How to shrink java heap space? [duplicate]

Question

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• Is there a way to lower Java heap when not in use? 7 answers

I have a Java console app that's processing big xml files using DOM. Basically it creates xml files from data it takes from the DB. Now, as you guess it's using large amount of memory but, to my surprise, it's not related to bad code but to "java heap space not shrinking". I tried running my app from Eclipse using these JVM params:

-Xmx700m -XX:MinHeapFreeRatio=10 -XX:MaxHeapFreeRatio=20

i even added the

-XX:-UseSerialGC

as i found out that parallel GC ignores "MinHeap" and "MaxHeap" options. Even with all those options graph of my app's memory use looks like this:

As you can see, at one point my app takes ~400 MB of heap space, heap grows to ~650 MB, but few seconds later (when xml generation is done) my app goes down to 12MB of used heap, but "heap size" remains at ~650 MB. It takes 650 MB of my ram! It's bizzare, don't you think?

**Is there a way to force JVS to shrink availabe heap size to, like 150% of current used heap?**Like, if my app needs 15 MB of ram, heap size is ~20MB, when my app asks for 400 MB of ram, heap grows to ~600 MB and DROPS back to ~20 MB as soon as my app finish heavy-lifting operation?

Answer 1

You should probably use Parallel collection and use -XX:-UseAdaptiveSizePolicy. Docs are for Java 1.5 but I can't find anything more recent.

The implementation of -XX:+UseAdaptiveSizePolicy used by default with the -XX:+UseParallelGC garbage collector has changed to consider three goals:

• a desired maximum GC pause goal
• a desired application throughput goal
• minimum footprint

The implementation checks (in this order):

1. If the GC pause time is greater than the pause time goal then reduce the generations sizes to better attain the goal.
2. If the pause time goal is being met then consider the application's throughput goal. If the application's throughput goal is not being met, then increase the sizes of the generations to better attain the goal.
3. If both the pause time goal and the throughput goal are being met, then the size of the generations are decreased to reduce footprint.

EDIT

Added "-" per OP's suggestion.