Make ThreadSafeMapBenchmark lookup index per-thread (remove shared-counter contention)#11988
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…unter contention) nextLookupKey used a shared static counter incremented from all @threads(8) — a cache-line-contended race that floors the fastest reads and masks the very differences the benchmark compares (mirrors the per-thread index SingleThreadedMapBenchmark already uses, and the set-benchmark fix in #11721). Maps stay static/shared; only the lookup index goes per-thread via @State(Scope.Thread). Existing Javadoc numbers predate this and need a rerun. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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What Does This Do
Makes
ThreadSafeMapBenchmark's lookup-key cursor per-thread instead of a shared static counter.Why
nextLookupKeyincremented astatic int sharedLookupIndexfrom all@Threads(8)— a cache-line-contended data race. That counter traffic is a fixed floor added to everyget_*measurement; for the fastest reads it dominates, compressing exactly the differences the benchmark exists to show.SingleThreadedMapBenchmarkalready uses a per-thread index, and the set benchmarks got this fix in #11721 — this brings the map suite in line (it was only ever done for sets).How
@State(Scope.Thread)on the class;static int sharedLookupIndex→ instanceint lookupIndex;nextLookupKey→ instance methods.static/shared — that's the point (concurrent reads of one shared map); only the index goes per-thread.Note
The committed Javadoc numbers predate this and need a rerun.
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