Partial NVLink Failure vs NVSwitch Failure (Localization)
NVLink bandwidth degradation can be a single failing link or a whole-node NVSwitch fault. Comparing per-GPU NVLink bandwidth across all 8 GPUs tells them apart.
NVLink bandwidth degradation can be a single failing link or a whole-node NVSwitch fault. Comparing per-GPU NVLink bandwidth across all 8 GPUs tells them apart.
What this failure is
Partial NVLink Failure vs NVSwitch Failure (Localization) is a Hardware failure seen during ML training runs. NVLink bandwidth degradation can be a single failing link or a whole-node NVSwitch fault. Comparing per-GPU NVLink bandwidth across all 8 GPUs tells them apart. Common tags: Nvlink, Nvswitch, Bandwidth, Hardware.
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Why it happens (the mechanism)
A single GPU's NVLink connection degraded (one GPU low). Or the shared NVSwitch failed (all 8 GPUs low). Link errors / flaps reduce effective bandwidth. 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
- Collective bandwidth drops on a node
- Unclear if one GPU or the whole switch is at fault
- Need to localize before draining hardware
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| DCGM_FI_DEV_NVLINK_BANDWIDTH_TOTAL low on one or all GPUs | A single GPU's NVLink connection degraded (one GPU low) |
| NVLink error counters incrementing | Or the shared NVSwitch failed (all 8 GPUs low) |
| Slow all-reduce localized to one node | Link errors / flaps reduce effective bandwidth |
Which systems are affected
- 8-GPU NVSwitch nodes (DGX/HGX)
- NVLink-heavy collectives
- Clusters monitoring DCGM NVLink fields
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: DCGM_FI_DEV_NVLINK_BANDWIDTH_TOTAL low on one or all GPUs
- ✓Verified signal present: NVLink error counters incrementing
- ✓Verified signal present: Slow all-reduce localized to one 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
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Root cause
- A single GPU's NVLink connection degraded (one GPU low)
- Or the shared NVSwitch failed (all 8 GPUs low)
- Link errors / flaps reduce effective bandwidth
The fix and how to prevent it
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