Memory usage of a java application

[ubertyp@xxxxxxxxxxx (ubertyp@xxxxxxxxxxx)]

The answer is...

pinning.

The actual 'Java' heap size may be 3 meg - that means that if you were to 
add up the individual sizes of all the objects you have allocated, you'd 
have 3 meg - but that doesn't mean the native heap size will be three meg. 
There's lots of empty space in the native heap, and it's space that quite 
often can't be reclaimed easily because of pinning. Under ordinary 
circumstances, the garbage collector will compact the heap to reclaim extra 
space whenver it can, but this obviously requires objects to be moved. The 
problem is that sometimes there are native pointers (within JNI stuff, 
peers, etc) that point at the java objects directly. You can't just go 
moving those objects all willy-nilly, since the pointers will be 
invalidated. So, any object that the garbage collector thinks might have a 
pointer pointed at it won't get moved, and that means heap compaction can 
only go so far, and that means that the native heap size gets a lot bigger 
than the java heap size.

Try this for kicks: Run the app, watching your heap size. Now minimize the 
window... Weird enough for you? Maximize it, then use the app. Windows NT 
does its own heap compaction - you can do that if you do address 
translation, which it does.





>Hello All,
>I am in a process of measuring the memory usage of a RMI based java
>application on Windows NT Server 4.0 and JDK 1.1.6.
>I have started the application with 32MB of minimum heap memory and 64 MB 
>of
>maximumum memory.
>At the application startup Windows NT task mamager shows 10MB of memory
>utilization for the JVM associated with this RMI
>application. And the Runtime.usedMemory() returns 2 MB. As a user connectes
>task manager memory usage increases substantially to
>16 MB(NT virtual memory size = 38MB), However JVM Runtime.usedMemory() 
>returns
>3 MB of heap memory used.
>Can anyone please explain me how to relate the JVM's heap memory usage with
>the Windows task managers' memory usage data.
>why does NT task manager memory increases by such a huge amount.
>
>Thanks in advance.
>Pradeep
>
>
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