RFR (M): 8228609: G1 copy cost prediction uses used vs. actual copied bytes

[Thomas Schatzl]

Hi all,

   can I have reviews for this change that makes G1 calculate and the 
use actual amount of bytes copied for Object Copy phase estimation?

The problem is that the "used" value that is currently used for this can 
differ a lot from the number of actually copied bytes during the 
parallel phases.

Sources for differences are:
  - TLAB sizing
  - TLAB/region fragmentation
  - all of that multiplied by the number of threads

Particularly if the amount of copied data is small compared to the 
number of regions all this can add up and disturb the prediction quite a 
lot, although overall it's not that bad.

It's only that this and other small inaccuracies add up.

CR:
https://bugs.openjdk.java.net/browse/JDK-8228609
Webrev:
http://cr.openjdk.java.net/~tschatzl/8228609/webrev/
Testing:
hs-tier1-5

Thanks,
   Thomas