GIL-slayers take note. Here are two papers about concurrent reference counting: An On-The-Fly Reference Counting Garbage Collector for Java Levanoni and Petrank, 2001. http://www.cs.technion.ac.il/~erez/Papers/refcount.ps Efficient On-the-Fly Cycle Collection Paz, Bacon, Kolodner, Petrank, and Rajan, 2005. http://www.cs.technion.ac.il/%7Eerez/Papers/CycleCollection.ps "On-the-fly" means the algorithm is neither "fully concurrent" nor "stop the world". Rather, each thread pauses occasionally to do some work. Instead of a GIL, you have a lock that covers this periodic bookkeeping. The details are awfully complex, but there may be insights worth gleaning regardless. Also -- I wrote some stuff at: http://wiki.python.org/moin/GlobalInterpreterLock in the hopes that future "Kill GIL" discussions can start from a better-informed base. -j