CMSPermGenSweepingEnabled vs CMSClassUnloadingEnabled

Question

I've kind of asked this question before - What does JVM flag CMSClassUnloadingEnabled actually do?, but this time it's slightly different!

If I start a Java VM with the -XX:+CMSPermGenSweepingEnabled flag set, the following message is printed:

Please use CMSClassUnloadingEnabled in place of CMSPermGenSweepingEnabled in the future

The -XX:+CMSPermGenSweepingEnabled flag is often used to mitigate against PermGen OutOfMemory errors, however I have read elsewhere that people have found that by following the above advice they have still had these errors, but by sticking with -XX:+CMSPermGenSweepingEnabled as well as -XX:+CMSClassUnloadingEnabled their VM has stayed up longer between restarts.

Does -XX:+CMSClassUnloadingEnabled really supersede -XX:+CMSPermGenSweepingEnabled or is there still some benefit in having them both?

Rich ps: I know that the root cause of perm gen issues is still usually Classloader leaks, this is more about the message that the JVM produces if we use the above options.

Anchor

Answer 1

From what I can tell, the only thing that the 1.6 JVM does with CMSPermGenSweepingEnabled is to print that message - it's not processed the way it is in 1.5. For CMSClassUnloadingEnabled to have any impact, UseConcMarkSweepGC must also be set.

Watters

Thanks - do you have any links or other ways to back this up? – Anchor

Download the JDK source code (which includes hotspot) and inspect hotspot/src/share/vm/runtime/globals.hpp. CMSPermGenSweepingEnabled is present in 1.5 and absent in 1.6. In addition, the 1.6 version of hotspot/src/share/vm/runtime/arguments.cpp contains new code for printing the message. – Watters

Ok looking to the jdk 1.6.23 source code didn't find any place of usage except globals.hpp where it is just set to false. So it is really used??? Sorry if i missed the place where it is used. – Uniocular

-XX:+CMSClassUnloadingEnabled and -XX:+CMSPermGenSweepingEnabled are unusable in java 1.7, see link – Kolomna

According to Oracle (see blogs.oracle.com/poonam/entry/…) option -XX:+CMSClassUnloadingEnabled is still valid for Java 1.7. – Auscultation

no longer valid for 1.8 - blogs.oracle.com/poonam/… – Breadth