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From "Caldarale, Charles R" <Chuck.Caldar...@unisys.com>
Subject RE: 100% cpu usage by "VM Thread" in tomcat
Date Fri, 21 Sep 2007 18:13:02 GMT
> From: Lindsay Patten [mailto:BLindsayPatten@netscape.net] 
> Subject: 100% cpu usage by "VM Thread" in tomcat
> 
> Expecting to find that I had some error in my webapp I did a 
> few thread dumps, but assuming that nid in the thread dump
> corresponds to pid from ps I found that the process that was
> spinning was "VM Thread":
> 
> "VM Thread" prio=1 tid=0x08098a48 nid=0x144 runnable

That's not likely to be the one actually spinning, unless your JVM is
very broken.  The VM thread will always show as runnable, since it's the
one displaying the thread dump.  You probably need to look a bit more
closely at the other threads.

> The thread dump does not give any additional information about this 
> thread. I presume it is the Virtual Machine itself.

It's the JVM's non-dedicated background thread; it does get involved in
garbage collection, but may or may not do the bulk of the work,
depending on the GC mechanism selected.

> If I look at the system status using the Tomcat manager 
> webapp there are often requests listed with ridiculously
> large values in the Time column, several hundred seconds
> for jsp pages that only take a fraction of a second to
> generate

That sounds like your webapp logic may be spinning somewhere after it
has sent the response.

> Sometimes it seems that reloading some of the webapps can "fix" the 
> problem, at least for a while

Another bit of evidence pointing to a problem in the webapp, not Tomcat
or the JVM.

 - Chuck


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