RFR 8204947: Port ShenandoahTaskTerminator to mainline and make it default

[Zhengyu Gu]

Hi Thomas,

>> I did not port it to CMS, as it is deprecated.
> 
> I submitted a job to push it through our testing infrastructure. I will
> also run it through our perf benchmarks checking for issues.
> 
Thanks!

> As for CMS support, I would prefer to provide it - I need to see the
> reasoning in the CR - while CMS is deprecated, we may not be able to
> remove it in 13, while I would like to remove this flag independently
> and earlier :)
> 
> With no CMS support for this task terminator we tie flag removal to CMS
> removal probably needlessly.

Okay, ported to CMS.

Webrev: http://cr.openjdk.java.net/~zgu/JDK-8204947/webrev.02/index.html

and passed hotspot_gc on Linux 64. I will run more tests.

-Zhengyu

> 
> I have no opinion on whether we should simply drop the
> ParallelTaskTerminator, but since we need to have it in the code for
> CMS anyway, it seems cheap to have for one release or so.
> 
> Thanks,
>    Thomas
> 
>