Generational ZGC

Key points

• ZGC already gave sub-millisecond pauses, but scanned the whole heap every cycle.
• Most objects die young, so scanning old survivors repeatedly is wasted work.
• JEP 439 added young and old generations to ZGC, collecting the young part far more often.
• The result is the same tiny pauses at meaningfully lower CPU and heap overhead.
• In 21 you opt in with -XX:+UseZGC -XX:+ZGenerational; later releases made it the default.

Example

public class Main {
    public static void main(String[] args) {
        Runtime rt = Runtime.getRuntime();
        System.out.println("max heap MB : " + rt.maxMemory() / (1024 * 1024));

        long before = rt.totalMemory() - rt.freeMemory();

        // Churn: almost all of this dies immediately - exactly what a young gen is for
        StringBuilder keep = new StringBuilder();
        for (int i = 0; i < 200_000; i++) {
            String tmp = "garbage-" + i;
            if (i % 50_000 == 0) { keep.append(tmp).append(' '); }
        }

        long after = rt.totalMemory() - rt.freeMemory();
        System.out.println("used before : " + before / 1024 + " KB");
        System.out.println("used after  : " + after / 1024 + " KB");
        System.out.println("survivors   : " + keep);
        System.out.println("GC in use   : " +
            java.lang.management.ManagementFactory.getGarbageCollectorMXBeans()
                .stream().map(b -> b.getName()).toList());
    }
}

Generational ZGC kept the pause guarantee and stopped paying for it with wasted scanning.