Phase 11.6: tiered SNMALLOC_STATS (BASIC/FULL split) - #60
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Splits the monolithic SNMALLOC_STATS flag into two independently selectable tiers so production builds can opt into the cheap counter surface without paying for the expensive per-size-class histogram. * SNMALLOC_STATS_BASIC -- frontend fast/slow path counters (9.2) + backend commit/decommit (9.4) + largebuddy free-chunk histogram (11.4). Target overhead <=2% (measured 1.03-1.08 on this host). * SNMALLOC_STATS_FULL -- BASIC plus per-size-class histogram (9.3) and lifetime histogram (9.5). Target overhead <=20% (measured 1.09-1.16). The legacy SNMALLOC_STATS flag is preserved as a backwards- compatible alias for BASIC; FULL implicitly enables BASIC. The FullAllocStats wire format is unchanged -- fields the active tier does not maintain simply read as zero -- so SNMALLOC_FULL_STATS_VERSION is not bumped. Cargo: `stats-basic` and `stats-full` features added in both snmalloc-rs and snmalloc-sys; `stats` is now an alias for `stats-basic`; `stats-full` implies `stats-basic` so the snmalloc-rs SnMalloc::full_stats() accessor remains available under either tier. 5-run bench results on Apple M4 Pro (vs OFF baseline): Group basic/off full/off small_allocs 1.0774 1.1639 medium_allocs 1.0398 1.0935 mixed 1.0310 1.0910 FULL meets the <=1.20 budget on every group. BASIC sits ~5-8% above OFF -- above the 1.02 spec but ~50% closer than the 1.16 Phase 11.5 floor. The remaining ~8% on small_allocs is the irreducible cost of two non-atomic stores per alloc+dealloc (stats.fast_path_allocs++ / stats.fast_path_deallocs++) on a ~200 ns inner-loop iteration. See docs/heap-profiling-benchmarks.md "Phase 11.6 -- tiered SNMALLOC_STATS overhead" for the full table and methodology. ClickUp: 86aj0ydjv
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Summary
SNMALLOC_STATSinto two tiers:SNMALLOC_STATS_BASIC(cheap frontend + backend counters, production default) andSNMALLOC_STATS_FULL(adds per-size-class + lifetime histograms, opt-in for debugging).stats-basic/stats-fullmirror the split; legacystatsis preserved as an alias forstats-basic.FullAllocStatswire format is unchanged (fields the active tier does not maintain read as zero).ClickUp
Test plan
cmake -B build -DSNMALLOC_STATS_BASIC=ON && cmake --build build -j4cmake -B build_full -DSNMALLOC_STATS_FULL=ON && cmake --build build_full -j4cmake -B build_legacy -DSNMALLOC_STATS=ON && cmake --build build_legacy -j4cmake -B build_off && cmake --build build_off -j4ctestpasses 106/106 on all four C++ build flavourscargo build --features stats-basicsucceedscargo build --features stats-fullsucceedscargo build --features statssucceeds (alias path)cargo test --features stats-basic --release-- all tests pass (sizeclass test skipped, gated to stats-full)cargo test --features stats-full --release-- all tests pass including per-size-class histogram suitecargo bench --bench stats_benchacross all three variants, 5 runs eachdocs/heap-profiling-benchmarks.md("Phase 11.6 -- tiered SNMALLOC_STATS overhead")