training) is using the port. Default master port 29500 is in use. Port range is exhausted. 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 with address already in use
- Different jobs conflict on the same port
- DDP hangs waiting for connection on master port
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Address already in use error in DDP | Another process (often previous training) is using the port |
| bind: address already in use | Default master port 29500 is in use |
| RuntimeError: Distributed connection setup failed | Port range is exhausted |
Which systems are affected
- Multiple training jobs on the same node
- Previous job didn't release the port
- Port range too narrow
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: Address already in use error in DDP
- ✓Verified signal present: bind: address already in use
- ✓Verified signal present: RuntimeError: Distributed connection setup failed
- ✓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
- Another process (often previous training) is using the port
- Default master port 29500 is in use
- Port range is exhausted
The fix and how to prevent it
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