RFR: 8049831: Metadata Full GCs are not triggered when CMSClassUnloadingEnabled is turned off

Stefan Karlsson stefan.karlsson at oracle.com
Thu Jul 10 09:38:41 UTC 2014

Hi all,

I decided to convert my adhoc test into a jtreg test that could be 
checked in:


On 2014-07-10 10:22, Stefan Karlsson wrote:
> Hi all,
> Please, review this fix to honor -XX:-CMSClassUnloadingEnabled 
> correctly in the metadata allocation failure path.
> http://cr.openjdk.java.net/~stefank/8049831/webrev.00
> https://bugs.openjdk.java.net/browse/JDK-8049831
> By default, CMS does class unloading after a concurrent marking cycle. 
> When the amount of metadata hits the high water mark 
> (capacity_until_GC), a concurrent cycle is initiated, the high water 
> mark is increased and the thread can proceed with the allocation. If 
> CMSClassUnloadingEnabled is turned off we don't unload classes after a 
> concurrent cycle, but instead relies on Full GCs to reclaim metadata 
> memory. The current code skips triggering a concurrent cycle if 
> CMSClassUnloadingEnabled is turned off, but it still allows the high 
> water mark to be increased and the thread can proceed with the 
> allocation without triggering a Full GC first. This defeats the entire 
> purpose of the high water mark.
> The suggested fix is to not allow the increase of the high water mark 
> and the allocation if CMSClassUnloadingEnabled is turned off.
> thanks,
> StefanK

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