RFR: Enforce GC workers constraint

[Roman Kennke]

Am 06.09.2018 um 21:25 schrieb 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.
> 

I was thinking the same.

The issues are probably:
- we need to keep testing this weird case to ensure it doesn't get broken
- IIRC, we have some extra code block to deal with that case and
re-balance the task queues when going from more conc-TQs to fewer
par-TQs at the beginning of final-mark/final-traversal. We could wipe
those blocks I think. Zhengyu probably knows better about this. However,
if this is the case, those code blocks should be wiped together with the
rest of the patch.

Roman