frequent CMS collections/ CPU spike/ Hotspot JRE1.4.2_17/

Jon Masamitsu Jon.Masamitsu at Sun.COM
Tue Apr 21 02:46:08 UTC 2009

With a total heap of 768m and a young gen pf 500m you
might not have enough room in the tenured (old) gen.
That is, the amount of free space left in the tenured gen
may be so small that CMS thinks it need to start
a collection immediately. Try a larger total heap
or a smaller yount gen. I see that you're turning on

-XX:MaxTenuringThreshold=0 -XX:SurvivorRatio=128

so everything that survives a young gen is being
promoted to the tenured gen. Those values reduce
the CMS pause times but also fill up the tenured
generation faster. If you don't have a specific reason
to use those values, 15 for the tenuring threshold and
6 or 8 for the survivor ratio may serve you better.

vasu ts wrote On 04/20/09 16:50,:

> Hi all,
> We have an application which is deployed on IBM websphere 5.1/Solaris
> 5.9/Sun hotspot JRE1.4.2_17/. We have 4 JVM's which are running on the
> same machine.
> These JVM's recieve xml messages from MQ queue which are processed (
> business logic stores the data from xml into database) and xml replies
> are sent back to the MQ queue.
> Hardware
> 8 dual core - sparc IV
> 4 single core - sparc III
> JRE options set on JVM's
> -server -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -Xmx768m -Xms768m
> -XX:MaxNewSize=500m -XX:NewSize=500m -XX:+UseParNewGC
> -XX:+CMSParallelRemarkEnabled -XX:+UseConcMarkSweepGC
> -XX:MaxTenuringThreshold=0 -XX:SurvivorRatio=128
> During our stress test we are seeing that CMS collector is trying to
> start old gen collection very frequently and the cpu usage spikes upto
> 99% -100% usage.
> Our stress test included adding a user per second until we reach 2500
> user limit and then we maintain steady user rate of 2500.
> Attached are the gc logs from one of the JVM, the PrintGCStats details
> and GCTimeline graph from GCHisto tool
> Is there anything I should set so that CMS collector doesn't start so
> frequently. Also, don't know if increasing the total heap size (to
> 1GB) will improve this situation.
> Please provide your comments.
> thanks
> vasu..
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