[abstractfactory]

In 2004 Bacon et al. showed that tracing GC and ref-counting GC are special cases of a more general framework, and that it is possible to combine them to get the benefits of both. See "A Unified Theory of Garbage Collection" here:

http://www.research.ibm.com/people/d/dfb/publications.html

Scholar cluster:

http://scholar.google.com/scholar?q=bacon+unified+theory+gar...

Incidentally, it is not even remotely true that reference counting is more efficient than tracing GC in all cases.