RFR: Enforce GC workers constraint

[Aleksey Shipilev]

On 09/06/2018 08:53 PM, Zhengyu Gu wrote:
> CMS and G1 enforce GC worker constraint:
>   Parallel workers >= Concurrent workers
> 
> Shenandoah should probably do so.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/gc_worker_constraints/webrev.00/

The patch is okay, and probably sensible. But do we need to do this? Does it allow for any
convenience in our code? I can mind-wrestle the example where having more concurrent threads (e.g.
4) than parallel (e.g. 2) is beneficial. Seems inconvenient to tie them together without the reason.

-Aleksey