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

Thomas Schatzl thomas.schatzl at oracle.com
Thu Jul 10 11:34:43 UTC 2014


Hi,

  looks good.

Thomas

On Thu, 2014-07-10 at 11:38 +0200, Stefan Karlsson wrote:
> Hi all,
> 
> I decided to convert my adhoc test into a jtreg test that could be 
> checked in:
> http://cr.openjdk.java.net/~stefank/8049831/webrev.01
> 
> StefanK
> 
> 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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