CMS Garbage collection eating up processor power

[Jon Masamitsu]

Late in this log I see

162417.510: [GC 162417.510: [ParNew: 370176K->11520K(381696K), 0.4232456 
secs] 3055096K->2715938K(3134208K), 0.4240171 secs]

At that point the heap is about 85% full (2715938K/3134208K).  The
tenured generation is almost completely full.

Do you have similar logs when using the default GC?  We could use them
to verify the amount of live data.


T.K wrote:
>  
> 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 
> <mailto: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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