In this blog post, we will delve into the concurrent and parallel phases in the Shenandoah Garbage Collector (GC) algorithm. Shenandoah is an advanced garbage collector designed for low-pause time and high throughput in OpenJDK. It aims to reduce the GC pause time to a minimum to maintain smooth application performance. To achieve this, Shenandoah GC divides its work into concurrent and parallel phases.
Concurrent Phases
The concurrent phase in Shenandoah GC is responsible for performing garbage collection concurrently with application threads, thereby minimizing the pause time. During this phase, the Shenandoah GC algorithm performs several tasks, including:
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Initial Mark - This phase quickly scans the heap to identify reachable objects and mark them as live. To do this, Shenandoah uses a combination of read and write barriers to track object references efficiently.
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Concurrent Mark - After the initial mark, Shenandoah continues scanning the heap and marking objects concurrent to the application threads. It employs a technique known as “snapshot-at-the-beginning” to capture a consistent state of object references.
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Concurrent Preclean - During this phase, Shenandoah concurrently processes objects that were marked grey (reachable but with pending references) in the concurrent mark phase. The goal is to remove redundant references early to reduce work during the evacuation phase.
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Final Mark - In this phase, Shenandoah takes a forced pause to complete the marking concurrently. It ensures that all references are marked correctly before proceeding to the evacuation phase.
Parallel Phases
In addition to the concurrent phases, Shenandoah GC includes parallelized phases to further optimize the garbage collection process. These phases take advantage of multiple threads to achieve higher throughput. The parallel phases in Shenandoah GC include:
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Parallel Evacuation - Once the concurrent phases complete, Shenandoah enters the parallel evacuation phase. In this phase, it moves live objects from the regions being collected to empty regions, thereby ensuring efficient memory compaction. This phase is performed in parallel across multiple threads, improving the overall garbage collection speed.
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Parallel Cleanup - After the parallel evacuation phase, Shenandoah performs a parallel cleanup to finalize the garbage collection process. It does the necessary cleanup tasks, such as returning empty regions to the system, resetting data structures, and preparing for the next garbage collection cycle.
Conclusion
Understanding the concurrent and parallel phases in the Shenandoah GC algorithm is crucial for comprehending how it achieves low pause times and high throughput. By leveraging concurrent marking and parallel evacuation, Shenandoah minimizes the impact of garbage collection on application performance. This makes it an excellent choice for modern Java applications that require both responsiveness and efficient memory management.
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