ZGC: A Scalable Low-Latency Garbage Collector

ZGC (Z Garbage Collector) is a scalable, low-latency garbage collector introduced in Java 11 (JEP 333) and production-ready in Java 15. Designed for applications requiring minimal pause times and handling large heaps (multi-terabyte), ZGC achieves pause times typically under 10ms, regardless of heap size, making it ideal for latency-sensitive systems. It’s enabled with -XX:+UseZGC.

Key Features

• Low Latency: Pause times are sub-millisecond to low milliseconds, even for large heaps, due to concurrent operations.
• Scalability: Handles small (MBs) to massive (TBs) heaps efficiently.
• Concurrent Processing: Performs most GC tasks (marking, compaction, relocation) concurrently with the application, minimizing pauses.
• Color-Pointers: Uses load barriers and colored pointers to track object references, enabling concurrent memory management.
• Heap Size Independence: Pause times don’t scale with heap size or live object count, unlike traditional GCs.
• Memory Efficiency: Supports up to 16TB heaps with minimal overhead (requires 64-bit JVM).

How ZGC Works

ZGC divides its work into concurrent and pause phases:

• Concurrent Phases:
  • Marking: Identifies live objects by traversing the object graph concurrently.
  • Relocation: Compacts the heap by moving objects to reduce fragmentation, done concurrently.
  • Reference Processing: Updates object references to reflect relocated objects.
• Pause Phases:
  • Brief pauses (e.g., for root scanning or marking termination) are kept minimal and don’t scale with heap size.
• Load Barriers: Instead of stop-the-world pauses, ZGC uses load barriers to manage object access, ensuring consistency during concurrent operations.

Configuration

• Enable ZGC: -XX:+UseZGC
• Set heap size: -Xmx<size> (e.g., -Xmx16g for 16GB).
• Optional tuning:
  • -XX:ConcGCThreads=<n>: Number of concurrent GC threads.
  • -XX:ZCollectionInterval=<seconds>: Controls proactive GC cycle frequency.
• Platform requirements: 64-bit JVM; supported on Linux, Windows, macOS.

Use Cases

1. Latency-Sensitive Applications: Ideal for real-time systems like financial trading platforms, gaming servers, or microservices requiring consistent response times.
2. Large-Scale Systems: Suited for big data applications (e.g., Spark, Hadoop) with multi-terabyte heaps.
3. Cloud Environments: Optimizes containerized workloads where low latency and high throughput are critical.
4. High-Throughput Services: Web servers or APIs handling millions of requests benefit from reduced GC pauses.