RFR: Generation resizing [v3]

[William Kemper]

> These changes have the generational mode track the minimum mutator utilization (percentage of process time used by mutators). When it falls below a configuration percentage (GCTimeRatio), a heuristic will transfer memory capacity to whatever generation has been using more CPU time. The assumption here is that by increasing capacity, we will decrease the collection frequency and improve the MMU.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Remove vestigial lock, do not enroll periodic task while holding threads_lock

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/177/files
  - new: https://git.openjdk.org/shenandoah/pull/177/files/41f057fa..d7a01946

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=177&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=177&range=01-02

  Stats: 8 lines in 3 files changed: 2 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/177.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/177/head:pull/177

PR: https://git.openjdk.org/shenandoah/pull/177