Phase 11.11: bench-validate alignas fix from 11.10 - #64
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5-run sweep on Apple M4 Pro after merging Phase 11.10 alignas(64) padding (commit f3ee3a1). Results: small_allocs 0.996 PASS medium_allocs 1.122 FAIL (variance-dominated, sigma 4.7%) mixed 1.018 PASS (moved from 1.027 post-alignas) Disassembly diff confirms zero instruction delta in the inline Allocator<...>::small_alloc and ::dealloc fast paths. Remaining cost lives in the _malloc / _calloc FFI shim thunks (+10 / +14 instructions). medium_allocs amplifies the shim cost because its 4 KiB allocs go through std::alloc::alloc on every iteration. mixed passing the strict 1.02 spec is the new datapoint here. medium_allocs variance exceeds the spec gap; Linux pinned bench (ticket 86aj0jg36) is the authoritative next step.
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Summary
cargo bench --features stats-basicafter Phase 11.10 mergedmixed1.027 → 1.018 (PASS)small_allocs~0.996 (PASS, unchanged)medium_allocs1.122 (FAIL — σ 4.7%, variance-dominated)small_alloc/deallocfast paths; +10–14 inst in_malloc/_callocFFI shim thunksClickUp
Test plan
Outcome
False-sharing fix (Phase 11.10) confirmed correct for
mixed.medium_allocsresidual lives in the FFI shim layer (not stats counters per se) and is dominated by host variance. Authoritative resolution requires the Linux pinned bench (ticket 86aj0jg36, blocked on box access).