RFR: 8295267: Improve G1 scan to merge ratio calculation [v3]

[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 with a new target base due to a merge or a rebase. The pull request now contains 15 commits:

 - Merge branch 'master' of https://git.openjdk.org/jdk into submit/8295267-improve-merge-to-scan-ratio
 - ayang review, merge_to_scan_ratio -> scan_to_merge_ratio
 - Improve naming, comments
 - Remove debug code
 - Fix test
 - initial version
 - ayang reviews
 - Initial version cost sample threshold
 - Initial version by walulyai
 - young gen revise trigger initial version
 - ... and 5 more: https://git.openjdk.org/jdk/compare/dfcd9d53...7fe993a9

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Changes: https://git.openjdk.org/jdk/pull/10694/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10694&range=02
  Stats: 50 lines in 8 files changed: 4 ins; 11 del; 35 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