Shenandoah is a garbage collector (GC) that provides low pause times for large heaps in Java applications. It incorporates several techniques to achieve this goal, including concurrent weak reference discovery and concurrent cycle termination. In this blog post, we will explore these two concurrent processes in Shenandoah GC and their importance in improving the overall performance of the garbage collector.
Table of Contents
- What are Weak References?
- Concurrent Weak Reference Discovery
- Concurrent Cycle Termination
- Conclusion
What are Weak References?
Before diving into concurrent weak reference discovery and concurrent cycle termination, let’s briefly discuss what weak references are. In Java, weak references allow objects to be garbage collected when they are no longer strongly reachable, i.e., there are no strong references pointing to them. Weak references are often used in scenarios where the object’s existence is optional, and it can be safely reclaimed by the GC if memory pressure arises.
Concurrent Weak Reference Discovery
Shenandoah GC performs weak reference discovery concurrently with the application threads without any stopping-the-world pauses. The purpose of this process is to find weak references and enqueue them for further processing. By performing this task concurrently, Shenandoah GC can reduce the pause times for applications, ensuring that they continue running smoothly without unnecessary interruptions caused by garbage collection.
Concurrent weak reference discovery utilizes a “snapshot-at-the-beginning” technique, where a snapshot of the reference fields is created at the beginning of the concurrent phase. Any weak references discovered during the concurrent phase are then added to the reference queue for later processing.
Concurrent Cycle Termination
Cycle termination, also known as cycle collection or cycle breaking, is a critical step in garbage collection, especially when dealing with objects involved in cyclic references (objects that reference each other in a closed loop). Shenandoah GC performs concurrent cycle termination to detect and break these cycles while the application threads continue running.
Concurrent cycle termination employs a multi-threaded approach, with each thread independently tracing and breaking cycles. By utilizing multiple threads, Shenandoah GC is able to parallelize the cycle termination process, improving the overall efficiency and reducing the pause times.
Conclusion
Concurrent weak reference discovery and concurrent cycle termination are two vital components of Shenandoah GC. These concurrent processes allow for efficient garbage collection without significant pauses, enhancing the performance of Java applications. By leveraging these techniques, Shenandoah GC achieves its goal of minimizing pause times, enabling applications to run smoothly and responsively.
Shenandoah GC, with its concurrent algorithms and concurrent processing, demonstrates the continuous advancements in garbage collection research and technology, helping to make Java applications faster and more efficient.