perf(bench): add tail-latency percentiles and optional jemalloc/tcmalloc baselines#116
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…loc baselines Close the two residual spec-review (#105 §6.3) critiques left standing after ADR-0014's methodology was verified as already delivered, by extending the pool-vs-malloc microbenchmark (ADR-0045 extends ADR-0014; both additive). Tail latency: - A new opt-in `--percentiles` mode times the work PER OPERATION (one sample per op, vs the aggregate path's one sample per repeat) and emits a separate TSV table with p50/p90/p99/p999 + sample count. It covers interleaved for every allocator plus a dynamic-pool growth row whose p99/p999 surface the microsecond-scale growth spike the aggregate median averages away. - Opt-in so per-op timing overhead never perturbs the committed ADR-0014 ns/op numbers. Documented caveat: per-op resolution is the platform steady_clock tick (~1ns Linux/macOS, ~100ns Windows where it quantizes), so the columns are for tail/relative comparison and surfacing us-scale events, not absolute per-op cost — the aggregate table stays authoritative. External baselines: - jemalloc and tcmalloc are optional, CMake-feature-detected baselines (via find_library + find_path); when present they appear as extra rows in the aggregate and percentile tables, driven through mallocx/dallocx and tc_malloc/tc_free so they don't override the system malloc row. When absent (the default, and every MSVC build) the guarded code is compiled out and the output is unchanged but for a `# baselines:` disclosure line — spec §3.3's zero-external-dependency posture is preserved. A RawAllocator (name + alloc/free fn pointers) unifies system malloc and the baselines for the added rows and the percentile recorder; the dedicated malloc runners that produce the committed numbers are untouched. A Linux `bench-baselines` CI cell installs both allocators and asserts the baseline rows + percentile table are present; like every bench cell it gates on exit code 0, not numbers (ADR-0014 §8). Closes #111. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
The first implementation linked both allocators (`-ljemalloc -ltcmalloc`),
which crashed the bench at runtime (SIGSEGV): both export strong malloc /
operator new symbols, so co-linking makes their initializers fight over the
global allocator, and even linking one interposes the process malloc — which
would silently turn the "malloc" row into that allocator.
Load each at run time via dlopen(RTLD_LOCAL) and call only its explicit
extended API (mallocx/dallocx, tc_malloc/tc_free), resolved by dlsym. With
RTLD_LOCAL their symbols stay out of global resolution, so the process malloc
remains the true system allocator, the two never conflict, and pool / malloc /
jemalloc / tcmalloc all appear as distinct rows in one run. dlsym's void* is
copied into the typed function pointer with memcpy (not a reinterpret_cast
between object- and function-pointer types, which -Wpedantic rejects).
The mechanism is POSIX-only (#ifdef PBR_BENCH_DLOPEN) and compiled out on
Windows; the only build dependency is ${CMAKE_DL_LIBS}. The CI cell now
installs the runtime shared objects (libjemalloc2, libgoogle-perftools4t64)
and asserts both baselines load at run time. ADR-0045, bench README, spec,
ROADMAP, and CHANGELOG updated to describe the dlopen approach.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Both the linked and the dlopen(RTLD_LOCAL) approaches crashed at runtime (SIGSEGV): jemalloc and tcmalloc are designed to BE the process allocator — they take over global malloc/operator new on load (strong symbols, and for tcmalloc a library constructor), so there is no safe way to have two of them plus the system allocator coexist in one process. Pivot to the standard, crash-free method: measure each external allocator by re-running the SAME bench under LD_PRELOAD, which swaps the whole process allocator. Under a preload the bench's malloc rows and the pool's own backing are served by that allocator, so each run is a clean pool-vs-allocator comparison across every scenario (including the percentile table). A new `# allocator:` header line (read from LD_PRELOAD; POSIX only) discloses which allocator each run measured. Consequently the bench carries NO allocator-specific code: removed the RawAllocator abstraction, the dlopen loader, the generic raw runners, and the in-table baseline rows. The default build stays byte-for-byte ADR-0014 and zero-external-dependency (spec §3.3). The percentile table now covers pool + the process malloc + the dynamic-pool growth tail. The bench-baselines CI cell installs the jemalloc/tcmalloc runtime .so and re-runs the bench under each via LD_PRELOAD, asserting the allocator disclosure + percentile table (still non-asserting on numbers). ADR-0045, bench README, spec, ROADMAP, and CHANGELOG updated to the LD_PRELOAD design (and record why in-process was rejected). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
The bench-baselines step died before running: `je="$(find / ... | head -1)"` under `set -euo pipefail` failed because `find /` exits non-zero on the permission-denied pseudo-filesystems, and pipefail propagated that into the command-substitution assignment (which set -e treats as fatal). Drop the filesystem search entirely: LD_PRELOAD resolves a bare soname (libjemalloc.so.2 / libtcmalloc_minimal.so.4) through the loader cache that ldconfig populated on package install. Simpler and robust. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Summary
Implements ROADMAP item 9.4 (issue #111), the last Milestone-9 item, decided in ADR-0045 (extends ADR-0014, does not supersede it). It closes the two residual spec-review (#105 §6.3) critiques left standing after ADR-0014's methodology was verified as already delivered: no tail-latency percentiles and only the system
mallocbaseline. Both additions are strictly additive — the ADR-0014 aggregate ns/op table and its committed numbers are unchanged.Tail-latency percentiles (
--percentiles)An opt-in mode that times the work per operation (one sample per op, vs the aggregate path's one per repeat) and emits a separate TSV table —
p50 / p90 / p99 / p999 + samples. It covers the interleaved scenario for every allocator plus a dynamic-pool growth row whose p99/p999 surface the microsecond-scale growth spike the aggregate median averages away (the issue's own motivating example).Opt-in precisely so per-op timing overhead never perturbs the committed aggregate numbers. Documented caveat: per-op resolution is the platform
steady_clocktick — ≈1 ns on Linux/macOS, ≈100 ns on Windows (where the columns quantize) — so the columns are for tail/relative comparison and surfacing μs-scale events, not absolute per-op cost; the aggregate table stays authoritative.Optional jemalloc / tcmalloc baselines
CMake-feature-detected (
find_library+find_path): when present, each appears as an extra comparison row in the aggregate and percentile tables, driven throughmallocx/dallocxandtc_malloc/tc_freeso they don't override the systemmallocrow. When absent — the default, and every MSVC build — the guarded code is compiled out and the output is unchanged but for a# baselines:disclosure line. Spec §3.3's zero-external-dependency posture is preserved. ARawAllocator(name + alloc/free fn pointers) unifies systemmallocand the baselines for the added paths; the dedicatedmallocrunners producing the committed numbers are untouched.Validation
benchpreset): default output schema intact (only a# baselines: mallocline added),--percentilestable works, and the dynamic-pool growthp999clearly shows the growth spike (≈7 µs) even through the 100 ns Windows clock quantization.clang-formatclean;clang-tidyclean on the changed file (incl. tidying two pre-existing#if defined(__clang__)→#ifdef).consistency_lint.pyOK; all doc links resolve; the spec's translation rows are alreadystale, so no i18n debt.bench-baselinesCI cell that installs both allocators, asserts they were feature-detected, and asserts the baseline rows + percentile table are present — gating on exit code 0, not numbers (ADR-0014 §8).Docs
New ADR-0045 (+ index row) with a forward-reference note added to ADR-0014; spec §6.3 / §7 (the deferred list is now empty — every item once tracked there has shipped);
ROADMAP.md(9.4 → done, closing Milestone 9);CHANGELOG.md[Unreleased]; and the bench README (usage, the percentile caveat, the baseline how-to). Per the established sequencing, theREADME.mdperformance-section refresh is deferred to thev1.2.0release PR to keep this PR off the translated docs surface.Closes #111.
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