In this blog post, we will discuss the concurrent cycle start and the concurrent cycle termination in the Shenandoah garbage collector (GC). The Shenandoah GC is an open-source garbage collector developed by Red Hat that aims to provide low pause times for large heaps.
What is Concurrent Cycle Start?
Concurrent cycle start is a phase in the Shenandoah GC where the concurrent marking of live objects begins. During this phase, the GC starts scanning the object graphs to identify live objects. The concurrent cycle start is performed while the application continues to execute, which helps reduce the pause time experienced by the application.
How Concurrent Cycle Start Works
- Root Scanning: The GC starts by scanning the root objects, which includes thread stacks, static variables, and JNI references.
- Object Scan: After scanning the roots, the GC scans the objects in the heap that were marked during the previous cycle.
- Concurrent Object Update: As the GC scans the objects, the application may also be modifying them concurrently. To ensure consistent marking, the GC uses special algorithms like snapshot-at-the-beginning (SATB) to handle concurrent updates.
What is Concurrent Cycle Termination?
Concurrent cycle termination is the phase in the Shenandoah GC where the concurrent marking of live objects ends, and the heap is ready for reclamation of garbage objects. This phase is crucial for ensuring the correctness of the garbage collection process and preparing the heap for the next cycle.
How Concurrent Cycle Termination Works
- Final Traversal: After the marking phase, the GC performs a final traversal of the object graph to catch any missed objects due to concurrent updates.
- Concurrent Object Update Freeze: To ensure consistency, the GC freezes object updates before proceeding with the termination phase.
- Release Abandoned Objects: The GC releases objects that are no longer live, reclaiming memory and preparing it for future allocations.
- Safepoint: Finally, the GC waits for all threads in the application to reach a safepoint to ensure that no object updates are in progress.
Conclusion
The Shenandoah GC’s concurrent cycle start and concurrent cycle termination play a vital role in achieving low pause times for garbage collection. By allowing the GC to scan objects and update them concurrently with the application, the Shenandoah GC minimizes the pause time experienced by the application, making it suitable for latency-sensitive workloads.
By understanding these phases, developers can appreciate the sophisticated techniques employed by the Shenandoah GC to provide efficient garbage collection in large heap scenarios.