RFR: 8312116: GenShen: make instantaneous allocation rate triggers more timely [v7]

[Kelvin Nilsen]

> After studying large numbers of GC logs with degenerated cycles that have resulted from "late" triggers, we propose the following general improvements:
> 
> 1. Track trends in GC times rather than always using the average GC time plus standard deviation.  In many situations, GC times trend upward due to, for example, increasing amounts of live data that must be marked as a workload builds up its working set of memory.
> 2. Sample allocation rates more frequently than once every 100 ms.
> 3. Track trends in allocation rates.  In some situations, the allocation rate trends upwards due to, for example, the start of a new phase of execution or a spike in client workload.
> 4. When we detect acceleration of allocation rate, predict consumption of memory based on accelerated allocation rates rather than assuming constant allocation rate.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Changes requested by reviewers
  
  1. Change coordination between ShenandoahAdaptiveHeuristics and
     ShenandoahController/ShenandoahRegulator.  Before,
     ShenandoahAdaptiveHeuristics would poll ShenandoahController or
     ShenandoahRegulator to obtain most recent wake time and planned sleep
     time.  Now ShnandoahController and and ShenandoahRegulator notify
     ShenandoahAdaptiveHeuristics each time the values of these variables
     change.
  
  2. Use available() instead of capacity() - used() when recalculating
     trigger threshold from within
     ShenandoahAdaptiveHeuristcs::resume_idle_span().

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29039/files
  - new: https://git.openjdk.org/jdk/pull/29039/files/ea38ec15..70b7ebf8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29039&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29039&range=05-06

  Stats: 81 lines in 9 files changed: 25 ins; 49 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/29039.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29039/head:pull/29039

PR: https://git.openjdk.org/jdk/pull/29039