问题
I'm currently monitoring my running java application with Visual VM: http://visualvm.java.net/
I'm stressing the memory usage by with -Xmx128m.
When running I see the heap size increasing to 128m (as expected) however the used heap converges to approximately 105m before I run into a java heap space error.
Why are these remaining 20m, not used?
回答1:
The heap is split up in Young-Generation (Eden-Space, and two Survivor-Spaces of identical size usually called From and To), Old Generation (Tenured) and Permanent Space.
The Xmx/Xms option sets the overall heap size. So a region (with a default size) is actually the Permanent Space - and maybe, we don't know details about your stress test, no objects are actually moved from eden to tenured or permanent, so those regions remain empty while Eden runs out of space.
回答2:
You need to understand a central fact about garbage collector ergonomics:
The costly part of garbage collection is finding and dealing with the objects that are NOT garbage.
This means: as the heap gets close to its maximum capacity, the GC will spend more and more time for less and less return in reclaimed space. If the GC was to try and use every last byte of memory, the net result would be that your JVM would spend more and more time garbage collecting, until ... eventually ... almost no useful work was being done.
To avoid this pathological situation, the JVM monitors the ratio of time is spent GC'ing and doing useful work. When the ratio exceeds a configurable threshold value, the GC raises an OutOfMemoryError ... even though (technically) there is free memory available. This is probably what you are seeing, though the other explanations are equally plausible.
You can change the GC thresholds, generation sizes, etc via JVM options, but it is probably better not to. A better idea is to figure out why your application's memory usage is continually creeping upwards. There are most likely memory leaks ... i.e. a bugs ... in your code that are causing this. Spend your effort finding and fixing those bugs, rather than worrying about why you are not using all of the memory.
(In fact, you are using it ... but not all of the time.)
回答3:
Java splits its memory into generations. You can get a heap space error if the tenured generation fills. Normally, they resize dynamically but if you have set a fixed size it won't.
来源：https://stackoverflow.com/questions/5882216/java-heap-size-not-entirely-used