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From Jesse Klaasse <>
Subject RE: Tomcat 100% CPU usage after moving from Java 5 to 6
Date Tue, 26 Jan 2010 15:50:05 GMT

Hi Chuck, thanks for your extensive comments about the JVM options.. 

n828cl wrote:
>>-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.)

This shouldn't be a problem, the system has 16 GB RAM, almost all of it
dedicated to Tomcat.

n828cl wrote:
>> Tomcat's CPU usage seems significantly lower than before 
>> (it's average seems about 10% now - which was 30-40% before)
> There are some improvements in the JDK/JRE 6 JIT and synchronization
> handling, but probably not enough to account for that large a difference.

Well, I don't know where exactly it's coming from, but after a full day very
closely monitoring the system, it turns out to be the truth. Which is not a
bad thing, of course :)

n828cl wrote:
>> I also added the -XX:+UseCompressedOops JVM option
> Be careful with that; it can impact CPU usage, since the compressed OOPs
> have to be expanded into actual pointers every time they're used, and
> pointers 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 compressed OOPs, look here:

I already found that article myself. Since CPU usage is as low as it is now,
I have decided to give it a try.
Anyway, thanks again!
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