Description: solves CoW friendly problem on MRI 2.2 and later MRI 2.1 employs generational GC algorithms that separate YOUNG objects and OLD objects. Young objects will be promoted to old objects when they survive 1 GC. . However, Ruby 2.2 employs an algorithm to promote young objects after 3 GCs. . nakayoshi_fork promotes most of young objects before fork by invoking GC some times.