Apple NVMe small random read latency under F_NOCACHE at queue depth 16
Parent: Mac local LLMs: MoE streaming and offload · Published reference · snapshot 2026-10-05
↓ Facts as markdownall context files
Existing benchmarks that skip F_NOCACHE can serve reads from the unified buffer cache, so results vary with installed RAM (author's claim; a forum reply disputes it on the basis that SATA and HDD speeds looked like device speeds).
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Facts
- Existing benchmarks that skip F_NOCACHE can serve reads from the unified buffer cache, so results vary with installed RAM (author's claim; a forum reply disputes it on the basis that SATA and HDD speeds looked like device speeds). [source]
- DiskTruth author: BlackMagic and AmorphousDiskMark do not use F_NOCACHE and are inflated by RAM caching on large-memory Macs. A MacRumors reply (bzgnyc2/!!!) says those tools gave device-consistent speeds on SATA SSD (<500 MB/s) and HDD (~200 MB/s), so caching is not evident. Shown side by side, not resolved. [source]
- DiskTruth (beta announced 2026-04-16) sets fcntl(fd, F_NOCACHE, 1) on every file descriptor before any I/O. [source]
- DiskTruth measures random 4K IOPS and latency at QD1 with 10,000 operations per pass and reports average and p99. [source]
- DiskTruth measures queue-depth scaling at QD1, QD4, QD16 and QD32 with 3,000 ops per level. [source]
- Its author states Apple Silicon internal NVMe has "extraordinary QD1 latency" and less dramatic scaling at higher depths than desktop NVMe, with no numbers given in the post. [source]
- DiskTruth uses a 16 GB test file, 3 iterations and the median for sequential tests. [source]
- The ds4 QD16 stall estimate remains incomplete: no measured latency number exists in the sources read. [source]
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