RFR (M): 8004687: G1: Parallelize object self-forwarding and scanning during an evacuation failure

Mikael Gerdin mikael.gerdin at oracle.com
Tue Jul 21 15:07:13 UTC 2015


Hi Thomas,

On 2015-07-16 11:40, Thomas Schatzl wrote:
> (resend with CR number in the subject)
> Hi all,
>
>    can I have reviews for the following change from a student (Walter
> Gugenberger; he has signed the OCA) who worked on this issue last
> semester?
>
> The change parallelizes object self-forwarding and scanning during an
> evacuation failure by:
> - reuse the entire existing evacuation copy closure and work queue
> mechanism in case of evacuation failure to store the work items.
> This allows evacuation failure to automatically benefit from work
> stealing etc.
> - remove the dedicate evacuation failure handling closure because it is
> not necessary any more.
>    - there is one subtle change in behavior: the old evacuation failure
> always had a NULL ReferenceProcessor, while now the standard reference
> processor for references is applied. Since the NULL ReferenceProcessor
> kept all references alive, now potentially less references will be kept
> alive after an evacuation failure. I do not see a problem here.

Right. Another difference is that the evac failure closure 
unconditionally called update_rs for each oop while the scanner closure 
only calls update_rs for non-cset oops and defers the cset updating to 
when a reference is popped from the work queue.

> - implementing per-thread preserved mark buffers
>
> As for actual performance improvements, there is none: the main problem
> is that in case of evacuation failure the code will simply serialize on
> the FreeList_lock when trying to get a new allocation region instead of
> the EvacFailureStack_lock.
>
> Evacuation failure and the early-exit for evac failure as suggested in
> the CR will only be really effective when the FreeList_lock can be
> avoided. This will be added in a follow-up change.
>
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8004687
>
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8004687/webrev/

Overall, a nice cleanup regardless of its lack of a general performance 
improvement.

I think that this change removes the last use of G1BarrierEvac, so the 
code for that in G1ParCopyClosure::do_oop_work can probably be removed.

/Mikael

>
> Testing:
> jprt, lots of internal evacuation failure testing, aurora runs with vm.quick/vm.gc nightly tests etc.
>
> Thanks,
>    THomas
>
>
>
>
>


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