RFR (S) 8242101: Shenandoah: coalesce and parallelise heap region walks during the pauses

[Roman Kennke]

Very nice!

Patch looks good.

Thank you!
Roman

Am 03.04.20 um 14:20 schrieb Aleksey Shipilev:
> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8242101
> 
> We are walking heap regions multiple times during final mark pause. We also walk them during final
> update refs pause without any parallelism. We should really fix those, to allow more regions per heap.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8242101/webrev.01/
> 
> I had to drop shenandoah_assert_heaplocked_or_safepoint from update_watermark, because it expects
> that we are called from VMThread at safepoint, and with this patch, we are not.
> 
> By my count, the whole thing saves about 50us in pauses with the default number of regions.
> 
> Testing: hotspot_gc_shenandoah, benchmarks
>