The native and Java heaps

The JVM maintains two memory areas, the Java™ heap, and the native (or system) heap. These two heaps have different purposes and are maintained by different mechanisms.

The Java heap contains the instances of Java objects and is often referred to as 'the heap'. It is the Java heap that is maintained by Garbage Collection, and it is the Java heap that is changed by the command-line heap settings. The Java heap is allocated using mmap, or shmat if large page support is requested. The maximum size of the Java heap is preallocated during JVM startup as one contiguous area, even if the minimum heap size setting is lower. This allocation allows the artificial heap size limit imposed by the minimum heap size setting to move toward the actual heap size limit with heap expansion. See Memory management in the J9 VM reference for more information.