RFR: 8311883: [Genshen] Adaptive tenuring threshold [v12]

[Y. Srinivas Ramakrishna]

> JDK-8311883 [GenShen] Adaptive tenuring
> 
> I am opening this previously draft PR for formal preliminary review. It has already benefited from review feedback from a code walkthrough of an earlier version of the code. Most of that feedback and the corrections thereof are to be found in the comments in this PR. I have addressed a large majority of those comments, and am working on the last one that I plan to address as part of this PR. For the ones that I don't plan to address in this PR, I will create follow up tickets. Those will be added in the responses for the remaining feedback comments recorded in this PR's conversation.
> 
> Preliminary testing w/SPECjbb didn't yield reliable performance data from which to infer any performance improvements stemming from enabling adaptive tenuring. I believe that was because of the way SPECjbb is run, which causes excessive degenerate and full gc's. I plan to collect SPECjbb numbers with a fixed lower max HBIR so as to be able to discern performance differences from this change, as well as Extremem workloads. Those will be added here once ready over the next few days.

Y. Srinivas Ramakrishna has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 83 commits:

 - Merge branch 'master' into adaptive_tenuring
 - Merge branch 'master' into adaptive_tenuring
 - Merge branch 'master' into adaptive_tenuring
 - Reposition a mis-positioned assert as well as surrounding if-test order.
 - Fix global flags to make adaptive tenuring with census at mark the
   default, along with ignoring mortality rates of cohorts older than the
   selected tenuring age at previous epoch.
 - Flag to ignore mortality rate of cohorts older than the tenuring age of
   most recent preious cycle when computing new tenuring threshold.
 - Clear partial census data before a new marking cycle.
   Allow consideration of mortality rate of youngest cohort for tenuring
   decisions. (It was previously excluded because census data didn't
   include the youngest cohort allocated in the current epoch.)
   
   Still to do: avoid tenuring revisionism a la Kelvin's suggestion.
 - jcheck whitespace.
 - Merge branch 'master' into adaptive_tenuring
 - Separate out clearing and testing of local and global portions of census
   data, towards expunging partial/interrupted census data as a next step.
 - ... and 73 more: https://git.openjdk.org/shenandoah/compare/867f558d...2604d78a

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Changes: https://git.openjdk.org/shenandoah/pull/289/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=289&range=11
  Stats: 994 lines in 30 files changed: 876 ins; 45 del; 73 mod
  Patch: https://git.openjdk.org/shenandoah/pull/289.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/289/head:pull/289

PR: https://git.openjdk.org/shenandoah/pull/289