RFR: Allow regulator to schedule global cycles for the purpose of class unloading

[Kelvin Nilsen]

On Thu, 28 Apr 2022 20:56:42 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Generational mode only supports class unloading during _global_ cycles. Global cycles can only be triggered in generational mode by an explicit `System.gc` call. With this change, the regulator is able to trigger periodic global cycles based on the value of `ShenandoahUnloadClassesFrequency`. This is currently disabled by default for the generational mode because we've not found a workload that shows a clear benefit, however, we expect it will be needed for longer running workloads and we want the ability to turn this on when necessary.

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/138