Java Memory Management with G1 GC

The Garbage-First (G1) Garbage Collector is a server-style garbage collector, introduced in Java 7 and made the default in Java 9. It is designed for multiprocessor machines with large memory, offering a good balance between throughput and pause times. This post will take you through the internals of the G1 GC, its phases, and how you can tune it for optimal performance.

A Quick Refresher on Java Garbage Collection

Before we dive into the specifics of the G1 GC, let's have a quick refresher on the basics of Java Garbage Collection. In Java, memory management is mostly automated through a process called garbage collection. The JVM automatically reclaims memory occupied by objects that are no longer in use by the application. This process prevents memory leaks and helps ensure the efficient use of memory.

The heap is the memory space where Java objects reside. The heap is typically divided into two main generations:

• Young Generation: This is where new objects are allocated. The Young Generation is further divided into an Eden space and two Survivor spaces.
• Old Generation: This is where long-lived objects are stored.

Garbage collection is the process of identifying and removing unused objects from the heap. Different garbage collectors use different algorithms to perform this task.

Introducing the G1 Garbage Collector

The G1 (Garbage-First) Garbage Collector is a generational, incremental, parallel, and mostly concurrent garbage collector. Unlike older garbage collectors that divide the heap into contiguous Young and Old generations, the G1 GC partitions the heap into a set of equal-sized heap regions.

This regionalized approach allows the G1 GC to avoid long "stop-the-world" pauses by performing garbage collection on a subset of regions at a time. The G1 GC gets its name from its strategy of collecting the "garbage-first" – that is, the regions with the most garbage.

Key Features of the G1 GC

• Parallel and Concurrent: The G1 GC uses multiple threads to scan and compact the heap, which significantly reduces pause times.
• Predictable Pause Times: The G1 GC aims to meet a user-defined pause time target, making it suitable for applications with soft real-time requirements.
• Efficiently Handles Large Heaps: The G1 GC is designed to work efficiently with large heap sizes (upwards of 4GB).
• No Manual Tuning Required: The G1 GC is designed to be self-tuning, but it can be manually tuned for optimal performance.

How G1 GC Works

The G1 GC operates in two main phases: the Young-Only phase and the Space-Reclamation phase.

Young-Only Phase

This phase starts with a few Normal young collections that promote objects to the Old generation. When the Old generation occupancy reaches a certain threshold (the Initiating Heap Occupancy threshold), the G1 GC transitions to the Space-Reclamation phase.

Space-Reclamation Phase

This phase consists of a series of mixed collections that collect both Young and Old generation regions. The Space-Reclamation phase consists of the following stages:

1. Initial Mark: This is a short "stop-the-world" pause during which the G1 GC marks the root objects.
2. Root Region Scanning: In this phase, the G1 GC scans the survivor regions for references to the Old generation.
3. Concurrent Marking: The G1 GC concurrently traverses the object graph and marks live objects.
4. Remark: This is another "stop-the-world" pause during which the G1 GC completes the marking process.
5. Cleanup: In this final phase, the G1 GC performs a "stop-the-world" cleanup of the heap, reclaiming the regions with the most garbage.

G1 GC Tuning

While the G1 GC is designed to be self-tuning, you can still tune it for optimal performance by providing a few key command-line options. Here are some of the most important options:

• -XX:+UseG1GC: This option enables the G1 Garbage Collector.
• -Xmx<size>: This option sets the maximum heap size.
• -Xms<size>: This option sets the initial heap size.
• -XX:MaxGCPauseMillis=<time>: This option sets a target for the maximum GC pause time. The G1 GC will try its best to meet this target.
• -XX:InitiatingHeapOccupancyPercent=<percent>: This option sets the heap occupancy threshold that triggers the Space-Reclamation phase.

Here's an example of how you can enable and configure the G1 GC:

java -Xmx4g -Xms4g -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:InitiatingHeapOccupancyPercent=45 -jar my-application.jar

Monitoring G1 GC

Monitoring the performance of the G1 GC is crucial for ensuring that your application is running optimally. You can use a variety of tools to monitor the G1 GC, including:

• jstat: A command-line tool that provides detailed information about the garbage collector's performance.
• Java Mission Control: A profiling and diagnostics tool that provides a wealth of information about the JVM, including the garbage collector.
• VisualVM: A visual tool that integrates several command-line JDK tools and lightweight profiling capabilities.

Conclusion

The G1 Garbage Collector is a powerful and versatile garbage collector that is well-suited for a wide range of applications. Its ability to handle large heaps with predictable pause times makes it an excellent choice for modern Java applications. By understanding how the G1 GC works and how to tune it, you can ensure that your application is running at its best.

Resources

• Getting Started with the G1 Garbage Collector
• Java Garbage Collection Handbook
• G1 Garbage Collector Tuning