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From: "Caldarale, Charles R" <Chuck.Caldarale@unisys.com>
To: Tomcat Users List <users@tomcat.apache.org>
Date: Tue, 26 Jan 2010 09:38:18 -0600
Subject: RE: Tomcat 100% CPU usage after moving from Java 5 to 6
Thread-Topic: Tomcat 100% CPU usage after moving from Java 5 to 6
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> From: Jesse Klaasse [mailto:jesse.klaasse@indicia.nl]
> Subject: Re: Tomcat 100% CPU usage after moving from Java 5 to 6
>=20
> I found the following article particularly useful:
> http://confluence.atlassian.com/display/DOC/Garbage+Collector+Performance=
+Issues

Unfortunately, that article is so full of mistaken assumptions and outright=
 errors that it must be taken with a large chunk of salt.  There are some g=
ood things in it, but also a lot of rubbish.  Among other things, the autho=
r is obviously unaware of how paging systems work, confusing contiguous vir=
tual memory (required for the heap) with contiguous real memory (not requir=
ed at all).
=20
> -XX:+UseConcMarkSweepGC

This will reduce overall throughput, but also reduce the GC pause time.  (G=
C pause time is pretty small even without that parameter these days.)  It d=
oes make the GC threads do a lot more work, but you appear to have CPU time=
 to burn.

>-XX:+UseTLAB

This has been the default for quite a few years now, so it's unnecessary to=
 specify it.

>-XX:+CMSClassUnloadingEnabled

The above slows down CMS collections.  You can leave it off if you never up=
date JSPs on the fly, or redeploy webapps without a Tomcat restart.

>-XX:+CMSPermGenSweepingEnabled

The above is the old name for CMSClassUnloadingEnabled and should not be us=
ed.

>-XX:+DisableExplicitGC

Always a good idea to set the above.

>-XX:NewSize=3D4096m

I would be wary of setting the above parameter, preferring to let the JVM d=
etermine the value as needed.  JDK/JRE 6 is much better at reacting to dyna=
mic changes in workload than 1.4 was.

>-Xms10240m -Xmx10240m

That's a very large heap; shouldn't hurt as long as you have enough RAM to =
support it all without paging.  (But see below for use with compressed OOPs=
.)

> Tomcat's CPU usage seems significantly lower than before=20
> (it's average seems about 10% now - which was 30-40% before)

There are some improvements in the JDK/JRE 6 JIT and synchronization handli=
ng, but probably not enough to account for that large a difference.

> I also added the -XX:+UseCompressedOops JVM option

Be careful with that; it can impact CPU usage, since the compressed OOPs ha=
ve to be expanded into actual pointers every time they're used, and pointer=
s compressed when stored in a field.  Works best with heap sizes less than =
4 GB, which avoids the decoding/encoding.  For a real explanation of compre=
ssed OOPs, look here:
http://wikis.sun.com/display/HotSpotInternals/CompressedOops

 - Chuck


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