Serial GC and Parallel GC both have two GC memory managers:
one is to represent the minor GC and collect its statistics which manages
the young generation, and the other is to represent the major GC and
collect its statistics which manages the young generation, old generation
and also the perm generation.

CMS has a concurrent collector executing while mutators are running
in addition to the stop-the-world minor and major GC.

CMS should be represented with 3 GC memory managers.