RFR: 8058568: GC cleanup phase can cause G1 skipping a System.gc()

Stefan Johansson stefan.johansson at oracle.com
Tue Oct 21 07:45:24 UTC 2014


Thanks for reviewing Mikael.

Stefan

On 2014-10-20 14:27, Mikael Gerdin wrote:
> Stefan,
>
> On 2014-10-20 13:08, Stefan Johansson wrote:
>> Hi,
>>
>> Please review this fix for:
>> https://bugs.openjdk.java.net/browse/JDK-8058568
>>
>> Webrev:
>> http://cr.openjdk.java.net/~sjohanss/8058568/hotspot.00/
>
> Looks good to me, thanks for fixing this.
>
> /Mikael
>
>>
>> Summary:
>> For G1, full GCs might be skipped if a young GC is executed after the
>> full GC is initiate but before it has been started. The cause for this
>> is that G1 Full GCs aren't marked as full collections when creating the
>> VM_GC_Operation. Changing this to actually mark it as a full collection
>> will cause it to do a second check in skip_operation() to make sure it
>> really should skip it. This check checks if the number of full
>> collections has changed since the operation was scheduled, and if not
>> continues.
>>
>> There was a second problem with this as well, the full gc before count
>> we passed in to the GC operation was not the correct one, we previously
>> passed in _old_marking_cycles_started which is not the same as number of
>> full collections. Changing this to instead pass in the
>> total_full_collections() will make the above check work as expected.
>>
>> Testing:
>> * Verified that a JTREG test that previously failed now passes
>> * Adhoc aurora run didn't show any regression
>>
>> Stefan



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