RFR: Add support for CollectedHeap::get_safepoint_workers()

[Per Liden]

On 05/08/2018 05:49 PM, Aleksey Shipilev wrote:
> On 05/08/2018 05:42 PM, Ramki Ramakrishna wrote:
>> Hi Per,
>>
>> is there a bug id that you can link to the webrev for background (for those
>> who came in late)?
> 
> This is the part of what we did in Shenandoah to exploit parallelism during safepoint cleanups:
>    https://bugs.openjdk.java.net/browse/JDK-8180932
> 
> You'll probably get better results if you can reuse GC worker threads to do the safepoint cleanups,

I wonder if there would really be a measurable difference, but I could 
be wrong.

> but that requires GC workers to be suspendable for non-GC safepoints. Having a separate thread pool
> is fine for now: Shenandoah does the separate thread pool, the proposed ZGC code also does it.

Long-term I'm hoping we work towards something like 
http://openjdk.java.net/jeps/8183909

/Per