Node Rank Mismatch at NCCL Init
The configured world size does not match the number of ranks that actually connect, so NCCL initialization hangs or errors during the communication-setup stage.
The configured world size does not match the number of ranks that actually connect, so NCCL initialization hangs or errors during the communication-setup stage.
What this failure is
Node Rank Mismatch at NCCL Init is a Distributed Training failure seen during ML training runs. The configured world size does not match the number of ranks that actually connect, so NCCL initialization hangs or errors during the communication-setup stage. Common tags: Nccl, Rank Mismatch, World Size, Ddp.
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Why it happens (the mechanism)
RANK/WORLD_SIZE/MASTER_ADDR misconfigured across nodes. A launcher started the wrong number of processes. A node failed to launch or register. 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
- Distributed init hangs or fails at startup
- world_size doesn't match connected peers
- Common with misconfigured launchers
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL init blocks waiting for ranks that never join | RANK/WORLD_SIZE/MASTER_ADDR misconfigured across nodes |
| Mismatch between WORLD_SIZE and connected peers | A launcher started the wrong number of processes |
| Hang at the first collective / barrier | A node failed to launch or register |
Which systems are affected
- Multi-node DDP/FSDP launches
- SLURM/torchrun/Kubernetes launchers
- Communication-setup stage (the most DDL-specific failure stage)
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 init blocks waiting for ranks that never join
- ✓Verified signal present: Mismatch between WORLD_SIZE and connected peers
- ✓Verified signal present: Hang at the first collective / barrier
- ✓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.
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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.
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
- RANK/WORLD_SIZE/MASTER_ADDR misconfigured across nodes
- A launcher started the wrong number of processes
- A node failed to launch or register
The fix and how to prevent it
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