In this blog post, we will explore the concurrent thread roots scanning and concurrent mark update techniques used in the Shenandoah garbage collector (GC). The Shenandoah GC is an open-source garbage collector designed for low pause times and large heaps. It is developed by the Java performance team at Red Hat.

1. Introduction to Shenandoah GC

The Shenandoah GC is a garbage collector that operates concurrently with the application threads, aiming to keep GC pauses short and predictable, even with large heaps. It is designed to reduce the impact of pause times on user applications, making it suitable for latency-sensitive workloads.

2. Concurrent Thread Roots Scanning

In traditional garbage collectors, the scanning of thread roots (such as thread local variables and stack frames) is performed during a stop-the-world pause. However, in Shenandoah GC, this scanning is done concurrently with the application threads, allowing the application to continue running during the scan.

The concurrent thread roots scanning process in Shenandoah GC involves:

1. Initialization: The GC thread starts by scanning the root set. It identifies the application threads’ root objects and creates a work queue for each thread.
2. Thread Local Scanning: Each application thread concurrently scans its own thread-local variables and pushes the discovered objects to its work queue.
3. Stack Frame Scanning: The GC thread periodically scans the stack frames of the application threads. It identifies the objects referenced by the stack frames and pushes them to the corresponding thread’s work queue.
4. Work Queue Processing: The GC thread processes the work queues in parallel, visiting the objects and marking them as live.

By performing the thread roots scanning concurrently, Shenandoah GC avoids lengthy pause times and keeps the application threads running for most of the collection cycle.

3. Concurrent Mark Update

In addition to concurrent thread roots scanning, Shenandoah GC also employs concurrent mark update. In traditional garbage collectors, marking objects involves stopping the application threads and recursively traversing the object graph to identify live objects. Shenandoah GC, however, performs marking concurrently with the application threads, minimizing pause times.

The concurrent mark update process in Shenandoah GC involves:

1. Initialization: The GC thread starts by creating a marking bitmap to track the marking progress.
2. Initial Marking: The GC thread performs the initial marking concurrently with the application threads. It marks the objects directly reachable from the root set and stores the marked objects in remembered sets.
3. Concurrent Marking: The GC thread continues to mark objects concurrently with the application threads. It traverses the object graph, following references and marking objects as it goes.
4. Final Marking: The GC thread performs a final marking step, ensuring that all objects reachable from the root set are properly marked. It also finalizes any remaining marking work.
5. Concurrent Sweep: After the marking phase is complete, the Shenandoah GC performs a concurrent sweep to reclaim the memory occupied by the unmarked objects.

By performing the marking process concurrently, Shenandoah GC reduces pause times and allows the application to continue running with minimal disruption.

Conclusion

Concurrent thread roots scanning and concurrent mark update are key techniques used in the Shenandoah GC to achieve low pause times and efficient garbage collection. By scanning thread roots concurrently with the application threads and marking objects concurrently, Shenandoah GC minimizes pause times and ensures smooth application performance.

To learn more about Shenandoah GC and its implementation, you can visit the official Shenandoah GC GitHub repository.

#Java #GarbageCollection