Generational GC makes most GC interruptions short, but you still have to
occasionally collect the larger old generation. With large heaps,
this can cause a considerable interruption; in some applications this
may be annoying or even unacceptable.

The strategy here is to have a separate GC thread that accomplishes
most of the old-generation collection concurrently with the mutator.
This mixes the Xerox PARC "mostly-parallel" collection idea with
generational collection, noting that the write barrier needed for
keeping track of old-to-young pointers also provides "dirty bits"
needed by the mostly-parallel technique, and at a relatively fine
grain.