Big GC with unknow reason

Jon Masamitsu Jon.Masamitsu at Sun.COM
Tue Jun 24 15:38:27 UTC 2008

Your perm gen is nearly full

CMS Perm : 23152K->23072K(23296K)

so that might be the cause for the collection.

Try increasing the initial size of the perm generation
with -XX:PermSize=<nnn>.  Or maybe the maximum
size of the perm gen with -XX:MaxPermSize=<nnn>.

The cms collector does not collect the perm
gen by default.  Add the flag


to turn on the collection of the perm gen.

Ken-- at wrote:
> First of all, I dunno is it appropriate to ask gc tuning question here.
> Please forgive me if I post in wrong place.
> Well, I was chat with Sun GC Tuning Team long time ago and seek for gc
> tuning advise. The team was dismissed. Today, I google Jon Masamitsu and
> finally get here. :>
> So, my question is: Any idea why there was a big gc at 51789?
> We are using jdk1.6.0_04 on windows32 with this set of vm option:
> c:\progra~1\Java\jdk1.6.0_04\bin\java -server -verbose:gc
> -XX:+PrintGCDetails -Xloggc:itrade_gc_gc.log -Xms1024M -Xmx1612M
> -XX:NewSize=128M -XX:+DisableExplicitGC -XX:+UseConcMarkSweepGC
> -XX:+UseParNewGC -XX:SurvivorRatio=6 -XX:CMSInitiatingOccupancyFraction=80
> -XX:TargetSurvivorRatio=90
> 51789.930: [Full GC 51789.930: [CMS: 403366K->126465K(917504K), 19.1967956
> secs] 430862K->126465K(1032192K), [CMS Perm : 23152K->23072K(23296K)],
> 19.1972027 secs] [Times: user=0.94 sys=0.17, real=19.19 secs] 
> The heap is around 4x% full at that time and the machine has 8G physical
> memory. From the logs, it show that it spend 19.19 secs (real), what is
> user/sys/real? Any info about it? I guess the vm is spending many time on
> memory swapping but task mgr show that the peak is around 4G.

See the unix command time(1) for the definitions of
user, sys, and real.  Not sure what it is on windows.

Basically they are

user = user application cpu time
sys = system cpu time
real = wall clock time

Yes, it looks like the VM is waiting on something.

> Thanks and Regards,
> Ken

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