RFR: 8333926: Shenandoah: Lower default immediate garbage threshold

[William Kemper]

After marking, when Shenandoah finds a certain percentage threshold of garbage within regions that contain no live objects it will skip the evacuation phase. This threshold is currently 90%, but we (Amazon) have found that many internal workloads benefit significantly by reducing this threshold to 70%. We have also seen improvements on dacapo/sunflow and specjbb2015. We have also not seen any performance degradation caused by lowering this threshold.

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Commit messages:
 - Default immediate threshold to 70

Changes: https://git.openjdk.org/jdk/pull/19641/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19641&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333926
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19641.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19641/head:pull/19641

PR: https://git.openjdk.org/jdk/pull/19641