Overhaul map JMH benchmarks: remove thread contention and split by use case

[dougqh]

What This Does

Overhauls the internal-api map microbenchmarks so each one isolates the dimension it actually claims to measure, and splits the general map benchmark by use case.

• TagMapAccessBenchmark — removes cross-thread contention: all shared state is immutable, while every bit of mutable state (the pre-populated read map, lookup index, reader flyweight) lives in a @State(Scope.Thread) holder, so threads never contend on a shared map. Now runs at @Fork(2) to match the rest.

• Splits UnsynchronizedMapBenchmark into two classes. The correct JMH @State scope differs by use case and can't vary by @Param, so one class can't host both threading models:

  • ImmutableMapBenchmark — precomputed / read-mostly maps shared across threads (@State(Scope.Benchmark)); sharing is realistic and contention-free because nothing mutates after construction. get / iterate across HashMap, LinkedHashMap, TreeMap, TagMap.
  • SingleThreadedMapBenchmark — per-thread mutable lifecycle (@State(Scope.Thread)): create / clone + reads. Adds a Collections.synchronizedMap case to measure the uncontended synchronization tax — each thread owns its synchronized map, so the monitor is only ever locked by one thread (bias never revoked). The unsynchronized HashMap get/iterate are the in-harness baseline; the tax is the delta.

• Fixes a latent bug: the old iterate_linkedHashMap iterated TREE_MAP.

Motivation

Before making further changes to TagMap, I want robust benchmarks in place. The previous shared mutable state (a cross-thread counter/index, shared maps) turned several "iteration / lookup" benchmarks into contention measurements rather than measurements of the map itself. Isolating state per thread — and separating the read-mostly-shared use case from the single-threaded-mutable one — fixes that.

Notes

• Run at default JVM flags (what customers run). The synchronized-map result is meant to be read across JVM versions: Java ≤ 11 has biased locking on by default (uncontended same-thread locking ≈ free), Java 15+ has it disabled (JEP 374), so a Java 11 → 17 comparison shows the biased-locking cliff directly.
• Result blocks are intentionally empty pending a fresh multi-JVM run.
• The -Pjmh.includes / -PtestJvm gradle wiring was moved to its dedicated PR (Wire -Pjmh.includes and -PtestJvm into internal-api JMH config #11703); this PR no longer touches build.gradle.kts.
• Independent of the LegacyTagMap removal (Remove LegacyTagMap; OptimizedTagMap is the sole TagMap implementation #11678) — uses only the public TagMap API, so it builds/runs on master and on that branch alike.
• First of a planned per-span-type benchmark suite (web/db/queue/…), giving instrumentation authors relatable numbers and the natural vehicle for per-type optimizations later.

🤖 Generated with Claude Code

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	13.97 s	13.91 s	[-0.6%; +1.5%] (no difference)
startup:insecure-bank:tracing:Agent	12.88 s	12.90 s	[-0.8%; +0.5%] (no difference)
startup:petclinic:appsec:Agent	16.29 s	16.71 s	[-6.9%; +1.9%] (no difference)
startup:petclinic:iast:Agent	16.76 s	15.94 s	[-0.9%; +11.2%] (unstable)
startup:petclinic:profiling:Agent	16.72 s	16.77 s	[-1.1%; +0.5%] (no difference)
startup:petclinic:sca:Agent	16.39 s	16.76 s	[-6.3%; +1.9%] (no difference)
startup:petclinic:tracing:Agent	15.97 s	16.13 s	[-2.1%; +0.1%] (no difference)

Commit: a9329e0b · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.

[dougqh]

Future intended changes will care about the specifics of the tags, so using real tags is preferable for future-proofing