In modern applications, off-heap memory usage is becoming increasingly common to optimize performance and reduce the impact of garbage collection on the application’s throughput. The Shenandoah GC, introduced in JDK 12, is a low-pause garbage collector that provides advanced features to handle off-heap memory efficiently. In this article, we will explore how to effectively manage off-heap memory usage with the Shenandoah GC.

Understanding off-heap memory

Off-heap memory refers to memory that is allocated outside of the Java heap managed by the garbage collector. It is typically used for storing large amounts of data or for accessing native resources using Java Native Interface (JNI). Off-heap memory is not subject to Java’s garbage collection, making it a suitable choice for scenarios where frequent garbage collection pauses are undesirable.

Shenandoah GC and off-heap memory

The Shenandoah GC provides a special heap region called the “Shenandoah Heap.” This heap region is specifically designed to handle off-heap memory allocation and deallocation. Applications can allocate off-heap memory directly from the Shenandoah Heap, which enables the garbage collector to track and manage the off-heap memory usage effectively.

To use the Shenandoah Heap for off-heap memory allocation, you can follow these steps:

1. Enable the Shenandoah GC by adding the -XX:+UseShenandoahGC flag to your JVM command line.
2. Allocate off-heap memory using the Unsafe class or any other method that allows direct memory allocation, specifying the Shenandoah Heap as the memory source.
3. Deallocation of the off-heap memory is handled automatically by the Shenandoah GC when the memory is no longer referenced.

It’s important to note that the Shenandoah Heap has its own memory limits, separate from the Java heap. You can configure the maximum size of the Shenandoah Heap using the -XX:+ShenandoahAllocLimit flag.

Monitoring off-heap memory usage

To monitor the off-heap memory usage in your application, you can use the following command-line flags:

• -XX:ShenandoahAllocSiteSamplingInterval=N: This flag enables allocation-site sampling, which provides information about off-heap memory allocations. Replace N with the desired sampling interval.
• -XX:PrintShenandoahAllocSiteStats: This flag prints statistics about off-heap memory allocations and deallocations.

By monitoring these statistics, you can get insights into how your application is utilizing off-heap memory and identify any potential issues or areas for optimization.

Conclusion

Effectively managing off-heap memory usage is essential for optimizing performance in modern applications. With the Shenandoah GC, Java developers can take advantage of its built-in support for allocating and deallocating off-heap memory efficiently. By enabling the Shenandoah Heap and monitoring off-heap memory usage, developers can ensure optimal performance and reduce garbage collection pauses in their applications.

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