Shenandoah: A pauseless GC for OpenJDK

June 10, 2013

Update: It seems like this post gets frequently linked to, but is fairly old. For current information on Shenandoah GC, please refer to the Shenandoah Wiki.

It’s been quite a while that I did not post anything. The reason is that I was busy with a very interesting new project in the last weeks, Shenandoah, a new GC for OpenJDK. Christine, a collegue of mine in Red Hat’s Java team, set out to build a next generation garbage collector for OpenJDK. I jumped into this project a while ago, and I’m very excited about it.

I guess I will not go into technical details in this post, but instead outline a few of the problems of existing garbage collectors, and Shenandoah’s goals and some highlevel ideas how to achieve them.

Pause times: this problem existed since the invention of GCs, and hasn’t been fully solved since then. The good old stop-the-world garbage collection is probably still in everybody Java programmers mind. Nowadays we have more modern garbage collectors, and most of them drastically reduce pause times, but nevertheless, pause times still exist, be it for evacuation of regions, for book keeping of references, etc. Shenandoah aims to reduce pause times even more. Realistically speaking, there will still be pause times, but we aim to minimize them as much as possible. Our goal is to reduce them to less than 10ms per GC cycle.

Scalability. The larger the heap gets, and the more processors/threads are working, the more garbage is produced. This means more work for the garbage collector to find and free all this garbage. In other words, the garbage collector needs to be able to keep up with ever-growing memory demands and processor capability. We are shooting for TB-sized heaps.

The idea now is that, in order to reduce pause times, the GC needs to do as much work as possible concurrently to mutator threads (mutators are all threads that modify the heap, i.e. the actual application running). In order to be scalable, the GC needs to be able to utilize that ever-growing processing power by doing as much work as possible using multiple threads in parallel. (Notice that the terminology is a bit confusing here. In the Java GC world, ‘concurrent’ often means ‘concurrently with mutators’ while ‘parallel’ means ‘using parallel worker threads to do GC work’).

While that sounds conceptually simple, I can tell you it’s not. Boy, are there a lot of hairy issues to resolve here. I think I will explain the ideas one blog post at a time here. Stay tuned!