RFR (S) 8241351: Shenandoah: fragmentation metrics overhaul

[Roman Kennke]

Hi Aleksey,

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241351
> 
> Current fragmentation computations have flaws:
>  - fragmentation metrics are there to drive mutator behavior, but it captures evac-reserve;
>  - external fragmentation do not count humongous objects spanning a single region
>  - external fragmentation takes "total space" over all regions, while using "free space" only for
> completely empty ones
> 
> These flaws make gc/oom/TestThreadFailure.java fail with JDK-8241139 changes.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8241351/webrev.01/
> 
> Testing: hotspot_gc_shenandoah {fastdebug,release}

Good. Just one small nit:

+ *   f) Heap has the small object per each region => IF =~ 1

I'd write 'has *one* small object per region'


Don't need another webrev for this.

Thank you!
Roman