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

Garbage Collection (GC) in Java is the automatic process of identifying and removing objects from memory that are no longer in use, thereby freeing heap space. While basic GC concepts cover simple memory cleanup, modern Java includes advanced garbage collectors such as G1, ZGC, and Shenandoah, each designed to address scalability, latency, and performance challenges in different ways.

1. G1 (Garbage First) Garbage Collector

G1 is designed for applications that require predictable pause times and handle large heap sizes (typically several gigabytes or more).

Key Concepts:

• The heap is divided into multiple equal-sized regions instead of fixed generations (like Young and Old).

• Regions can belong to Eden, Survivor, or Old spaces dynamically.

• G1 prioritizes collecting regions with the most garbage first, hence the name “Garbage First.”

Working Mechanism:

• It performs both minor and major garbage collection cycles.

• Uses a concurrent marking phase to identify live objects across the heap.

• After marking, it selects regions with high garbage percentage for cleanup.

• Compacts memory during evacuation to reduce fragmentation.

Advantages:

• Predictable pause times through user-defined goals.

• Efficient for large heap applications.

• Reduces fragmentation through compaction.

Limitations:

• Slightly higher CPU overhead due to concurrent operations.

• Not always optimal for very low-latency applications.

2. ZGC (Z Garbage Collector)

ZGC is designed for ultra-low latency and scalability, capable of handling very large heaps (even terabytes) with minimal pause times.

Key Concepts:

• Region-based memory layout called ZPages.

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

• Almost all GC phases run concurrently with the application.

Working Mechanism:

• Performs marking, relocation, and remapping concurrently.

• Uses load barriers to ensure that application threads always see the correct object references.

• Pause times are typically less than 10 milliseconds, regardless of heap size.

Advantages:

• Extremely low pause times.

• Scales efficiently with large heaps.

• Minimal impact on application responsiveness.

Limitations:

• Higher memory overhead due to metadata and pointer coloring.

• Requires modern hardware and newer Java versions.

3. Shenandoah Garbage Collector

Shenandoah is another low-latency garbage collector focused on minimizing pause times by performing most work concurrently.

Key Concepts:

• Concurrent compaction: unlike many collectors, it compacts memory while the application is running.

• Uses Brooks pointers (forwarding pointers) to manage object movement safely.

Working Mechanism:

• Performs marking, evacuation, and updating references concurrently.

• Pause times are independent of heap size.

• Reduces long pauses by avoiding stop-the-world compaction phases.

Advantages:

• Consistent low pause times.

• Reduces memory fragmentation.

• Suitable for latency-sensitive applications.

Limitations:

• Slightly reduced throughput due to concurrent processing.

• Additional memory overhead for forwarding pointers.

Comparison Overview

When to Use Each

• Use G1 when you want a balance between throughput and pause time, especially for general-purpose server applications.

• Use ZGC when your application requires extremely low latency and runs on large heaps.

• Use Shenandoah when you want low pause times with concurrent compaction but may not need the extreme scalability of ZGC.

Conclusion

Modern garbage collectors like G1, ZGC, and Shenandoah represent a shift from simple memory cleanup to highly optimized, concurrent systems designed for large-scale and real-time applications. Choosing the right GC depends on application requirements such as latency sensitivity, heap size, and throughput expectations.