RFR (S) 8220153: Shenandoah does not work with TransparentHugePages properly

[Roman Kennke]

Ok good. Thanks!

Roman

> Bug:
>    https://bugs.openjdk.java.net/browse/JDK-8220153
> 
> Fix:
>    http://cr.openjdk.java.net/~shade/8220153/webrev.02/
> 
> There are several troubles in current code. First, we need to make sure heap is aligned to make it
> madvise'able by shared code during commits. Second, we have to pretouch with smaller pages when THP
> is enabled, because we need to tell kernel all these smaller pages can be for coalesced into a huge
> one. Third, when pretouching, it is apparently important to pretouch contiguous spaces separately (I
> haven't verified why, but it is empirically better).
> 
> Unfortunately, constructing a reliable regression test proved hard, so I had to resort to manual
> testing.
> 
> Testing: adhoc runs with THP enabled, hotspot_gc_shenandoah {with and without -XX:+UseTHP}
> 
> Thanks,
> -Aleksey
>