RFR: 8241670: Enhance heap region size ergonomics to improve OOTB performance

[Stefan Johansson]

Hi,

Please review this enhancement to improve the out of the box performance 
of G1.

Webrev: http://cr.openjdk.java.net/~sjohanss/8241670/00/index.html
JBS: https://bugs.openjdk.java.net/browse/JDK-8241670

Summary
The default heap region size determined at startup used the initial and 
max heap size to calculate a region size so that the heap would have at 
least 2048 regions (if possible). This proposed patch will change this to:
1) Only consider Max to make it easy to explain and avoid strange 
situations where -Xms or the lack of it will cause different region size 
for the same max heap size.
2) Round up the region size to next power of 2, since we've seen many 
cases where a larger region size is beneficial.
3) Keep the 2048 target for now since the other two changes, will have 
good effect on choosing a larger region size for heaps above 2G.

Testing
Mach5 tier1-4, aurora performance run for sanity and manual performance 
testing to verify results.

Thanks,
Stefan