Garbage collection (GC) is a critical component of any modern programming language runtime. It helps manage memory usage by identifying and reclaiming memory that is no longer needed by the application. One popular GC algorithm is the Shenandoah GC, developed by Red Hat.

Introduction to Shenandoah GC

Shenandoah GC is a low-pause garbage collector that aims to keep GC pause times short and predictable, even for large heaps. It achieves this by performing concurrent garbage collection work alongside the running application threads.

One of the key features of Shenandoah GC is its use of hybrid collection cycles, which combines concurrent and stop-the-world phases. This hybrid approach helps to minimize pause times and improve application responsiveness.

Hybrid Collection Cycles

Hybrid collection cycles in Shenandoah GC involve a combination of concurrent and stop-the-world phases. During the concurrent phase, the GC threads work concurrently with the application threads to find and mark live objects. This phase is designed to be concurrent and does not require stopping the application threads.

Once the concurrent phase completes, Shenandoah GC enters a stop-the-world phase known as the final evacuation pause. In this phase, the GC threads pause the application threads and evacuate the remaining live objects to a new memory location. This relocation process allows for efficient compaction of the memory.

Following the final evacuation pause, the heap is fully evacuated, and Shenandoah GC returns to a concurrent phase where the application threads resume execution. This concurrent phase might include reclaiming any remaining garbage objects and preparing the heap for future allocations.

Concurrent Work-Stealing

Another notable feature of Shenandoah GC is its use of concurrent work-stealing. During the concurrent phase, the GC threads perform concurrent marking and evacuation tasks. However, if the GC threads are falling behind, they can steal work from application threads to catch up.

Using work-stealing, the GC threads can offload some of the marking and evacuation work to idle application threads. This allows for efficient utilization of available resources and helps to keep the GC pause times short, even during high-load scenarios.

Conclusion

Shenandoah GC’s hybrid collection cycles and concurrent work-stealing techniques contribute to its ability to provide low and predictable GC pause times. By combining concurrent and stop-the-world phases, it minimizes pause times while still achieving efficient memory reclamation. Concurrent work-stealing further enhances its responsiveness by offloading GC work to idle application threads.

These features make Shenandoah GC a popular choice for applications that require low-latency, such as cloud services, big data applications, and virtual machine environments.

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