CMS Full GC

[Shivkumar Chelwa]

Thanks Ramki. The perm gen size is well below the max setting. Only 70-80M is being used out of 256M, so I don't think it is an issue.

Will -XX:+PrintHeapAtGC prints heap only when there is Full GC?

Regards,
Shiv

________________________________
From: Srinivas Ramakrishna [mailto:ysr1729 at gmail.com]
Sent: 17 April 2012 15:07
To: Shivkumar Chelwa
Cc: hotspot-gc-use at openjdk.java.net
Subject: Re: CMS Full GC

Is it possible that you are GC'ing here to expand perm gen. Check if permgen footprint changed between the two JVM releases (when running yr application).
Now, CMS should quietly expand perm gen without doing a stop-world GC, but there was a temporary regression in that functionality before it was fixed again.
I can't however recall the JVM versions where the regression was introduced and then fixed. But all of this is handwaving on my part.
If you run 6u22 and 6u29 both with -XX:+PrintHeapAtGC, you might have more visibility into why the GC is kicking in. A longer log would allow
the community to perhaps provide suggestions as well.

Which reminds me that there is a bug in the printing of GC cause (as printed by jstat) which needs to be fixed. HotSpot/GC folk, have you noticed that we never
see a "perm gen allocation" as the GC cause even when that's really the reason for a full gc? (not that that should happen here where CMS is being used.)

-- ramki
On Tue, Apr 17, 2012 at 8:08 AM, Shivkumar Chelwa <schelwa at tibco.com<mailto:schelwa at tibco.com>> wrote:
Hi,

Till date I was using JRE 6u22 with following garbage collection parameters and the CMS cycle use to kick-in appropriately (when heap reaches 75%)


-server -d64 -Xms2048m -Xmx2048m -javaagent: my-instrumentation.jar -XX:MaxPermSize=256m -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC -XX:CMSInitiatingOccupancyFraction=75 -XX:+UseCMSInitiatingOccupancyOnly -verbose:gc -Xloggc:LB01.log -XX:+PrintGCTimeStamps -XX:+PrintGCDetails -Djava.library=/usr/java/jre/lib/sparcv9/server/libjvm.so -Xmx8192M -Xms8192M -Xss256K

But I switched to JRE 6u29 and see the CMS Full GC happening randomly. Can you please help me undercover this mystery. Here is one of the log message from gc log file.

13475.239: [Full GC 13475.239: [CMS: 4321575K->3717474K(7898752K), 54.0602376 secs] 4412277K->3717474K(8339648K), [CMS Perm : 73791K->73339K(74048K)], 54.0615557 secs] [Times: user=53.97 sy
s=0.12, real=54.06 secs]


Kindly help.


Regards,
Shiv




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