Java JVM Tuning: Cheat Sheet

JVM tuning can dramatically improve performance. A practical cheat sheet.

Memory¶

-Xms4g -Xmx4g -XX:MaxMetaspaceSize=512m

Garbage Collectors¶

-XX:+UseG1GC -XX:+UseZGC # Java 17+ -XX:+UseShenandoahGC

GC Logging¶

-Xlog:gc*:file=gc.log:time,uptime

JFR¶

jcmd JFR.start duration=60s filename=recording.jfr

Diagnostics¶

jstack # thread dump jmap -dump:live,format=b,file=heap.hprof -XX:+HeapDumpOnOutOfMemoryError

Containers¶

-XX:MaxRAMPercentage=75.0

Rules¶

• Heap = 50-75% RAM
• Xms = Xmx
• G1GC for most workloads
• ZGC for ultra-low latency

Choosing a Garbage Collector¶

G1GC is the default GC since Java 9 and a good choice for most applications. It divides the heap into regions and collects regions with the most garbage first. ZGC (Java 17+) offers ultra-low latency with GC pauses under 1ms, ideal for real-time systems and latency-sensitive microservices. Shenandoah GC offers similar characteristics to ZGC and is available in OpenJDK.

Java Flight Recorder (JFR) is a production profiler with minimal overhead (typically under 1%). It records CPU samples, allocations, GC events, I/O, and thread activity. Combining JFR with JDK Mission Control provides detailed performance analysis without needing to reproduce the problem. For containerized environments, always set -XX:MaxRAMPercentage instead of fixed -Xmx values so the JVM properly adapts to container memory limits.

Tip¶

Always measure before and after changes. JFR is the best tool for Java performance analysis.