JVM Memory information

[nabieh]

Dears,
I have explored the JBoss Web-Console and found these information:

JVM Environment
Free Memory: 137 MB
Max Memory: 988 MB
Total Memory: 220 MB


I am wondering what is the difference between these parameters, thanks in advance.

Nabieh.

[visolvejboss]

Hello,

The Free Memory gives the details of free heap memory space, Max Memory gives the details of maximum allocated heap space and Total memory gives the details of total heap memory space.

[nabieh]

- Total memory gives the details of total heap memory space.

We have set the maximum heap size to 1 GB, I am asking why I have only 137 free? And what do you mean by "Details of Total space"? I am lost! Total of what?
Let us refer to a simple case, when I have 100 space, and the current free space is 30, then the used space must be 70. Where is the case in our example of Heap Size details? Thanks in advance.

Nabieh.

[nabieh]

Dears,
Any help please? That answer confused me instead of giving me the answer. Thanks in advance.

Nabieh.

[jaikiran]

All these values are obtained by invoking APIs provided by Java's java.lang.Runtime class. So the javadoc of the following methods might help you in understanding the same:

Free Memory: http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Runtime.html#freeMemory()

Max Memory : http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Runtime.html#maxMemory()

Total Memory : http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Runtime.html#totalMemory()

[peterj]

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.