CUDA Unified Memory Error
CUDA Unified Memory (UVM) errors cause page faults and performance issues when memory oversubscription occurs.
CUDA Unified Memory (UVM) errors cause page faults and performance issues when memory oversubscription occurs.
What this failure is
CUDA Unified Memory Error is a Memory failure seen during ML training runs. CUDA Unified Memory (UVM) errors cause page faults and performance issues when memory oversubscription occurs. Common tags: Cuda, Uvm, Unified Memory, Page Fault.
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Why it happens (the mechanism)
Page fault when GPU accesses CPU memory. Memory transfer overhead between CPU and GPU. UVM oversubscription causes thrashing. UVM page size not optimal for workload. 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
- Page fault errors during training
- Training is slower when using UVM
- CUDA error related to managed memory
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| CUDA error: page fault or illegal memory access | Page fault when GPU accesses CPU memory |
| CUDA out of memory with UVM | Memory transfer overhead between CPU and GPU |
| Performance degradation with UVM | UVM oversubscription causes thrashing |
Which systems are affected
- Models larger than GPU memory
- Training with managed memory allocation
- Multi-GPU training with unified memory
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: CUDA error: page fault or illegal memory access
- ✓Verified signal present: CUDA out of memory with UVM
- ✓Verified signal present: Performance degradation with UVM
- ✓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 extensionCUDA errors in context
CUDA reports errors asynchronously, so the traceback usually points at whatever line synchronised next rather than the one at fault. The hub covers every common CUDA error and how to make it report honestly.
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Root cause
- Page fault when GPU accesses CPU memory
- Memory transfer overhead between CPU and GPU
- UVM oversubscription causes thrashing
- UVM page size not optimal for workload
The fix and how to prevent it
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