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Java heap size not entirely used

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.

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Why are these remaining 20m, not used?

Solution

  • 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.