Java - Java Garbage Collection Algorithms (G1, ZGC, Shenandoah)

 

Garbage collection in Java is the automatic process of identifying and removing objects from memory that are no longer in use, thereby freeing up space in the heap. This eliminates the need for manual memory management and helps prevent memory leaks and crashes. Over time, Java has evolved to include several advanced garbage collection algorithms designed to improve performance, reduce pause times, and handle large-scale applications efficiently.

Basics of Garbage Collection

Java uses a generational heap model, which divides memory into different regions:

  • Young Generation: Where new objects are allocated

  • Old Generation: Where long-lived objects are moved

  • Metaspace: Stores class metadata

Most garbage collection activity happens in the Young Generation, where objects are frequently created and destroyed. Objects that survive multiple collection cycles are promoted to the Old Generation.

G1 Garbage Collector (Garbage First)

The G1 collector is designed for applications that require predictable performance and moderate pause times. It divides the heap into many small regions instead of fixed generations. These regions can belong to either young or old generations dynamically.

Key features of G1:

  • Region-based memory management allows more flexible allocation

  • Performs garbage collection in parallel and concurrently with application threads

  • Prioritizes regions with the most garbage, hence the name “Garbage First”

  • Aims to meet user-defined pause time goals

G1 is suitable for large heap sizes and applications where consistent response time is important, such as web servers and enterprise systems.

Z Garbage Collector (ZGC)

ZGC is a low-latency garbage collector designed to keep pause times extremely short, typically in the range of a few milliseconds, regardless of heap size.

Key features of ZGC:

  • Concurrent garbage collection, meaning most of the work happens while the application is running

  • Uses colored pointers to track object states without stopping application threads

  • Supports very large heaps, even in terabytes

  • Pause times are independent of heap size

ZGC is ideal for applications that require very low latency, such as real-time systems, financial trading platforms, and large-scale cloud services.

Shenandoah Garbage Collector

Shenandoah is another low-pause-time garbage collector that focuses on reducing pause times by performing most of its work concurrently with the application.

Key features of Shenandoah:

  • Concurrent compaction, which reduces memory fragmentation without long pauses

  • Pause times are short and consistent

  • Designed to handle large heaps efficiently

  • Works by evacuating live objects while the application continues running

Shenandoah is suitable for applications where responsiveness is critical and long pauses are unacceptable.

Comparison of G1, ZGC, and Shenandoah

  • G1 focuses on balancing throughput and pause time, making it a general-purpose collector

  • ZGC prioritizes ultra-low latency with minimal pause times, even for very large heaps

  • Shenandoah also aims for low pause times but uses a different approach with concurrent compaction

In simple terms, G1 is widely used for general applications, while ZGC and Shenandoah are preferred when minimizing latency is the top priority.

Performance Considerations

Choosing the right garbage collector depends on several factors:

  • Heap size: Larger heaps may benefit from ZGC or Shenandoah

  • Application type: Real-time applications need low latency collectors

  • Throughput requirements: Some collectors may reduce overall throughput to achieve lower pause times

  • Hardware resources: Advanced collectors may require more CPU for concurrent processing

Tuning parameters such as heap size, pause time goals, and thread counts can significantly impact performance.

Challenges and Limitations

  • More advanced collectors like ZGC and Shenandoah may not be fully supported in older Java versions

  • They can consume more CPU due to concurrent processing

  • Improper configuration can lead to performance degradation

  • Monitoring and tuning require a deeper understanding of JVM internals

Conclusion

Java garbage collection algorithms have evolved to meet the needs of modern applications. G1 provides a balanced approach suitable for most use cases, while ZGC and Shenandoah are designed for systems where low latency is critical. Understanding how these collectors work helps developers choose the right strategy and optimize application performance effectively.