RFR: Move periodic GC decision making to base GC heuristics class

[Aleksey Shipilev]

On 07/19/2018 08:34 PM, Zhengyu Gu wrote:
> Please review this simple cleanup.
> 
> Currently, the same periodic GC decision is made by heuristics subclasses overall the places, while
> it can be made by base class.
> 
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/periodic_gc/webrev.00/

Looks good. You can avoid excess branches like this:

bool ShenandoahCompactHeuristics::should_start_normal_gc() {
   ...
   if (available < threshold_bytes_allocated || bytes_allocated > threshold_bytes_allocated) {
     ...
     return true;
   }

   return ShenandoahHeuristics::should_start_normal_gc();
 }

It also highlights we fallback to super-implementation if nothing is taken in this method.

-Aleksey