MMap File Handle Exhaustion
Memory-mapped (mmap) file handles are exhausted when datasets have many large files, causing data loading failures.
Memory-mapped (mmap) file handles are exhausted when datasets have many large files, causing data loading failures.
What this failure is
MMap File Handle Exhaustion is a Data Pipeline failure seen during ML training runs. Memory-mapped (mmap) file handles are exhausted when datasets have many large files, causing data loading failures. Common tags: Mmap, Ulimit, File Handles, Hdf5.
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Why it happens (the mechanism)
Ulimit -n too low for number of files. Each worker holds file handles. HDF5 file handles not released. Long-running training accumulates handles. 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
- DataLoader fails with mmap error
- Too many open files for mmap
- OSError: [Errno 24] Too many open files
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| OSError: [Errno 24] Too many open files | ulimit -n too low for number of files |
| ulimit -n shows low value | Each worker holds file handles |
| mmap can't open file for data loading | HDF5 file handles not released |
Which systems are affected
- Training with HDF5 datasets
- LMDB databases with mmap
- NumPy memmap for large arrays
- WebDataset with many tar files
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: OSError: [Errno 24] Too many open files
- ✓Verified signal present: ulimit -n shows low value
- ✓Verified signal present: mmap can't open file for data loading
- ✓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
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Root cause
- ulimit -n too low for number of files
- Each worker holds file handles
- HDF5 file handles not released
- Long-running training accumulates handles
The fix and how to prevent it
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References
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