perf(kernel): cache filesystem usage for quota checks; fix WASI hot paths

[NathanFlurry]

The WASI syscall dispatch tax (3.1, the biggest known optimization) was
dominated by the KERNEL, not the shim: every fd_open/fd_write ran a full
filesystem usage scan for quota accounting (~10ms each on a populated VFS),
plus redundant WASI post-open stats and per-op fixture churn.

• Kernel: filesystem usage is cached and maintained incrementally — old/new
  size deltas for file writes/pwrite/truncate (path and fd paths), inode
  deltas for create/remove/symlink; rename and mount/import/snapshot/host-dir
  events invalidate (overlay copy-up topology is not locally delta-able;
  invariants documented per mutation path). The cache populates lazily via
  the RAW filesystem so quota bookkeeping never fires guest-attributable
  permission checks.
• WASI shim: removed redundant path_open create/truncate stat round-trips;
  per-syscall metrics kept (sub-phase timings) for future attribution.
• native-baseline: readdir fixtures marker-cached (setup out of timing);
  fs_stat_x32 measures identical work on every lane (batching one lane
  harder than the others distorts the differential — comment documents the
  quantized sub-ms wasm reading).
• Stale DEFAULT_WASM_EXECUTION_TIMEOUT_MS limits-inventory entry removed
  (constant retired earlier for typed max_fuel + V8 CPU watchdog).

Release results (guest = JS lane, wasm lane):

• fs_write_small 0.24 -> 0.10ms guest, 25 -> 4ms wasm
• fs_write_big 20.5 -> 4.4ms guest, 15 -> 14ms wasm
• stat_storm 0.24 -> 0.07ms guest, 12 -> 5ms wasm
• readdir_big 10.9 -> 6.1ms guest, 165 -> 14ms wasm (documented floor:
  the op stats all 1000 entries; fd_readdir itself is ~1ms)
• ecosystem: ls_100 vmCmd 918 -> 490ms, git_init_commit 8.9s -> 1.4s

Kernel/limits/limits_audit/tls/http2 suites green; git + shell-redirect
semantics oracle 20/20; baseline regenerated; bench gate passes.

[NathanFlurry]

Stack for rivet-dev/secure-exec

• perf(kernel): cache filesystem usage for quota checks; fix WASI hot paths #208 suolmrts · 2026-07-02 15:42:19 👈
• main

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Push local edits: forklift submit
Merge when ready: forklift merge 208