Generational Shenandoah currently has the notion of a tenuring threshold but it isn't dynamically adapted, but rather kept fixed at 7. Adapt the tenuring threshold based on object demographics as determined by a recent GC that visits objects in the young generation.

The basic idea is to keep track of age-cohort populations at each minor GC epoch and use the historical data to determine if it would be a good idea to tenure or not based on recently measured mortality rates. A few tunables are exposed to play with these to determine some good settings.

Other variants are possible, and may be implemented / tested in the future as opportunity and data permit.

A design doc (draft) is attached.