Introduction

Garbage collection is a crucial process in managed programming languages that helps reclaim memory occupied by objects that are no longer in use. One of the leading garbage collection algorithms used in modern JVMs (Java Virtual Machines) is the Shenandoah garbage collector. Shenandoah GC is designed to provide low pause times and high throughput for large heap sizes.

In this blog post, we will explore the Object Graph Tracing algorithm employed by Shenandoah GC and understand how it contributes to its efficient garbage collection process.

Object Graph Tracing

Object Graph Tracing is a technique used by garbage collectors to identify and mark objects that are still reachable (live) from the root of the object graph. The basic idea behind this algorithm is to start from the root objects, traverse their references, and recursively follow the references of the referenced objects until all reachable objects are identified.

Shenandoah GC utilizes a concurrent marking approach for Object Graph Tracing, meaning it performs the tracing process concurrently with the application’s execution. This allows it to minimize pause times and seamlessly integrate garbage collection into the normal execution of the application.

Concurrent Marking Phases

The Object Graph Tracing algorithm used by Shenandoah GC consists of several phases, each executed concurrently with the application. Let’s briefly walk through these phases:

Initial Marking

The Initial Marking phase is where Shenandoah GC identifies the root objects from which the tracing process will start. This includes objects such as thread stacks, static variables, and other GC-specific data structures. During this phase, the GC pauses the application briefly to identify these root objects.

Concurrent Marking

Once the root objects are identified, the Concurrent Marking phase begins. Shenandoah GC concurrently traverses the object graph starting from the root objects, following references and marking the reachable objects. This phase operates concurrently with the application’s execution, ensuring minimal impact on the application’s responsiveness.

Remark

During the Concurrent Marking phase, if any objects are modified or new objects are created, there is a possibility that some objects might have been missed during marking. The Remark phase is responsible for identifying and marking these missed objects. Unlike the Initial Marking phase, this phase involves a short pause to ensure accurate marking of objects.

Cleanup

After the Remark phase, the Cleanup phase takes place, where Shenandoah GC performs any necessary cleanups and prepares for the subsequent garbage collection cycle. This includes freeing memory occupied by unmarked objects and preparing data structures for the next marking cycle.

Conclusion

The Object Graph Tracing algorithm used by Shenandoah GC plays a vital role in achieving the low pause times and high throughput that Shenandoah GC is known for. By performing concurrent marking and gracefully integrating into the application’s execution, Shenandoah GC is able to minimize the impact of garbage collection on the overall performance of Java applications.

Understanding the internals of garbage collection algorithms like the Object Graph Tracing is crucial for developers and JVM practitioners to optimize memory management and ensure smooth application execution.

#garbagecollection #shenandoahgc