GC tuning

Learn G1 and ZGC garbage collector settings.

Garbage collectors

Java has several main garbage collectors (GC). G1 is the default since Java 9; it offers balanced throughput and latency. ZGC and Shenandoah do concurrent relocation for low latency; they target sub-millisecond pauses even on large heaps.

The GC is chosen with -XX:+UseG1GC, -XX:+UseZGC or -XX:+UseShenandoahGC. The choice depends on whether the workload prioritises throughput or latency.

Heap and GC tuning

Heap size is set with -Xmx (max) and -Xms (initial). A small heap triggers frequent GC; a very large heap can cause long pauses. A target pause time is given with a flag such as -XX:MaxGCPauseMillis=200.

For latency-sensitive applications ZGC or Shenandoah is preferred; for throughput-oriented batch work G1 is chosen. Before tuning, the real load must be measured with JFR.

Exercise Write the GcConfig class

Define class GcConfig with a static String config(String gc, int heapMb) method that returns "-XX:+Use" + gc + " -Xmx" + heapMb + "m". In main print System.out.println(config("G1GC", 2048));.

Show example solution

class GcConfig {
    static String config(String gc, int heapMb) {
        return "-XX:+Use" + gc + " -Xmx" + heapMb + "m";
    }
    public static void main(String[] args) {
        System.out.println(config("G1GC", 2048));
    }
}

Test yourself

Match the garbage collectors

G1

Default collector
ZGC

Low latency
Shenandoah

Concurrent relocation
-Xmx

Max heap
Complete the GC flag
```
-XX:+Use____ -Xmx2g
```
Show answer
```
-XX:+UseG1GC -Xmx2g
```
Which GC is preferred for low latency?
- Serial GC
- ZGC
- Parallel GC
- G1 GC
Show answer

ZGC — ZGC targets very short pauses with concurrent relocation.

Next lesson: Heap dump

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