RFR (M): 8226913: Scale cards per chunk used during heap root scanning with region

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that scales the amount of cards
processed per chunk depending on heap/heap region size to decrease some
unnecessary overhead when clearing it on larger heaps.

It is basically the answer to this comment in the code:
  96   // Random power of two number of cards we want to claim per
thread. This corresponds
  97   // to a 64k of memory work chunk area for every thread.
  98   // We use the same claim size as Parallel GC. No particular
measurements have been
  99   // performed to determine an optimal number.

I.e. the chosen number simply seemed suboptimal, and should actually be
determined adaptively.

CR:
https://bugs.openjdk.java.net/browse/JDK-8226913
Webrev:
http://cr.openjdk.java.net/~tschatzl/8226913/webrev
Testing:
hs-tier1-5, manually checking that correct chunk sizes are returned for
all possible region sizes

Thanks,
  Thomas