Understanding Java Garbage Collection: A Guide to ZGC and Generational Mode

Garbage collection (GC) is a fundamental component of memory management in Java, automating the process of reclaiming memory occupied by objects that are no longer in use. This allows Java applications to run efficiently without needing to manually manage memory allocation and deallocation. As Java has evolved, so too have its garbage collectors, with the Z Garbage Collector (ZGC) representing one of the most advanced GC options available.

In this article, we’ll explore the basics of Java garbage collection, the key features of ZGC, and the recent introduction of Generational Mode by Default in ZGC, as outlined in JEP 474.

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What is Garbage Collection in Java?

Garbage collection in Java is a process by which the Java Virtual Machine (JVM) identifies and discards objects that are no longer needed, freeing up memory for future use. This process is crucial in long-running applications to prevent memory leaks and ensure optimal resource usage.

Java provides several garbage collection algorithms, each with different characteristics:

  • Serial GC – A simple, single-threaded garbage collector suitable for small applications.
  • Parallel GC – A multi-threaded collector that speeds up the GC process by utilizing multiple threads.
  • Concurrent Mark-Sweep (CMS) – A collector that aims to reduce pauses by performing most of its work concurrently.
  • Garbage-First (G1) GC – A collector designed for low-latency and high-throughput applications.
  • Z Garbage Collector (ZGC) – A low-latency garbage collector designed to handle large heaps with minimal pause times.

Introduction to the Z Garbage Collector (ZGC)

ZGC is a modern garbage collector introduced in Java 11, aimed at applications requiring low latency and high throughput. ZGC is particularly advantageous in environments with large heaps, offering pause times in the range of just a few milliseconds, regardless of heap size. Its low pause times make ZGC ideal for latency-sensitive applications, such as financial services, gaming, and web applications.

Key Features of ZGC:

  • Concurrent Processing: ZGC does most of its work concurrently, which minimizes pause times.
  • Region-based Memory Management: ZGC divides the heap into regions, making it easier to handle and optimize memory.
  • Colored Pointers: ZGC uses a unique approach to track object references through colored pointers, allowing it to manage memory without “stop-the-world” pauses.

ZGC Generational Mode by Default: What It Means for Java

Generational garbage collection is based on the observation that most objects in a Java application are short-lived. By organizing objects by their lifespan, generational garbage collectors can optimize memory management and reduce the frequency of garbage collection events for long-lived objects.

With JEP 474, ZGC introduces a Generational Mode by Default in Java 21, making it a generational garbage collector. This approach allows ZGC to separate objects into young and old generations, optimizing garbage collection based on object life expectancy.

Benefits of Generational Mode in ZGC:

  1. Improved Throughput: By collecting short-lived objects more frequently, ZGC can allocate more resources to managing long-lived objects, improving overall efficiency.
  2. Reduced Memory Fragmentation: Generational mode helps reduce memory fragmentation by grouping similarly aged objects, which in turn improves memory locality and allocation speed.
  3. Better Heap Utilization: Generational GC helps ZGC make better use of the heap, making it more efficient even as memory demand increases.

How Generational ZGC Affects Application Performance

The introduction of generational mode by default in ZGC impacts Java application performance in several key ways:

  1. Lower Latency for Short-Lived Objects: Generational ZGC can quickly reclaim memory from objects in the young generation, reducing latency for frequently allocated objects.
  2. Longer Lifetime for Old Generation: With generational ZGC, long-lived objects don’t need to be checked as frequently, reducing the processing load on the collector and potentially extending application uptime.
  3. Efficient Memory Reclamation: Generational mode allows ZGC to optimize memory usage by targeting specific generations, leading to a more balanced and effective garbage collection process.

Configuring ZGC in Java

To use ZGC in generational mode, you can simply enable it with the JVM option:

				
					java -XX:+UseZGC -XX:+ZGenerational

				
			

Java 21 automatically sets ZGC to generational mode by default, so no extra configuration is needed if you are running on this version or later.

Conclusion

ZGC has transformed Java garbage collection by providing a low-latency solution that can scale with high-memory applications. With the generational mode introduced in JEP 474, ZGC combines the best of both worlds: the efficiency of generational garbage collection with ZGC’s hallmark low pause times. For developers building high-performance, latency-sensitive applications, ZGC in generational mode offers a powerful tool for effective memory management and enhanced performance.

 

 

Understanding Java Garbage Collection: A Guide to ZGC and Generational Mode
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