RFR (S): 8049341: Parallelize clearing the next mark bitmap

[Marcus Larsson]

Hi,

I would like reviews for the following patch to parallelize the clearing 
of the next mark bitmap in G1.

Short summary:
The heap is divided into as many parts as there are workers and each 
worker will clear its corresponding part of the bitmap. Workers will 
join the suspendible thread set instead of the concurrent mark thread to 
properly allow them to yield during clearing work. Added support for 
applying heap region closures to a specific part of the heap. 
SPECjbb2013 shows a slight performance gain with this change (4%, using 
4 concurrent threads).

Webrev:
http://cr.openjdk.java.net/~brutisso/webrev-8049341/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8049341

Testing:
jprt, SPECjbb2013, SPECjbb2005, SPECjvm2008

Thanks,
Marcus