java-performance

SKILL.md

Java Performance Skill

Optimize JVM performance through profiling, GC tuning, and memory analysis.

Overview

This skill covers JVM performance optimization including garbage collection tuning, memory analysis, CPU profiling, and benchmarking with JMH.

When to Use This Skill

Use when you need to:

Tune GC for low latency or throughput

Profile CPU hotspots

Analyze memory leaks

Benchmark code performance

Optimize container settings

Quick Reference
GC Presets

# High-throughput
-XX:+UseG1GC
-XX:MaxGCPauseMillis=200
-Xms4g -Xmx4g
-XX:+AlwaysPreTouch

# Low-latency
-XX:+UseZGC
-XX:+ZGenerational
-Xms8g -Xmx8g

# Memory-constrained
-XX:+UseSerialGC
-Xms512m -Xmx512m
-XX:+UseCompressedOops

# Container-optimized
-XX:+UseContainerSupport
-XX:MaxRAMPercentage=75.0
-XX:+ExitOnOutOfMemoryError

Profiling Commands

# Thread dump
jstack -l <pid> > threaddump.txt

# Heap dump
jmap -dump:format=b,file=heap.hprof <pid>

# GC analysis
jstat -gcutil <pid> 1000 10

# Flight recording
jcmd <pid> JFR.start duration=60s filename=app.jfr

# Async profiler
./profiler.sh -d 30 -f profile.html <pid>

JMH Benchmark

@BenchmarkMode(Mode.Throughput)
@Warmup(iterations = 3, time = 1)
@Measurement(iterations = 5, time = 1)
@State(Scope.Benchmark)
public class MyBenchmark {

    @Benchmark
    public void testMethod(Blackhole bh) {
        bh.consume(compute());
    }
}

GC Comparison

GC
Latency
Throughput
Heap Size

G1
Medium
High
4-32GB

ZGC
Very Low
Medium
8GB-16TB

Shenandoah
Very Low
Medium
8GB+

Parallel
High
Very High
Any

Troubleshooting

Problem
Cause
Solution

GC thrashing
Heap too small
Increase heap

High latency
GC pauses
Switch to ZGC

Memory leak
Object retention
Heap dump + MAT

CPU spikes
Hot loops
Profile + optimize

Usage

Skill("java-performance")