RFR: JDK-8114823 - G1 doesn't honor request to disable class unloading (original) (raw)

Mikael Gerdin mikael.gerdin at oracle.com
Thu Mar 31 14:21:50 UTC 2016

Hi Jesper,

On 2016-03-31 15:51, Jesper Wilhelmsson wrote:

Hi,

Please review this change to make the GCs disable class unloading if disabled on the command line. There are a few parts in this change: 1. The flag ClassUnloadingWithConcurrentMark was not set to false if ClassUnloading was disabled on the command line. This is now done. 2. CMS was using CMSClassUnloadingEnabled instead of the more generic ClassUnloadingWithConcurrentMark. I changed so that ClassUnloadingWithConcurrentMark is used all over and CMSClassUnloadingEnabled is only used as an alias when initializing the arguments. At some point it would be nice to deprecate CMSClassUnloadingEnabled but it is a fairly well known flag so that's not part of this change. 3. CMS, Serial, and PS did correctly disable class unloading when told so, but the outermost code witch included some logging was still enabled making class unloading output present in the log file. I changed so that this code is also disabled. 4. G1 did not disable class unloading during full GCs at all. This is now done. Testing: I used SecureDBBTest.java as suggested in the bug while fixing to verify that class unloading was happening before the fix and not after. I also ran it through JPRT. Bug: https://bugs.openjdk.java.net/browse/JDK-8114823 Webrev: http://cr.openjdk.java.net/~jwilhelm/8114823/webrev.00/

I think this change seems good overall. I have one small concern regarding CMSClassUnloadingEnabled, and that is that if I set -ClassUnloading then CMSClassUnloadingEnabled will still be true, would it make sense to still disabled it in the !ClassUnloading block even if nobody's supposed to look at CMSClassUnloadingEnabled?

Another approach could be to just add CMSClassUnloadingEnabled as an alias for ClassUnloadingWithConcurrentMark but I haven't researched exactly how the aliased_jvm_flags table works.

/Mikael

Thanks, /Jesper

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