bench/bench.c (`make bench`) measures create/read/stat/readdir/unlink/
mkdir on mem, dir, and raw fs; large sequential I/O; random-access pack
reads via mmap vs raw fs; compaction throughput; and 8-thread
concurrent mixed workloads. Results from one full run, with honest
analysis (what each "raw fs" vs "raw+fsync" vs "dir" label actually
measures, so they aren't misread as interchangeable), are in BENCH.md.
The benchmark surfaced a real, quantitatively-confirmed finding, not
just favorable numbers: bulk sequential create/unlink on mem/dir is
O(n^2) in file count (~9-22x slower than raw fs at N=20,000), because
every structural write copies the entire snapshot entry array before
publishing it (Section 5.3). concept.md itself names the exact trigger
condition for reconsidering this ("a persistent structurally-shared
tree structure is not required until this assumption is empirically
violated") — this benchmark is that violation, measured rather than
hypothesized: mkdir at N=4,000 vs create at N=20,000 (same mechanism,
5x the N) shows a 28.4x slowdown, matching the O(n^2) prediction (25x)
far better than O(n) (5x).
Also found: dir-backend stat() costs ~2x raw stat() (open+fstat vs one
syscall, the direct cost of openat2 containment on the metadata path);
mem-backed large writes lose to raw fs above ~16MB (Section 5.7's
buffer-growth discipline re-copies prior writes on every capacity
doubling, the price of never exposing a reader to a freed buffer).
Recorded the O(n^2) finding in CLAUDE.md's new "Known performance
characteristics" section, per the same pattern used for the
reclaim_gate use-after-free discovery, since it's exactly the kind of
fact that would otherwise have to be rediscovered by benchmarking
again from scratch. Linked from README and CONTRIBUTING.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UqJpkdJ6Njnt1pw3CbghzB
retoorandClaude Sonnet 5
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