Is it worth migrating our legacy monolith from Java 8 to Java 21 for the ZGC performance?

All answers to this question.

The jump from Java 8 to 21 is significant, mostly due to the Module System introduced in Java 9. Once you get past the dependency issues, ZGC is like magic for large heaps. It is designed to keep pause times under 1 millisecond, regardless of whether your heap is 1GB or 1TB. In our migration, our 99th percentile latency dropped from 1500ms to under 10ms. You don't even need to do much tuning; just set -XX:+UseZGC and let the JVM handle the rest. It’s the single biggest reason to upgrade your legacy enterprise systems today

   Answered 2025-01-15 by Barbara King

Are you using any third-party libraries that rely on 'Unsafe' or internal Sun packages? Java 21 is much stricter about "Illegal Access," which could be a bigger blocker than the GC config.

   Answered 2025-01-17 by Jeffrey Vance

• Jeffrey, we did find some issues with an old caching library. We had to use some --add-opens flags in our startup script to get it to run, but it’s a temporary fix while we look for a modern alternative. The performance boost from ZGC on our 64GB heap was so massive that management is okay with these minor configuration workarounds for now. It has completely solved our "jitter" issues during peak trading hours, which was our primary goal.

     Commented 2025-01-20 by Kevin Adams

Don't forget the 'Generational ZGC' feature in Java 21. It handles short-lived objects much more efficiently than the original version, further reducing the CPU overhead of the collector.

   Answered 2025-01-22 by Maria Hernandez

• Maria is right. Generational ZGC makes the collector viable for even more workloads, as it handles the "infant mortality" of objects without scanning the whole heap.

     Commented 2025-01-25 by Barbara King