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

[Thomas Schatzl]

Hi,

On 07.06.20 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

   looks good. Good find! :)

Thomas