Perf Impact of CMSClassUnloadingEnabled

[Martin Hare Robertson]

Hi,

I recently encountered an interesting GC issue with a Tomcat application. I
came up with a simple repro scenario which I posted to StackOverflow:
http://stackoverflow.com/questions/8017193/when-does-the-perm-gen-get-collected

To solve this issue I have been encouraged to use
-XX:+CMSClassUnloadingEnabled.
I currently use the following GC configuration.

-XX:+UseMembar
-XX:+UseConcMarkSweepGC
-XX:+UseParNewGC
-XX:CMSInitiatingOccupancyFraction=80
-XX:+UseCMSInitiatingOccupancyOnly

Is enabling CMSClassUnloadingEnabled likely to have a negative perf impact?
If not, why is it disabled by default?

Thanks

Martin
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