CMS Garbage collection eating up processor power

Jon Masamitsu Jon.Masamitsu at Sun.COM
Thu Mar 27 17:39:52 UTC 2008

Using CMS sometimes needs some tuning (especially
with the 1.4.2 jdk).  Do you have any gc logs
(-XX:+PrintGCDetails) so we can see what's happening?

T.K wrote:
> Hi All,
> We got 5 Sun Web Servers running on Java 1.4.2, and used to use the 
> default GC for Tenured space. The problem with that is that it takes 
> 60-80 seconds everytime the GC happens, and the latency on the site goes 
> crazy. So we decided to change it to use the Concurrent Mark Sweep 
> Collector on one server to test it out. Here's the setting:
> -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -Xms3G -Xmx3G -XX:NewSize=384M 
> -XX:MaxNewSize=384M -XX:PermSize=64M -XX:MaxPermSize=64M 
> -XX:CMSInitiatingOccupancyFraction=60
> With that setting, the server runs great. But eventually, when the 
> server reach a medium load (around 100-200 users), the tenured space is 
> always around half full, and the CMS collector starts to run 
> continuously one after another. It doesn't hurt the application for now, 
> but it's taking 25% of processing time (we got 4 cpu, so one web server 
> always keep 1 cpu power). I don't see that much cpu utilization on other 
> web server that don't have CMS, and they have more users than the one 
> with CMS. If we got CMS on all 5 web servers, I'm wondering if that will 
> crash the server or not.  What should I do to decrease the processor 
> utilization caused by GC? 
> Also, I'm thinking to use i-CMS on the JVM, and maybe that might slow 
> down the CMS and reduce the amount of CPU utilization by CMS. Any thought?
> Thanks,
> TK
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