Garbage Collection Tuning Basics

The JVM reclaims memory automatically, but the collector you choose trades throughput against pause time. Understanding that trade-off is the first step before touching a single flag.

Pick the right collector

G1 is the default and balances pauses with throughput. For large heaps that need very low pauses, ZGC is worth a look.

# G1 with a pause-time goal
java -XX:+UseG1GC -XX:MaxGCPauseMillis=100 -Xmx4g -jar app.jar
 
# ZGC for very large heaps and sub-millisecond pauses
java -XX:+UseZGC -Xmx16g -jar app.jar

Read the logs before tuning

Enable unified GC logging and measure — never tune blind.

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

Takeaways

• Set -Xms equal to -Xmx in containers to avoid heap resizing pauses.
• Measure allocation rate before assuming the collector is the problem.
• Most "GC problems" are actually memory leaks or oversized caches.

Reducing allocation — reusing buffers, avoiding needless boxing — usually beats any collector flag you can set.