java - Encourage the JVM to GC rather than grow the heap?

Question

(Note that when I say "JVM", I really mean "Hotspot", and I'm running the latest Java 1.6 update.)

Example situation:

My JVM is running with -Xmx set to 1gb. Currently, the heap has 500mb allocated, of which 450mb is used. The program needs to load another 200 mb on the heap. Currently, there is 300mb worth of "collectable" garbage in the heap (we'll assume it's all in the oldest generation.)

Under normal operation, the JVM will grow the heap to 700 mb or so, and garbage collect when it gets around to it.

What I would like in that situation is for the JVM to gc first, then allocate the new stuff, so that we end up with the heap size staying at 500mb, and the used heap at 350mb.

Is there a JVM parameter combo that does that?

Answer 1

You could try specifying -XX:MinHeapFreeRatio and -XX:MaxHeapFreeRatio to control heap expansion and shrinking:

• -XX:MinHeapFreeRatio - when the percentage of free space in a generation falls below this value the generation will be expanded to meet this percentage. Default is 40.
• -XX:MaxHeapFreeRatio - when the percentage of free space in a generation exceeded this value the generation will shrink to meet this value. Default is 70.

You might also want to experiment with concurrent GC by specifying -XX:+UseConcMarkSweepGC. Depending on your application it could keep the heap size lower at the cost of additional CPU cycles.

The JVM is otherwise going to use the memory you specify as available when it's optimal do to so. You could specify a lower amount like -Xmx768m to keep it contained and it might run fine, although you would increase your risk of running out of memory in a heavy load scenario. Really the only way to use less memory overall is to write code that uses less memory :)