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

Stefan Karlsson stefan.karlsson at oracle.com
Thu Jul 10 11:36:44 UTC 2014

On 2014-07-10 13:08, Bengt Rutisson wrote:
> Hi Stefan,
> Looks good!



> Bengt
> On 2014-07-10 11:38, 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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