Comment by EvanMiller

A "50/50" rule such as the one described is a clear signal that the memory manager's cache eviction policy is an ad-hoc hack. This sort of problem would benefit from some old-fashioned operations research-style statistical and mathematical modeling. What's the probability that a page will be needed? What's the cost of keeping it around? A proper model would be able to answer these questions. A "recency cache" and "frequency cache" is missing the bigger picture.