Summary

This should give one a sufficient basic understanding of how G1 functions. There are, of course, still quite some implementation details that we have left out for brevity, like dealing with humongous objects. All things considered, G1 is the most technologically advanced production-ready collector available in HotSpot. On top of that, it is being relentlessly improved by the HotSpot Engineers, with new optimizations or features coming in with newer java versions.

As we have seen, G1 addressed a wide range of problems that CMS has, starting from pause predictability and ending with heap fragmentation. Given an application not constrained by CPU utilization, but very sensitive to the latency of individual operations, G1 is very likely to be the best available choice for HotSpot users, especially when running the latest versions of Java. However, these latency improvements do not come for free: throughput overhead of G1 is larger thanks to the additional write barriers and more active background threads. So, if the application is throughput-bound or is consuming 100% of CPU, and does not care as much about individual pause durations, then CMS or even Parallel may be better choices.

The only viable way to select the right GC algorithm and settings is through trial and errors, but we do give the general guidelines in the next chapter.

Note that G1 will probably be the default GC for Java 9: http://openjdk.java.net/jeps/248