The Shenandoah garbage collector (GC) is a low-pause, concurrent, and region-based garbage collector designed for OpenJDK. It aims to reduce the pause times experienced by applications during garbage collection by allowing concurrent execution of the relocation phase.
Relocation is an essential step in garbage collection, where live objects are moved to new memory locations to reclaim memory occupied by dead objects. Shenandoah GC introduces two techniques to improve the efficiency of the relocation phase: Concurrent Relocation and Parallel Relocation.
Concurrent Relocation
Concurrent Relocation in Shenandoah GC allows the relocation phase to be executed concurrently with the application threads, reducing the duration of garbage collection pauses. This is achieved by utilizing a dedicated set of threads to perform relocation work in parallel with the mutator threads.
During the concurrent relocation phase, the GC threads scan the heap, identify live objects, and move them to new memory locations. The mutator threads continue running concurrently, allowing the application to make progress while the relocation is happening. This concurrent approach significantly reduces the impact of garbage collection pauses on application performance.
Parallel Relocation
Parallel Relocation takes the performance of Concurrent Relocation a step further by introducing parallelism to the relocation process. In Shenandoah GC, parallel relocation is achieved by dividing the heap into multiple regions and assigning individual GC threads to relocate objects in parallel.
Each GC thread is responsible for relocating objects in a specific region, and they work in parallel with other GC threads. This parallelization of relocation helps to better utilize the available CPU resources and further reduces the time required for garbage collection pauses.
Benefits of Concurrent and Parallel Relocation
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Reduced Pause Times: Concurrent relocation in Shenandoah GC enables the garbage collection process to happen concurrently with application execution, minimizing the impact of pauses on the application’s responsiveness and performance.
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Improved Throughput: By introducing parallel relocation, Shenandoah GC distributes the relocation work among multiple threads, effectively utilizing available CPU resources and enhancing the overall throughput of the garbage collection process.
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Better Scalability: With concurrent and parallel relocation, Shenandoah GC scales well with the increasing size of heaps and the number of available CPU cores. It allows efficient utilization of resources in multi-core environments, minimizing the impact of garbage collection on the overall system performance.
Shenandoah GC’s concurrent and parallel relocation techniques offer significant benefits in reducing pause times and improving the performance of garbage collection in Java applications. By harnessing the power of concurrent and parallel execution, Shenandoah GC enables applications to run smoothly even under high memory pressure and heavy workloads.
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