RFR: JDK-8114823 - G1 doesn't honor request to disable class unloading

[Mikael Gerdin]

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