AbstractA program’s distribution of object lifetimes is one of the factors that determines whether and how much it will benefit from generational garbage collection, and from what kind of generational collector. Linear combinations of radioactive decay models appear adequate for modelling object lifetimes in many programs, especially when the goal is to analyze the relative or theoretical performance of simple generational collectors.The boundary between models that favor younger-first generational collectors and models that favor older-first generational collectors is mathematically complex, even for highly idealized collectors. For linear combinations of radioactive decay models, non-generational collection is rarely competitive with idealized generational collection, even at that boundary.