RFR: 8246718: ParallelGC should not check for forward objects for copy task queue

[Kim Barrett]

> On Jun 8, 2020, at 2:59 AM, Erik Österlund <erik.osterlund at oracle.com> wrote:
> 
> Hi Kim,
> 
> Looks good.

Thanks.

> 
> Thanks,
> /Erik
> 
> On 2020-06-07 07:08, Kim Barrett wrote:
>> Please review this change to the handling of copy tasks by ParallelGC.
>> Formerly, before adding a task entry to the queue it would check
>> whether the referenced object was already forwarded.  If so, it would
>> handle the reference immediately, inline, rather than pushing the task
>> onto the queue.
>> 
>> Measurements show that a no-worse and sometimes better (depending on
>> the hardware configuration) approach for most applications is to not
>> do the forwarding check, but to instead prefetch the start of the
>> referenced object and then always push the task.
>> 
>> The corresponding G1 code does a for-write prefetch on the mark word
>> of the object, and a for-read prefetch past the object header.
>> Measurements with ParallelGC found the for-read prefetch not very
>> productive, and possibly even slightly counter-productive, so this
>> change only does the for-write prefetch on the mark word.
>> 
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8246718
>> 
>> Webrev:
>> https://cr.openjdk.java.net/~kbarrett/8246718/
>> 
>> Testing:
>> mach5 tier1-5
>> various performance tests
>> 
>