RFR: Initial sizing refactor [v2]

[William Kemper]

On Wed, 21 Dec 2022 22:15:58 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> I agree we can wire up more signals to the resizing mechanism. In the scenario you describe, where old generation has become _too small_ and old collections are running _too frequently_, the MMU based resizing would enlarge the old generation.
>
> What do you do before any data is available for one of the MMU trackers? (for example before the first old collection cycle has happened.) I'd assume that the control algorithm wouldn't kick in until the data driving the control signal was valid and available. Where is that done?

We generally have a flurry of "learning cycles" at startup, but it makes sense to add an explicit check to avoid premature resizing. To Kelvin's earlier point, it may also make sense to re-initiate the learning phase after resizing the generations.

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PR: https://git.openjdk.org/shenandoah/pull/185