RFR (S/M): 8136678: Implement adaptive sizing algorithm for IHOP

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that adds a G1IHOPControl instance
that adaptively adjusts the current IHOP based on allocation rate and
marking cycle length?

Instead of statically setting the IHOP value (by the user at VM
startup), this change adds adaptive IHOP control similar to CMS.

The main change is in G1IHOPControl lines 106-121, the rest is just
setup changes and a unit test.

This feature, enabled by setting G1UseAdaptiveIHOP, is disabled by
default for now. It is planned to be enabled by default in JDK-8136680,
when more thorough testing has been conducted by SQE.

Generally it boosts G1 throughput significantly due to the low IHOP
default value of 45, which with that change typically gets >70, if not
up to 80-90, decreasing pause times significantly.

It depends on JDK-8136681 which is also out for review.

CR:
https://bugs.openjdk.java.net/browse/JDK-8136678
Webrev:
http://cr.openjdk.java.net/~tschatzl/8136678/webrev/
Testing:
jprt, vm.gc with G1UseAdaptiveIHOP disabled and enabled, unit test

Thanks,
  Thomas