# Jmh

> Write Java microbenchmarks with JMH (Java Microbenchmark Harness) that produce trustworthy numbers — not numbers distorted by JIT dead-code elimination, constant folding, insufficient warmup, or single-fork JIT contamination. Use this skill whenever the user writes `@Benchmark`, mentions JMH, microbenchmark, throughput measurement, latency measurement, or compares two implementations performance-…

- **Type:** Skill
- **Install:** `agentstack add skill-umit-skills-jmh`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [umit](https://agentstack.voostack.com/s/umit)
- **Installs:** 0
- **Category:** [AI & ML](https://agentstack.voostack.com/c/ai-and-ml)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [umit](https://github.com/umit)
- **Source:** https://github.com/umit/skills/tree/main/skills/jmh
- **Website:** https://umitunal.net/

## Install

```sh
agentstack add skill-umit-skills-jmh
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# JMH — trustworthy Java microbenchmarks

## Workflow

1. **Pre-flight before running** — read `references/pitfalls.md` and apply its 15-point checklist to the benchmark source. Catch DCE, constant folding, missing `Blackhole`, `@Fork(0)`, `@Warmup < 5`, `final` constants in the op, missing `@State`, raw loops without `@OperationsPerInvocation`. Most "fast" results come from broken benchmarks; catching this before running saves hours.
2. **Identify the build system** — Maven (`pom.xml` with `jmh-core`) or Gradle (`me.champeau.jmh` plugin). Setup differs; running differs. See `references/maven.md` or `references/gradle.md`.
3. **Pick the right `Mode`** — `Throughput` for ops/sec, `AverageTime` for ns/op, `SingleShotTime` for cold-path / startup, `SampleTime` for distribution (p50/p99). Wrong mode → wrong question answered. See `references/modes.md`.
4. **Write the benchmark** — annotate class with `@State(Scope.Benchmark)`, `@BenchmarkMode`, `@OutputTimeUnit`, `@Fork(value=3, jvmArgs={"-Xmx2g","-Xms2g"})`, `@Warmup(iterations=5)`, `@Measurement(iterations=10)`. Every `@Benchmark` method either returns a value or takes a `Blackhole` parameter. Use `@Param` for matrices instead of separate methods.
5. **Re-check the source against the checklist** after edits.
6. **Run with profilers attached** — never run benchmarks without `-prof gc` (allocation rate context) and ideally `-prof async:output=flamegraph` (flame graph per benchmark). See `references/profilers.md`.
7. **Output JSON** (`-rf json -rff results.json`) — never trust the console table alone; JSON is what diffing and visualization tools consume.
8. **Analyze** — drag `results.json` to https://jmh.morethan.io for charts, or use Bencher/Codspeed in CI for continuous diff. See `references/analysis.md`.
9. **Report with confidence intervals** — JMH prints `Score ± Error (99.9%)`. Two means are *not* different if their confidence intervals overlap. Don't claim "10% faster" inside the noise band.

## Quick reference

```bash
# Maven — build + run a single benchmark class
mvn clean verify -DskipTests
java -jar target/benchmarks.jar MyBench -wi 10 -i 10 -f 3 -prof gc -rf json -rff result.json

# Gradle (me.champeau.jmh plugin) — run all benchmarks in jmh source set
./gradlew jmh

# Run only matching benchmarks (regex)
java -jar target/benchmarks.jar 'com\.acme\..*Hash.*'

# Profile per-benchmark with async-profiler
java -jar target/benchmarks.jar MyBench -prof async:output=flamegraph;dir=profiles
```

## Common modes

| Mode | Unit | When |
| --- | --- | --- |
| `Throughput` | ops/time | "how many per second" — default for hot-path code |
| `AverageTime` | time/op | "how long per call" — typical for latency-sensitive ops |
| `SampleTime` | time/op (sampled) | distribution incl. p50/p95/p99 — outlier-aware |
| `SingleShotTime` | time/op (one-shot, no warmup-loop) | cold start, init code, single-event measurement |

## References

| File | When to read |
| --- | --- |
| `references/intro.md` | **Read first** — what JMH is, why naive benchmarks lie, minimal example, how to read the score table + GC columns + percentiles, golden-default checklist |
| `references/pitfalls.md` | Always before reviewing/writing a benchmark — 15 antipatterns (DCE, constant folding, false sharing, etc.) + 15-point pre-flight checklist + minimal correct template |
| `references/maven.md` | Maven `pom.xml` setup, archetype, run command, multi-module projects |
| `references/gradle.md` | `me.champeau.jmh` plugin config, `jmh {}` block, source set, IDE integration |
| `references/modes.md` | Mode + State + Scope + `@OperationsPerInvocation` deep dive |
| `references/profilers.md` | `-prof gc`, `-prof async`, `-prof perfasm`, `-prof jfr`, `-prof stack` — when to use which |
| `references/analysis.md` | JSON schema, `jmh.morethan.io`, statistical interpretation, CI integration (Bencher, Codspeed) |

## Output format

- Raw run: console table + `result.json` (always emit JSON with `-rf json -rff`).
- For sharing: upload JSON to https://jmh.morethan.io and share the URL.
- For CI: integrate with Bencher (`bencher run`) or Codspeed (`codspeed run`); both have JMH adapters.
- For deep analysis: pair with `async-profiler` JFR per benchmark; render flame graphs with `jfrconv`.

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [umit](https://github.com/umit)
- **Source:** [umit/skills](https://github.com/umit/skills)
- **License:** MIT
- **Homepage:** https://umitunal.net/

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-umit-skills-jmh
- Seller: https://agentstack.voostack.com/s/umit
- Browse the marketplace: https://agentstack.voostack.com/browse

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Listed on AgentStack — the marketplace for AI agent skills and MCP servers. Every listing is security-reviewed. Creators keep 70%.