r/kernel • u/Semiramis67 • 12d ago
NSD: LKML RFC - Kernel readahead learning prefetcher
Posting benchmark results from the ongoing LKML RFC discussion.
The patchset adds a learning prefetcher to the page cache that
observes access patterns and issues prefetch only when the kernel's
own window is insufficient.
Test methodology: interleaved ON/OFF, drop_caches before each run,
3 passes, 4 workloads, 5 read_ahead_kb values.
seq64 (8 GB sequential, 64k requests, n=131K/run):
| kb | OFF avg | ON avg | diff | p50 OFF->ON | p99 OFF->ON | overhead |
|-------|---------|---------|--------|-------------|--------------|----------|
| 128 | 22.02s | 17.11s | -22% | 50->23 us | 812->805 us | 5.94->4.33s |
| 256 | 17.08s | 17.22s | +1% | 20->15 us | 824->1129 us | 4.07->4.13s |
| 512 | 16.81s | 16.96s | +1% | 16->14 us | 1092->1903 us| 4.24->3.60s |
| 1024 | 17.25s | 17.03s | -1% | 15->16 us | 1903->1883 us| 3.67->3.74s |
SQLite full-table scan (6.2 GB):
| kb | OFF avg | ON avg | diff | overhead |
|-------|---------|---------|--------|----------|
| 128 | 24.51s | 20.48s | -16% | +0.95s |
| 256 | 20.05s | 16.60s | -17% | +1.26s |
| 512 | 15.31s | 12.18s | -20% | +1.06s |
| 1024 | 12.22s | 12.09s | -1% | +0.26s |
Key takeaways:
- 128K: seq64 -22%, SQLite -16%
- 512K: NSD reaches the same throughput as kernel+1M (12.18s vs 12.22s)
- 1M: NSD is completely silent, no regression
- random4k: neutral
- random_repeat: slight improvement
- Disk I/O is byte-identical between ON and OFF in all 96 runs