AWS EFA Driver Issue
AWS EFA (Elastic Fabric Adapter) driver issues cause NCCL to fall back to slower TCP/IP for inter-node communication.
AWS EFA (Elastic Fabric Adapter) driver issues cause NCCL to fall back to slower TCP/IP for inter-node communication.
What this failure is
AWS EFA Driver Issue is a Communication failure seen during ML training runs. AWS EFA (Elastic Fabric Adapter) driver issues cause NCCL to fall back to slower TCP/IP for inter-node communication. Common tags: Efa, Aws, Nccl, Inter Node.
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Why it happens (the mechanism)
EFA driver not installed. EFA-enabled NCCL not installed. EFA not enabled in container. Security group blocks EFA traffic. 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
- Inter-node bandwidth is low
- EFA not used by NCCL
- NCCL falls back to TCP/IP
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL WARN NET/IB: No IB devices found | EFA driver not installed |
| EFA device not visible | EFA-enabled NCCL not installed |
| fi_info fails | EFA not enabled in container |
Which systems are affected
- AWS GPU training with EFA
- Multi-node training on P4d/P5 instances
- High-performance distributed training
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 WARN NET/IB: No IB devices found
- ✓Verified signal present: EFA device not visible
- ✓Verified signal present: fi_info fails
- ✓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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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
- EFA driver not installed
- EFA-enabled NCCL not installed
- EFA not enabled in container
- Security group blocks EFA traffic
The fix and how to prevent it
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