CMS Garbage collection eating up processor power

T.K thankhar at gmail.com
Thu Mar 27 18:33:13 UTC 2008


  Hi Jon,

Here's the attached gclog.  It starts out fine when we put in the change in
the evening.  Eventually, the next morning (around 60000 seconds on gc time)
when the load starting to get in, CMS starts to run consectively.  The first
CMS fails occur when we bump up the users to 700+, and almost kill the
server.  Ever since then, I don't see the CMS ever stops, even when the load
goes down to 50 users.

I cut off the logs in between into 3 portion so that I can attach it.  :D

Thanks,
TK


On 3/27/08, Jon Masamitsu <Jon.Masamitsu at sun.com> wrote:
>
> 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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> > hotspot-gc-use at openjdk.java.net
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>
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