test: canary for the flaky-benchmark fixes in #8742 (do not merge)#8743
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Extends a cold-path error string in vortex-array, mimicking #8681 - a PR that changed only this kind of string and still got five false CodSpeed flags under glibc malloc. The change shifts the vortex-array bench binaries' layout without touching any benchmarked code path, so this stacked PR's CodSpeed run (compared against the flaky-microbenchmarks-cleanup branch) should report no changes if the mimalloc fixes hold. Signed-off-by: Claude <noreply@anthropic.com> Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01T6PPcdrqcNeUkfi1EGd4oC
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A small pool of microbenchmarks flips ±10-35% between two fixed values on unrelated PRs (including docs-only and lockfile-only changes), spamming every CodSpeed report. This PR fixes them (and removes only 1 benchmark). No `#[cfg(codspeed)]` gating; CI and local runs stay identical. One commit per benchmark. ## Changes - **mimalloc as global allocator** in the 5 flaky `vortex-array` bench files: `chunk_array_builder`, `dict_compress`, `varbinview_compact`, `compare`, `binary_ops` - **`bitwise_not_vortex_buffer_mut`**: drop the 128/1024/2048 sizes (measured only harness overhead) - **`slice_empty_vortex`**: rewrite as a 1024-iteration tight loop, renamed `slice_empty_tight_loop_vortex` - **`rebuild_naive` (vortex-zstd)**: the one benchmark removed instead of fixed — its cost is dominated by zstd-internal copies (glibc `ifunc`-resolved `memcpy`) that no bench-level change can stabilize, its fixture is degenerate (a 4-string dictionary with all-zero offsets), and ListView rebuild is already benchmarked across element types and list shapes in `vortex-array/benches/listview_rebuild.rs`. The crate's now-unused `divan` dev-dependency goes with it. <details> <summary>Which benchmarks were flaky, and the evidence</summary> Identified by reading the CodSpeed comments on the ~47 PRs merged since June 25 (post-#8490). The tell: the same benchmark flipping between the same two values, in both directions, on PRs that can't have affected it — including deny.toml-only (#8712, #8716), uv.lock-only (#8732), docs-only (#8737, #8728, #8685), and CI-YAML-only (#8660, #8683) changes. | Benchmark | Evidence | |---|---| | `bitwise_not_vortex_buffer_mut[128/1024/2048]` | ~half of all PRs — worst offender | | `chunked_varbinview_*` ×4 | ~20 PRs, both directions | | `chunked_bool_canonical_into[(1000,10)]` | ~2× flips (16µs ↔ 35µs) on 4 PRs | | `encode_varbin`, `encode_varbinview` | ~12 PRs; `encode_varbinview[(10000,4)]` also flipped on an earlier revision of this PR | | `compact`, `compact_sliced` (90%-utilization args) | ~8 PRs | | `compare_int_constant` | ±11.1% verbatim on ≥9 PRs | | `eq_i64_constant` | same ±11% signature incl. docs-only PR | | `slice_empty_vortex` | -14.66% verbatim on ~13 PRs | | `rebuild_naive` (vortex-zstd) | ~10 PRs, both directions | Watch list (left alone, below the ≥3-independent-sightings bar): `copy_nullable`/`copy_non_nullable[65536]` in `cast_decimal.rs`, `true_count_vortex_buffer[128]`. </details> <details> <summary>Root causes and why each fix matches</summary> - **Allocation in the timed region** → glibc malloc's code differs across CodSpeed runner images, so alloc-heavy benchmarks trace differently for byte-identical Vortex code. Vendored mimalloc removes glibc malloc from the trace. Empirical support: the only three bench files already using mimalloc (`single_encoding_throughput`, `common_encoding_tree_throughput`, `row_encode`) are the most alloc-heavy suites in the repo and were never flagged once in the 47-PR window. - **Sub-microsecond work** → the measurement is fixed harness overhead plus binary code layout, which shifts with any unrelated change. Fix by making the operation dominate: the in-place NOT (no alloc, no copy) keeps only sizes where the loop dominates; the empty slice runs 1024× per iteration, mirroring the neighboring `slice_tight_loop_vortex`. - **Environment-bound and low-signal** → `rebuild_naive`, per the justification above: unfixable at the bench level and redundant with the vortex-array ListView rebuild suite, so removed. </details> <details> <summary>Validation: A/A reruns and a stacked canary PR</summary> - **Expected one-time step changes on this PR**: swapping the allocator changes the trace of every benchmark in the touched files, so this PR's report shows a few ±10-20% level shifts (including on never-flaky `encode_primitives`, which just shares a file). These need a one-time acknowledgment on the CodSpeed dashboard; after merge every PR compares mimalloc-vs-mimalloc. - **A/A reruns** (same bench binaries measured three times, on separate runners and commits): every value reproduced exactly — 211.5µs, 137.1µs, 14.6µs, 26.3µs — with zero new flags across 1656 benchmarks. - **Canary #8743** (a #8681-style cold-string change stacked on this branch — the class of change that used to collect five false flags): **Performance Gate Passed**, `✅ 1660 untouched`, zero changes reported. </details> No public API changes; benchmark-only. https://claude.ai/code/session_01T6PPcdrqcNeUkfi1EGd4oC --------- Signed-off-by: Claude <noreply@anthropic.com> Co-authored-by: Claude <noreply@anthropic.com>
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Rationale for this change
Validation canary for #8742 — served its purpose, closed without merging.
This PR stacked a deliberately layout-shifting, perf-irrelevant change (a cold-path error-string tweak in
vortex-array, mimicking #8681 — a real PR that collected five false CodSpeed flags under glibc malloc) on top of the flaky-benchmark fixes, so CodSpeed would compare mimalloc-vs-mimalloc across different binaries — the comparison every future PR gets after #8742 merges.Result: Performance Gate Passed ✅
✅ 1660 untouched benchmarks, ⏩ 47 skipped— zero regressed, zero improved, zero new, comparingclaude/flaky-bench-canary-u68he8(40c5e76) withclaude/flaky-microbenchmarks-cleanup-u68he8(4efeb0d). CodSpeed posts no comment when there are no changes to report; the green check is the report.Together with the A/A rerun on #8742 (identical tree measured twice on separate runners: all values reproduced exactly), this validates the fixes from both directions.
https://claude.ai/code/session_01T6PPcdrqcNeUkfi1EGd4oC