Java Garbage Collection: Comparing Serial, Parallel, CMS, G1, ZGC, and Shenandoah

by marjavamitjava · July 30, 2024

Here are detailed descriptions of the different types of Java garbage collectors, including some technical differences:

Description: The simplest garbage collector implementation in Java, using a single thread for all garbage collection activities.
Technical Differences: It performs all garbage collection tasks in a stop-the-world manner, pausing all application threads. It’s best suited for small applications with a single processor, as it doesn’t benefit from multiple CPUs. The serial nature makes it less efficient for high-throughput or low-latency requirements.

Description: Uses multiple threads to speed up garbage collection, aiming to reduce pause times in multi-threaded applications.
Technical Differences: Also known as the “throughput collector,” it uses multiple threads for both minor and major garbage collection phases. It prioritizes high throughput by maximizing the amount of time spent by the application and minimizing the overhead of garbage collection. It can lead to longer pause times compared to low-latency collectors but is efficient for CPU-intensive applications.

Description: A low-latency garbage collector that performs most of its work concurrently with the application threads to minimize pause times.
Technical Differences: The CMS collector uses multiple threads to perform the marking and sweeping phases concurrently with application execution. However, it still has stop-the-world phases during the initial mark and final remark. It can suffer from fragmentation issues because it doesn’t compact the heap, which can lead to degraded performance over time.

Description: Designed for large heap applications, G1 divides the heap into regions and focuses on collecting the regions with the most garbage first.
Technical Differences: The G1 GC uses a region-based memory layout and performs both concurrent and parallel garbage collection. It aims to achieve predictable pause times by controlling the amount of work done during each collection cycle. G1 also compacts the heap to reduce fragmentation, making it suitable for applications with large heaps that require predictable performance.

Description: A low-latency garbage collector that aims to handle very large heaps (multi-terabyte) with minimal pause times.
Technical Differences: ZGC is a concurrent, region-based collector designed to maintain pause times under 10 milliseconds, regardless of the heap size. It uses load barriers and colored pointers to manage object references and perform most garbage collection tasks concurrently with application threads. ZGC is ideal for applications requiring very low latency and capable of handling large heaps efficiently.

Description: Another low-latency garbage collector, Shenandoah aims to provide very short pause times independent of heap size.
Technical Differences: Shenandoah performs concurrent garbage collection and compaction. It uses region-based heap management and concurrent evacuation, meaning it moves objects while the application is running. Shenandoah aims to keep pause times consistent and low, even as heap sizes grow, making it suitable for applications with real-time requirements.

For Low Latency: ZGC and Shenandoah GC are considered the best options for applications requiring minimal pause times due to their advanced concurrent collection techniques.
For High Throughput: Parallel GC is often chosen for applications where maximizing throughput is more important than minimizing pause times, due to its efficient use of multiple threads during garbage collection.
General Purpose: G1 GC is a good all-around choice, balancing low pause times with good throughput, making it suitable for a wide range of applications. Its region-based approach and concurrent phases make it versatile for various use cases.