RFR: 8321816: GenShen: Provide a minimum amount of time for an old collection to run [v3]

[William Kemper]

> Young collections are allowed to interrupt old collections. Under heavy load, this may result in starvation of old collections - which may in turn lead to even more memory pressure on the young generation. Adding a minimum guaranteed amount of time for the old gc to make progress will help avoid this situation.

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

  Only start global cycles when metaspace is out of memory

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/371/files
  - new: https://git.openjdk.org/shenandoah/pull/371/files/8a40a397..25de1177

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=371&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=371&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/371.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/371/head:pull/371

PR: https://git.openjdk.org/shenandoah/pull/371