RFR: 8311883: [Genshen] Adaptive tenuring threshold [v6]

[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 incrementally with one additional commit since the last revision:

  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.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/289/files
  - new: https://git.openjdk.org/shenandoah/pull/289/files/4a1d6c8d..93591592

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=289&range=05
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=289&range=04-05

  Stats: 8 lines in 2 files changed: 0 ins; 1 del; 7 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