Modern Java Garbage Collection: G1 vs. ZGC
The days of massive "stop-the-world" pauses in Java are largely over. With the introduction of modern collectors like G1 and ZGC, Java can now handle heaps from a few gigabytes to several terabytes with sub-millisecond pause times.
1. G1 GC (Garbage First)
The default collector since Java 9. It is a regional, generational collector.
- How it works: It divides the heap into equal-sized regions. It prioritizes collecting regions with the most garbage (hence "Garbage First").
- Pros: Great balance between throughput and latency. You can set a goal pause time (e.g.,
-XX:MaxGCPauseMillis=200). - Best for: Most standard business applications with heap sizes up to 32GB.
2. ZGC (Z Garbage Collector)
Introduced as a production-ready feature in Java 15. It is a scalable, low-latency collector.
- How it works: It performs almost all its work concurrently with the application threads. It uses colored pointers and load barriers to track object movement without stopping the world.
- Pros: Guaranteed pause times of less than 1ms, regardless of heap size. It can handle heaps up to 16TB.
- Cons: Slightly lower throughput (CPU overhead) compared to G1.
- Best for: Low-latency trading systems, real-time data processing, and huge memory-intensive caches.
3. The Generational ZGC (Java 21+)
The latest evolution. By introducing generations (Young/Old) to ZGC, Java 21 has significantly improved its efficiency, making it the most powerful GC in Java's history. It maintains sub-millisecond latency while drastically reducing CPU overhead for short-lived objects.
4. How to Choose?
- Scenario A: You care about maximum throughput and can tolerate 100-200ms pauses.
- Choice: G1 GC.
- Scenario B: You need extremely consistent response times (.9 < 10ms$) and have a large heap.
- Choice: ZGC.
- Scenario C: You have a small heap (< 2GB) and want the simplest setup.
- Choice: Serial GC.
5. Key Tuning Parameters
- G1:
-XX:+UseG1GC -XX:MaxGCPauseMillis=200 - ZGC:
-XX:+UseZGC -XX:+ZGenerational(Java 21+)
Summary
Garbage collection is no longer the bottleneck it once was. By choosing G1 for general-purpose workloads or ZGC for low-latency requirements, you can ensure your Java application remains responsive and efficient at any scale.
