RFR(M) Enable UseDynamicNumberOfGCThreads flag support in Shenandoah

[Zhengyu Gu]

Resend with proper subject line.

Thanks,

-Zhengyu



On 02/02/2017 07:39 AM, Zhengyu Gu wrote:
> The heuristics for selecting number of workers probably still need to 
> be tweaked, as I did not see
> improvement with flag on/off with SPECjbb2015.
>
> The patch fixes a couple of potential issues:
>
> - Without the patch, enable UseDynamicNumberOfGCThreads can crash VM 
> due to uninitialized gclab of
>   dynamic created workers.
>
> - Imbalance of task queues and workers that potential cause misses of 
> final mark. The problem is hidden
>   because of currently implementation utilizing max parallel workers 
> for the final mark from root.
>
> I would like to get this push if there are not problems with the patch 
> itself, so we can find more
> systems to tweak the algorithm.
>
> Webrev: 
> http://cr.openjdk.java.net/~zgu/shenandoah/dynamic_workers/webrev.00/
>
> Tests:
>   - SPECjbb2015 fastdebug and release
>   - SPECjvm     Debry fastdebug and release
>
> Thanks,
>
> -Zhengyu
>