Shenandoah is a low-pause garbage collector introduced in OpenJDK to reduce the impact of garbage collection pauses on application performance. One key aspect of Shenandoah GC is its concurrent marking phase, which enables garbage collection to be performed concurrently with the application’s execution.
What are Concurrent Mark References?
Concurrent Mark References (CMR) is a technique used by Shenandoah GC to track object references without the need for stopping the application threads during the marking phase. CMR allows the garbage collector to maintain an up-to-date view of the object graph without impacting the application’s execution.
During the CMR phase, Shenandoah GC scans the object graph starting from the roots (static variables, thread stacks, and JNI global references) and follows object references to determine the reachable objects. This process is performed concurrently with the application threads, which ensures that the application can continue its execution without significant pauses.
Benefits of Concurrent Mark References
The use of CMR in Shenandoah GC provides several benefits:
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Reduced Pause Times: By performing the marking phase concurrently, Shenandoah GC significantly reduces the pause times experienced by the application. This allows applications to maintain low-latency and responsiveness, making it ideal for latency-sensitive use cases.
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Improved Throughput: With shorter pause times, applications can achieve higher overall throughput as more time is spent executing the application code.
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Scalability: Concurrent mark references allow Shenandoah GC to scale efficiently with increasing heap sizes and thread counts, making it suitable for large-scale applications.
Concurrent Phase Scheduling in Shenandoah GC
Concurrent phase scheduling is an additional feature of the Shenandoah GC that allows the collector to overlap the different phases of the garbage collection cycle. It aims to further reduce the pause times and minimize the impact on application performance.
The phases in Shenandoah GC include the initial-mark, concurrent-mark, final-mark, and concurrent-evacuation. These phases are typically executed in sequential order. However, with concurrent phase scheduling, the collector can start overlapping the concurrent-mark phase with the initial-mark phase, and the concurrent-evacuation phase with the final-mark phase. This overlap helps in reducing the overall garbage collection pause time.
Conclusion
Concurrent mark references and concurrent phase scheduling are key techniques employed by Shenandoah GC to achieve low-pause garbage collection in OpenJDK-based applications. By performing the marking phase concurrently with the application’s execution and overlapping different phases, Shenandoah GC minimizes the impact on application performance and ensures low-latency and responsiveness.
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