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From "Ralph Einfeldt" <ralph.einfe...@uptime-isc.de>
Subject AW: Tomcat process takes 246M whereas total java heap is 127M
Date Wed, 10 Apr 2002 06:59:44 GMT
There is some data that is not stored in the heap:
- The Stack
  Each thread has at least one stack. The Sun VM has two stacks 
  per thread. (One for the java code, one for the c code)
  The size of the java stack is not specied, the size of the c 
  stack is controlled by the option -Xss (the default depends on 
  the underlying OS: typical value between 2 and 16 MB per thread)
- Static data 
- The VM iself

I'm not shure wether classes that are loaded at runtime are stored 
in the heap. (Never had enough interest in this to look in the java 
spec)

A memory leak is unlikely to explain what you see as
the memory leak just increases the heap and not the 
difference between heap and total memory.

Other possible explanations: 
- The physical memory gets fragmented over time.
- In a real bad implementation of a VM it is possible that the 
  size of allocation blocks don't match the size of the os. (e.g. 
  VM grows in 2KB blocks and the os has minimal blocksize of 4KB)

> -----Urspr√ľngliche Nachricht-----
> Von: Christophe Reynaud [mailto:creynaud@one2team.com]
> Gesendet: Dienstag, 9. April 2002 20:52
> An: Tomcat Users List
> Betreff: Tomcat process takes 246M whereas total java heap is 127M
> 
> 
> Hello,
> 
> We use tomcat 3.3.1 on Linux RH 7.2.  JVM : IBM 1.3
> 
> "top -c" tells that the tomcat threads take 246M but if we 
> use the Java
> function "totalMemory()", it tells that the JVM uses 127M for 
> the heap.
> Where is the rest of memory ?  I expected that the JVM used more than
> only the total of the heap, but in this case it is almost the double !
> Any ideas about the reasons of this problem ?  Are there some momory
> leaks in the JVM ?
> 
> Thanks.
> 
> Christophe
> 
> 
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