NCCL socketStartConnect: Software caused connection abort
A stale socket file descriptor is reused on retry after a failed connect, triggering ECONNABORTED during NCCL communicator init; fixed in NCCL 2.24.
A stale socket file descriptor is reused on retry after a failed connect, triggering ECONNABORTED during NCCL communicator init.
What this failure is
NCCL socketStartConnect: Software caused connection abort is a Communication failure seen during ML training runs. A stale socket file descriptor is reused on retry after a failed connect, triggering ECONNABORTED during NCCL communicator init; fixed in NCCL 2.24. Common tags: Nccl, Socket, Connection Abort, Retry.
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Why it happens (the mechanism)
First connect attempt fails (ECONNREFUSED/ETIMEDOUT), leaving the socket half-closed. NCCL retries on the same stale fd. Retry on the half-closed fd triggers ECONNABORTED. 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
- Intermittent failures during NCCL init
- Connection abort on retry
- Timing-dependent (depends on first failure)
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Nccl socketStartConnect: Connect to x.x.x.x failed : Software caused connection abort | First connect attempt fails (ECONNREFUSED/ETIMEDOUT), leaving the socket half-closed |
| Intermittent during communicator init | NCCL retries on the same stale fd |
| Only on retry after an initial failure | Retry on the half-closed fd triggers ECONNABORTED |
Which systems are affected
- NCCL multi-node TCP/IP connections
- Networks with transient ECONNREFUSED/ETIMEDOUT
- Pre-2.24 NCCL
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: Nccl socketStartConnect: Connect to x.x.x.x failed : Software caused connection abort
- ✓Verified signal present: Intermittent during communicator init
- ✓Verified signal present: Only on retry after an initial failure
- ✓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
- First connect attempt fails (ECONNREFUSED/ETIMEDOUT), leaving the socket half-closed
- NCCL retries on the same stale fd
- Retry on the half-closed fd triggers ECONNABORTED
The fix and how to prevent it
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