RDMA Configuration Issue
RDMA configuration issues prevent high-bandwidth, low-latency inter-node communication in GPU clusters.
RDMA configuration issues prevent high-bandwidth, low-latency inter-node communication in GPU clusters.
What this failure is
RDMA Configuration Issue is a Communication failure seen during ML training runs. RDMA configuration issues prevent high-bandwidth, low-latency inter-node communication in GPU clusters. Common tags: Rdma, Infiniband, Mellanox, Communication.
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Why it happens (the mechanism)
RDMA not enabled in BIOS. Mellanox OFED driver not installed. Subnet manager not running. RDMA device not in container. PCIe not configured for RDMA. 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
- RDMA not being used
- Training is slow on multi-node setup
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| NCCL not using RDMA | RDMA not enabled in BIOS |
| RDMA device not detected | Mellanox OFED driver not installed |
| ibstat shows no active ports | Subnet manager not running |
Which systems are affected
- Multi-node training on GPU clusters
- High-performance computing
- GPU cluster setup
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 not using RDMA
- ✓Verified signal present: RDMA device not detected
- ✓Verified signal present: ibstat shows no active ports
- ✓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.
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Root cause
- RDMA not enabled in BIOS
- Mellanox OFED driver not installed
- Subnet manager not running
- RDMA device not in container
- PCIe not configured for RDMA
The fix and how to prevent it
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