Paul Rubin wrote: > As I understand it, Python primarily uses reference counting, with a > mark and sweep scheme for cycle breaking tacked on as an afterthought.
It's not mark-and-sweep, it's a cycle detector. It goes through all allocated objects of certain types, and all objects reachable from those objects, trying to find sets of objects whose reference counts are all accounted for by references within the set. Then it picks an arbitrary object in each set and clears all the references from that object. This breaks the cycle, and allows all the objects in it to be reclaimed by their reference counts dropping to zero. (There's some kind of generational scheme in there as well, I think, but I don't know the details.) In a sense, this is the opposite of mark-and sweep. A mark-and- sweep collector assumes that everything is garbage unless it can be proven that it's not. Python's cycle detector, on the other hand, assumes that nothing is garbage unless it can be proven that it is. An advantage of this refcount/cycle detection scheme is that there is no "root set" that must be maintained at all costs. This makes it fairly easy to interface Python with external libraries that have their own notions of how to manage memory. > It doesn't move objects in memory during GC so you can get > fragmentation. It never moves PyObject structs, but variable-sized objects such as lists have their contents stored in a separate block of memory, that can be moved when its size changes. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
