Practical Garbage Collection

Observing garbage collector behavior

I encourage readers to experiment with the behavior of the garbage collector. Use jconsole (comes with the JDK) or VisualVM (which produced the graph earlier on in this post) to visualize behavior on a running JVM. But, in particular, start getting familiar with garbage collection log output, by running your JVM with (updated with jbellis’ feedback – thanks!):

• -XX:+PrintGC
• -XX:+PrintGCDetails
• -XX:+PrintGCDateStamps
• -XX:+PrintGCApplicationStoppedTime
• -XX:+PrintPromotionFailure

Also useful but verbose (meaning explained in later posts):

• -XX:+PrintHeapAtGC
• -XX:+PrintTenuringDistribution
• -XX:PrintFLSStatistics=1

The output is pretty easy to read for the throughput collector. For CMS and G1, the output is more opaque to analysis without an introduction. I hope to cover this in a later update.

In the mean time, the take-away is that those options above are probably the first things you want to use whenever you suspect that you have a GC related problem. It is almost always the first thing I tell people when they start to hypothesize GC issues; have you looked at GC logs? If you have not, you are probably wasting your time speculating about GC.