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From "Robinson, Eric" <>
Subject RE: Is IBM Right About Java?
Date Wed, 09 Feb 2011 16:31:04 GMT
> > The JVM always reserves the maximum heap size as virtual space, but 
> > does not allocate more of the heap than the current limit; 
> the limit 
> > will be adjusted up or down within the -Xms : -Xmx range as load 
> > dictates.  The unused virtual space is not available for 
> any other use.
> >
> > Chuck, d'you happen to know how the virtual space 
> reservation impacts 
> > swap
> space requirements (if at all)?  Although somewhat less 
> unpleasant than buying RAM, 200 * 0.5Gbytes is still a lot of 
> disk to allocate for swap if you expect it's never going to 
> be used.  I therefore assume there's not a direct mapping of 
> reserved virtual space to reserved space in the swap file.

My server should be a good test case. We have 203 instances of java. 95%
of them are configured with -Xms64M -Xmx64M. A handful are configured
with higher values, but none with more than 256M. 

A quick...

	# ps axo rsz,vsz,comm,user|grep java|sort -n

..shows the java processes are consuming a total of 20GB of resident
memory and 56GB of virtual memory. 

'top' shows zero swapping, by the way.

top - 08:24:57 up 128 days,  5:52,  5 users,  load average: 1.31, 1.49,
Tasks: 424 total,   1 running, 422 sleeping,   0 stopped,   1 zombie
Cpu(s):  4.3%us,  1.1%sy,  0.0%ni, 93.6%id,  0.8%wa,  0.0%hi,  0.3%si,
Mem:  33265832k total, 22393776k used, 10872056k free,   297752k buffers
Swap:  2031608k total,        0k used,  2031608k free,  3961280k cached


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