Shenandoah is a garbage collector in OpenJDK that aims to reduce pause times by performing certain tasks concurrently with the application threads. In this article, we will explore two important features of Shenandoah GC: Concurrent Pre-evacuation and Concurrent Pre-relocation.

What is Concurrent Pre-evacuation?

Concurrent Pre-evacuation is a concurrent phase in Shenandoah GC where the collector threads concurrently scan the heap, identify objects that need to be relocated, and update the references to those objects. This is done while the application threads are still running, reducing the overall pause times.

During Concurrent Pre-evacuation, the collector threads traverse the object graph, starting from the root objects, and identify objects that need to be relocated. The references to these objects are updated in a manner that is safe and consistent with the concurrent changes happening in other threads. This ensures that the application threads are not affected by the relocation process.

Benefits of Concurrent Pre-evacuation

Concurrent Pre-evacuation offers several benefits:

1. Reduced pause times: By performing the relocation of objects concurrently, Shenandoah GC can significantly reduce pause times, leading to better application performance and responsiveness.

2. Better throughput: By allowing the application threads to run concurrently with garbage collection tasks, Shenandoah can achieve higher throughput.

3. Improved scalability: Concurrent Pre-evacuation enables better scalability by utilizing multiple collector threads to perform the relocation of objects.

What is Concurrent Pre-relocation?

Concurrent Pre-relocation is another concurrent phase in Shenandoah GC. In this phase, the collector threads scan the heap and preemptively relocate objects that are likely to be evacuated during the evacuation phase. By relocating objects proactively, the impact on the application threads during the actual evacuation phase is minimized.

During Concurrent Pre-relocation, the collector threads identify objects that are likely to be evacuated and relocate them to a different memory location. This is done while the application threads are still running, allowing for concurrent execution and reduced pause times.

Benefits of Concurrent Pre-relocation

Concurrent Pre-relocation offers the following benefits:

1. Faster evacuation: By preemptively relocating objects that are likely to be evacuated, Shenandoah GC can reduce the time taken during the actual evacuation phase, resulting in faster overall garbage collection.

2. Lower pause times: By distributing relocation tasks across multiple collector threads, Shenandoah GC can parallelize and overlap the work, leading to reduced overall pause times.

3. Reduced interference: By relocating objects proactively, Concurrent Pre-relocation reduces the interference with the application threads during the evacuation phase, resulting in smoother execution and better performance.

In conclusion, Concurrent Pre-evacuation and Concurrent Pre-relocation are two powerful features of Shenandoah GC that enable concurrent execution of garbage collection tasks, resulting in reduced pause times, improved throughput, and better overall application performance. These features contribute to making Shenandoah GC an attractive choice for applications that require low pause times and increased scalability.

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