RFR (S): 8135200: Add White Box method that enumerates G1 old regions with less than specified liveness and collects statistics.

[Thomas Schatzl]

"Hi Kirill,

On Wed, 2015-09-09 at 21:10 +0300, Kirill Zhaldybin wrote:
> Hi!
> 
> Could you please review a patch for JDK-8135200?
> 
> I added White Box method that enumerates G1 old regions with less than 
> specified liveness and collects statistics.
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8135200
> 
> Webrev:
> http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8135200/webrev.00/
> hotspot: 
> http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8135200/hotspot/webrev.00/
> 

  some comments about the change:

- there is some wrong assumption in the code:

370         if (size == reg_size) { // non-full regions are not included
to the mixed GC
 371           totalMemToFree += size - prev_live;
 372         }

This is not true. G1 may try to evacuate non-full regions (regions
recently allocated into), but it is just unlikely.

- lines 374-376 look like debug code. Are they required for some reason?

As for the follow-up JDK-8129579 New tests for G1 Mixed GC are required"
I am not sure if it is actually useful to try to test them. I mean,
these are artifacts of the current implementation, and as long as mixed
gc goals are met imo mixed gc may do anything to achieve its goal.

Thanks,
  Thomas