GPU Fallen Off the Bus (Xid 79)
A GPU drops off the PCIe bus (Xid 79) and disappears from the system. A critical hardware/connectivity fault that ends training on that node until the GPU is recovered.
A GPU drops off the PCIe bus (Xid 79) and disappears from the system. A critical hardware/connectivity fault that ends training on that node until the GPU is recovered.
What this failure is
GPU Fallen Off the Bus (Xid 79) is a Hardware failure seen during ML training runs. A GPU drops off the PCIe bus (Xid 79) and disappears from the system. A critical hardware/connectivity fault that ends training on that node until the GPU is recovered. Common tags: Xid 79, Fallen Off Bus, Pcie, Hardware.
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Why it happens (the mechanism)
PCIe link lost (power instability, thermal, or physical seating). Failing GPU hardware. Riser/connector fault. 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
- A GPU vanishes from nvidia-smi mid-run
- Training crashes with the GPU unavailable
- Xid 79 in the kernel log
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Xid 79: GPU has fallen off the bus | PCIe link lost (power instability, thermal, or physical seating) |
| nvidia-smi: GPU is lost / not listed | Failing GPU hardware |
| ERR! or missing entry for the GPU | Riser/connector fault |
Which systems are affected
- Any PCIe-attached GPU
- Nodes with marginal power/thermal or seating
- Dense multi-GPU servers
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: Xid 79: GPU has fallen off the bus
- ✓Verified signal present: nvidia-smi: GPU is lost / not listed
- ✓Verified signal present: ERR! or missing entry for the GPU
- ✓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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Xid 79 in context
Xid 79 is one of a small set of codes the NVIDIA driver uses to report GPU faults, and the number is most of the diagnosis: it tells you whether you are looking at your own code, the driver, or a board that needs replacing.
Compare every Xid code side by sideDiagnose this failure in VS Code
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Root cause
- PCIe link lost (power instability, thermal, or physical seating)
- Failing GPU hardware
- Riser/connector fault
The fix and how to prevent it
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