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

Bengt Rutisson bengt.rutisson at oracle.com
Fri Jul 11 08:16:06 UTC 2014


On 2014-07-10 14:28, Stefan Karlsson wrote:
> Dmitry Fazunenko had a couple of offline comments.
>
> Latests version:
> http://cr.openjdk.java.net/~stefank/8049831/webrev.03/

Looks good.

Bengt

>
> 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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