[jboss-user] [Beginners Corner] - Re: JVM Memory information

PeterJ do-not-reply at jboss.com
Wed Jul 26 10:54:40 EDT 2006

The javadoc doesn't expalin the difference between max memory and total memory, and assumes that you undertsand how the heap works, which you can find at http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html.

But to summarize.  The heap is broken up into several areas.  One of those areas (the permanent generation) stores class-related intormation.  Also, you can specifiy both a min (using -Xms) and a max (useing -Xmx) heap size.  The heap is automatically resized between those values, based on object allocation and retention (usign a fairly complex algorithm).  So, the differen heap statistics mean:

maxMemory - the maximum size of the heap minus the permanent generation
totalMemory - the amount of memeory currently allocated to the heap, once again minus the permanent generation
freeMemory - the amount of heap space available (this is out of 'totalMemory', not out of 'maxMemory')

If you check these stastics often while running you will see that 'max' doesn't change, 'total' changes occasionnally, and 'free' is probably different every time.

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