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From David kerber <>
Subject Re: CPU usage quirk with multiple tomcat instances
Date Fri, 02 May 2008 12:49:16 GMT
Lists wrote:
> Hi,
> We have a Tomcat app that is loadbalanced by hardware.  I have 4 boxes
> available in our test env, all of which vary slightly but not massively in
> spec.
> Anyway I moved from 2 tomcat instances on each machine ( listening on
> different ports ) down to 1 and I noticed for the same load the cpu
> utilisation dropped by 20%.  Interesting, how can this be explained?  Is
My guess (and that's all it is) is that you have eliminated much of the 
context-switching overhead that two instances would have compared to one.

> this the extra effort in tomcat or something?  Or is it memory related? the
> 2 instances had 1gb each, and the single one has been given 4gb.  However I
> have jprofiled the instance and it has never seemed memory hungry.  Anyway I
> suspect even at 2gb for 1 instance it would still be 20% less cpu - i can
> always do a run to prove this if people think that necessary..
> Another thing i've found is that on one server I have abnormally high CPU
> utilisation for the same load.  I've checked the logging levels are all the
> same as are the java VM and the VM config settings.  I wonder what else
> could cause this?  All i can think is some sort of OS or hardware
> malfunction! maybe the CPU cache is duff or something.
What is this cpu (type and speed) compared to the others?  If it's a 
slower or less-capable (no numeric coprocessor, etc) cpu, the same load 
would be a high percentage of its capability.

> Many thanks to any input to my Friday weird-ness!
> Dan

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