Question

Can someone explain how the G1 Garbage Collector works please? I haven’t been able to find any comprehensive, easy-to-understand descriptions anywhere yet.

Thanks

Answer 1

The collector splits the heap up into fixed-size regions and tracks the live data in those regions. It keeps a set of pointers — the “remembered set” — into and out of the region. When a GC is deemed necessary, it collects the regions with less live data first (hence, “garbage first”). Often, this can mean collecting an entire region in one step: if the number of pointers into a region is zero, then it doesn’t need to do a mark or sweep of that region.

For each region, it tracks various metrics that describe how long it will take to collect them. You can give it a soft real-time constraint about pause times, and it then tries to collect as much garbage as it can in that constrained time.

There is JavaOne talk about G1 and few articles on the topic:

• http://developers.sun.com/learning/javaoneonline/j1sessn.jsp
• http://www.fasterj.com/articles/G1.shtml
• http://www.drdobbs.com/java/219401061
• http://www.infoq.com/articles/G1-One-Garbage-Collector-To-Rule-Them-All