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

Mikael Gerdin mikael.gerdin at oracle.com
Mon Oct 20 12:27:06 UTC 2014


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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