FSDP ncclSystemError on InfiniBand (works on TCP)
FSDP all-gather fails with ncclSystemError on one node during collective init over InfiniBand, while a TCP fallback succeeds. Pointing to a per-node fabric or plugin issue.
FSDP all-gather fails with ncclSystemError on one node during collective init over InfiniBand, while a TCP fallback succeeds. Pointing to a per-node fabric or plugin issue.
What this failure is
FSDP ncclSystemError on InfiniBand (works on TCP) is a Communication failure seen during ML training runs. FSDP all-gather fails with ncclSystemError on one node during collective init over InfiniBand, while a TCP fallback succeeds. Pointing to a per-node fabric or plugin issue. Common tags: Nccl, Fsdp, Infiniband, System Error.
Is this what broke your run? Paste your log.
You're reading about FSDP ncclSystemError on InfiniBand (works on TCP). Paste your own crash log or traceback below and get the real root cause for YOUR run, not this generic entry. No account, no card. Logs are masked at ingress and never saved to account history.
Want 14 days on the Scale plan?
Request an evaluation code. A verified workplace organization activates up to 50 diagnoses a day, alerts, history, and follow-up questions. No credit card or automatic subscription.
Why it happens (the mechanism)
Inter-node socket bootstrap fails on one node before the IB path is used. NCCL RDMA-SHARP plugin symbol/version mismatch. Container network isolation blocks the bootstrap socket. 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
- Multi-node FSDP hangs/aborts at the first all-gather
- One node reports the failure, the other looks fine
- Works when IB is disabled (TCP fallback)
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| ncclSystemError: System call (e.g. socket, malloc) or external library call failed | Inter-node socket bootstrap fails on one node before the IB path is used |
| misc/socket.cc returns error code 2 (ENOENT) on node 2 bootstrap | NCCL RDMA-SHARP plugin symbol/version mismatch |
| Abort COMPLETE across all 8 GPUs on the failing node | Container network isolation blocks the bootstrap socket |
Which systems are affected
- 2+ node DGX A100 with Mellanox InfiniBand
- Containerized training with the NCCL RDMA-SHARP plugin
- Mismatched NCCL plugin versions (v5 vs v6 symbols)
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: ncclSystemError: System call (e.g. socket, malloc) or external library call failed
- ✓Verified signal present: misc/socket.cc returns error code 2 (ENOENT) on node 2 bootstrap
- ✓Verified signal present: Abort COMPLETE across all 8 GPUs on the failing node
- ✓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
The root cause is on this page and stays free. A free account adds the exact remediation steps, saved history, and the fix on every entry in the encyclopedia.
Sign up free. Unlock the full analysisNo credit card. Daily allowance follows verified trust tier. Instant access.
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.
Install the free VS Code extensionNCCL errors in context
NCCL is where a distributed job reports failure, which is not the same as where it failed. The hub lists every common NCCL error next to what it actually indicates, and the environment variables that tell them apart.
Compare every nccl error side by sideRelated failures to investigate next
Root cause
- Inter-node socket bootstrap fails on one node before the IB path is used
- NCCL RDMA-SHARP plugin symbol/version mismatch
- Container network isolation blocks the bootstrap socket
The fix and how to prevent it
Evaluate Denpex on your own logs
Request a Scale evaluation code. A verified workplace organization activates 14 days with up to 50 diagnoses a day. Every account keeps its current diagnosis allowance and gets a verification path. No card or automatic subscription.
Don't just read the fix, diagnose your run
The encyclopedia tells you what went wrong. Denpex tells you what went wrong in YOUR training run. With your logs, your config, and your stack.