Concurrent class unloading and class metadata with Shenandoah GC

10 Oct 2023

Shenandoah is a garbage collector (GC) that aims to minimize pause times by providing concurrent class unloading and class metadata operations. In this article, we will explore how Shenandoah GC handles class unloading and class metadata updates concurrently, leading to improved performance in Java applications.

What is Class Unloading?

Class unloading refers to the process of removing a class and its associated metadata from the Java virtual machine (JVM) when it is no longer needed. This typically occurs when a class is no longer used or has been dynamically loaded and is now no longer required.

Concurrent Class Unloading with Shenandoah GC

Shenandoah GC allows for concurrent class unloading, which means that the GC can remove unused classes while the application is still running. Unlike traditional stop-the-world collectors, Shenandoah GC does not require a pause to unload classes.

The concurrent class unloading mechanism in Shenandoah GC is achieved through the use of concurrent marking. Shenandoah GC performs concurrent marking to determine which classes are still in use and which can be unloaded. By doing so, it eliminates the need for a stop-the-world pause specifically for class unloading.

Class Metadata Updates

In addition to concurrent class unloading, Shenandoah GC also supports concurrent class metadata updates. Class metadata refers to the information about a class, such as its structure, fields, and methods.

Traditionally, updating class metadata involves a stop-the-world pause, as the JVM needs to ensure consistent class definitions across all running threads. However, Shenandoah GC allows for concurrent class metadata updates, which means that these updates can happen concurrently with the application execution.

Concurrent class metadata updates in Shenandoah GC are achieved using a technique called “bubbling”. Bubbling involves recording the metadata changes in a separate data structure and applying them during GC safepoints. This allows the GC to update the class metadata without interfering with the application’s execution.

Benefits of Concurrent Unloading and Class Metadata Updates

The concurrent unloading of classes and concurrent class metadata updates offered by Shenandoah GC provide several benefits:

Reduced pause times: By allowing class unloading and class metadata updates to happen concurrently, Shenandoah GC significantly reduces pause times that can impact application responsiveness.
Improved scalability: Shenandoah GC’s ability to perform these tasks concurrently eliminates the need for long stop-the-world pauses, which can lead to improved scalability in multi-threaded applications.
Increased application throughput: With reduced pause times, applications running with Shenandoah GC can achieve higher throughput by minimizing the impact of garbage collection on overall execution time.

Conclusion

Shenandoah GC’s concurrent class unloading and class metadata update features offer significant performance improvements for Java applications. By eliminating stop-the-world pauses for these operations, Shenandoah GC enhances application responsiveness, scalability, and throughput.

With its innovative approach to garbage collection, Shenandoah GC is a valuable option for applications requiring low-latency and highly responsive execution. Take advantage of concurrent class unloading and class metadata updates in Shenandoah GC to optimize the performance of your Java applications.