RFR: 8295267: Improve G1 scan to merge ratio calculation [v2]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews for this change that fixes the merge-to-scan ratio calculation ("which fraction of the remembered sets creates unique cards") for predictions to make it more accurate and understandable?
> 
> Previously the denominator has been the total number of scanned cards; this seems to be a good approximation (trying to make a case for it, it's wrong) if the number of log buffers is low at GC start, but tends to get broken otherwise (i.e. with [JDK-8137022](https://bugs.openjdk.org/browse/JDK-8137022)).
> 
> This change fixes this to use the number of cards that were merged in.
> 
> This change depends on PR#10656. Please review that one first if not done otherwise.
> 
> Testing: tier1, checking that no tests use the "Dirty Cards" string now changed.
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  ayang review, merge_to_scan_ratio -> scan_to_merge_ratio

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10694/files
  - new: https://git.openjdk.org/jdk/pull/10694/files/3ff69963..b648df4a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10694&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10694&range=00-01

  Stats: 16 lines in 3 files changed: 0 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/10694.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10694/head:pull/10694

PR: https://git.openjdk.org/jdk/pull/10694