RFR: JDK-8320336: GenShen: Reduce proactive humongous defragmentation efforts

[Y. Srinivas Ramakrishna]

On Fri, 17 Nov 2023 17:15:32 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> Only trigger defragmentation of old-gen memory if JVM is configured to reserve a certain amount of the heap to support humongous objects.  Even when the JVM is so configured, be slightly less aggressive in triggering defragmentation of old-gen memory.

Sorry for the delay. LGTM.

One question relating to the performance changes stemming from this fix:
- Effectively, it seems that the density/span based triggering is now disabled by default (because of the change at line 3129 and the default setting of humongous reserve at 0)
- Does this effectively win back the difference caused by the original change?
- However, were you to enable it, say by an appropriate positive value for the humongous reserve, does the less aggressive triggering result in a performance improvement?
- Are there any (rough) performance numbers, by chance, to share for the two changes separately?

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Marked as reviewed by ysr (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/361#pullrequestreview-1740872293