RFR (S) 8252660: Shenandoah: support manageable SoftMaxHeapSize option

[Zhengyu Gu]

Looks fine to me in general.

Tests' @requires are not consistent, some miss  '!vm.graal.enabled'

-Zhengyu

On 9/1/20 2:55 PM, Aleksey Shipilev wrote:
> RFE:
>    https://bugs.openjdk.java.net/browse/JDK-8252660
> 
> Since JDK 13, there is a SoftMaxHeapSize option that basically instructs 
> GC heuristics to target that particular heap size, while leaving the 
> headroom for allocation spikes and other contingencies. It makes sense 
> to support this in Shenandoah.
> 
> Implementation:
>    https://cr.openjdk.java.net/~shade/8252660/webrev.01/
> 
> Testing: hotspot_gc_shenandoah; tier{1,2} with Shenandoah; new tests; 
> adhoc experiments with jcmd
>