RFR: 8312116: GenShen: make instantaneous allocation rate triggers more timely [v8]

[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 two additional commits since the last revision:

 - Respond to reviewer feedback
 - Remove unneeded functions

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29039/files
  - new: https://git.openjdk.org/jdk/pull/29039/files/70b7ebf8..1ed1ba66

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29039&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29039&range=06-07

  Stats: 21 lines in 6 files changed: 0 ins; 19 del; 2 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