RFR 8204947: Port ShenandoahTaskTerminator to mainline and make it default

[Zhengyu Gu]

Updated version also passed submit tests.

-Zhengyu

On 11/28/18 10:26 PM, Zhengyu Gu wrote:
> Webrev updated to suppress multiple copy constructors and assignment 
> operators warnings on Window.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8204947/webrev.01/
> 
> Thanks,
> 
> -Zhengyu
> 
> On 11/28/18 2:08 PM, Zhengyu Gu wrote:
>> Hi,
>>
>> ShenandoahTaskTerminator is based on Google's work stealing protocol, 
>> described in paper [1]. The implementation diverges in a way to 
>> emphasize termination latency vs. power efficiency (see comments in 
>> code for details).
>>
>> Shenandoah switched to ShenandoahTaskTerminator as default terminator, 
>> shortly after it was implemented, and has been very stable since.
>>
>> On performance side, it showed better termination latency on almost 
>> all tests/benchmarks I ran, comparing to ParallelTaskTerminator. As 
>> the result, we (Shenandoah) purged fallback branches and uses it as 
>> the sole task terminator.
>>
>> This patch introduces a new experimental flag "UseOWSTTaskTerminator" 
>> (default: true), and switches G1 and Parallel GC to use it as default 
>> terminator. You can switch back to ParallelTaskTerminator with JVM 
>> option "-XX:+UnlockExperimentalVMOptions -XX:-UseOWSTTaskTerminator"
>>
>> I did not port it to CMS, as it is depreciated.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8204947
>> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8204947/webrev.00/
>>
>> Test:
>>    hotspot_gc and vmTestbase_vm_gc on Linux 64.
>>
>>
>> [1] Understanding and improving JVM GC work stealing at the data 
>> center scale (http://dl.acm.org/citation.cfm?id=2926706)
>>
>> Thanks,
>>
>> -Zhengyu