JConsole

=> open jconsole
=> Normally port should be 8686 if running in admin server directly.
=> In memory always concentrate on "memory pool PS Old Gen" because this is the area where large objects are retained even after GC...
=> First make sure performance test is done and observe the "memory pool PS Old Gen".
=> It is normal that the graphs go higher at this point of time since we are running high load tests.
=> Now hit "perform gc" multiple times and wait for some time.
=> Make sure the graph (line) goes to the same position of where it is before the load test.
=> Then fire the performance test again this time increasing the time-load.
=> Dont get too much worried when the 1st request takes lots of time to execute its because the GC might not have completed its job
=> "Memory pool PS Old Gen" is the 3rd tab in the heap tabs at the bottom of that jconsole screen.

Few internet definitions
Eden Space (heap): The pool from which memory is initially allocated for most objects.
Survivor Space (heap): The pool containing objects that have survived the garbage collection of the Eden space.
Tenured Generation (heap): The pool containing objects that have existed for some time in the survivor space.
Here Tenured Generation (heap) is what we are interested in and is called "memory pool PS Old Gen".
The Java HotSpot VM defines two generations: the young generation (sometimes called the "nursery") and the old generation. The young generation consists of an "Eden space" and two "survivor spaces." The VM initially assigns all objects to the Eden space, and most objects die there. When it performs a minor GC, the VM moves any remaining objects from the Eden space to one of the survivor spaces. The VM moves objects that live long enough in the survivor spaces to the "tenured" space in the old generation. When the tenured generation fills up, there is a full GC that is often much slower because it involves all live objects. The permanent generation holds all the reflective data of the virtual machine itself, such as class and method objects.

Source: http://docs.oracle.com/javase/7/docs/technotes/guides/management/jconsole.html