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From Ken Bowen <>
Subject Tomcat heap memory behavior question
Date Sun, 24 Jan 2010 19:06:15 GMT
Hi all,

I'm trying to understand Tomcat memory usage as observed with  
jconsole.  I'm using Tomcat 6.0.18 on Mac OS X 10.5.8;
Tomcat is downloaded from the Apache site.  I'm starting it with /no/  
web applications
at all: just an empty webapps folder, and I'm not hitting it with any  

jconsole shows the following behavior:
Shortly after startup with the heap at 15MB, a GC drops the heap to  
just above 5MB.
Over 5 minutes, the heap grows to about 21 MB, at which time another  
GC occurs, and it drops to about 6MB.
Over the second 5 minute interval, it rose to a little over 22MB, and  
then dropped to just under 5MB.
Then it settles into consistent 5 minute intervals, rising from about  
4.8MB to 21.5 MB, then a GC.

So I have three questions:

1)  Is this normal Tomcat behavior?  Since this is a vanilla Tomcat  
download, I assume the answer is yes, but maybe not?

2)  What Tomcat activity is consuming the heap?

3)  Presumably this means that in planning for a memory restricted  
Tomcat + apps, one needs to apportion about 22MB to the Tomcat heap,  
apart from heap allocated to applications.

BTW, I verified Chuck's statement that top is bad tool for looking at  
Tomcat's heap:  at the first pea k rising from 5MB to 21MB, GC dropped  
the heap value in jconsole to 6MB, but the RSIZE in top jumped from  
54M to 57M.

Thanks in advance,

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