Phase 11.10: diagnose + pad backend atomics for false-sharing - #63
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`BackendFragCounters::bytes_committed` + `bytes_decommitted_to_os` shared a cache line, as did `StatsRange::current_usage` + `peak_usage`. Every `notify_using` invalidated the line that the matching `notify_not_using` had just read, and the `current_usage`/`peak_usage` CAS dance bounced the line for no reason. Add `alignas(64)` to each global atomic so each lives on its own cache line. Cost: ~96 bytes of additional BSS per template instantiation. Correctness unchanged. Diagnostic write-up + recommended next steps in docs/heap-profiling-diagnostic-11-10.md.
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Summary
medium_allocs(~1.05) andmixed(~1.03): false-sharing on process-global backend atomicsBackendFragCounters(bytes_committed + bytes_decommitted_to_os) andStatsRange::Type(current_usage + peak_usage)alignas(64)to each atomic so each lives on its own cache lineClickUp
Test plan
-DSNMALLOC_STATS_BASIC=ON -DCMAKE_BUILD_TYPE=ReleaseDiagnostic
Full write-up:
docs/heap-profiling-diagnostic-11-10.md. Cost of padding: ~96 bytes BSS per template instantiation. Correctness unaffected.Follow-up if bench shows no improvement
File Phase 11.11 — disassembly diff (objdump -d) BASIC vs OFF on Allocator<...>::alloc to identify the actual hot-path delta. False-sharing was the highest-probability hypothesis; if disproved, codegen drift is next.