This extend reference counting to be able to identify cycles of garbage, if such cycles are the only objects in a group. The algorithm is known as group reference counting. Don t do it collection When the available storage is extremely large, garbage collection could be postponed for years. Kung, S. W . In gigantic LISP systems the adding of a third level backing storage may be used for old unused objects instead of doing garbage collector.
Bakers copying collection is used to move the old objects out of primary and into tertiary storage.
Daniel G. Bobrow. Managing reentrant structures using reference counts . The Efficiency Problem. The efficiency problem with reference counting is that its cost is generally proportional to the amount of work done by the running program, with a fairly large constant of proportionality. Reference counting with unreclaimable cycle.