Java heap won't shring

Question

java version "1.6.0_11"
Java(TM) SE Runtime Environment (build 1.6.0_11-b03)
Java HotSpot(TM) 64-Bit Server VM (build 11.0-b16, mixed mode)
I have an application that sees bursts of activity and long periods of no work. Recently I checked and the memory usage was 104,856,816 (used)/ 3,145,924,608 (total). Why didn't the jvm shrink the heap? The default value of MaxHeapFreeRatio is supposed to be 70.

Answer 1

The HotSpot JVM prefers to leave the heap expanded, rather than shrinking it to your current usage and then having to expand it back up when work comes in. So there's some hysteresis built into the heap shrinking algorithm. If your heap usage stayed small across several full collections, I would expect the heap to shrink. On the other hand, if you aren't doing any work, then I wouldn't expect there to be any full collections.
You don't say what your command line parameters are. If, for example you used "-Xms3g", you've told the JVM that you want the heap to start at and shrink no smaller than 3GB. In that case, I would expect the heap to start out at 3GB and stay at least that large no matter how little of it you were using at any time.