Java heap size not entirely used?

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.

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.

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