RFR (S): 8227719: G1 pending cards estimation too conservative for cost prediction

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that instead of using a very
conservative estimate of the number of pending cards in the DCQS, that
is assuming that all completed buffers are full, simply calculates it
as buffers are completed, i.e. giving the exact value.

Having a better input values to the algorithms seems to be desirable :)

Anecdotally, in my tests this makes a significant difference in cases
where G1 has formerly been unable to utilize the allowed pause time
goal fully, i.e. use significantly larger eden, decreasing total pause
time and number of GCs. There is also the risk that the results are
less conservative now, leading to more overshooting though.

There are imho a lot issues with the prediction heuristics remaining,
but at least one input value is a bit better now.

CR:
https://bugs.openjdk.java.net/browse/JDK-8227719
Webrev:
http://cr.openjdk.java.net/~tschatzl/8227719/webrev/
Testing:
hs-tier1-3, self verification, (manual) perf testing

Thanks,
  Thomas