Garbage collecting a circular list in ruby

I'm learning about ruby's mark and sweep approach to garbage collecting. I bumped into a few threads here and there (and this article via a SO thread which I can no longer spot), but they seemed to apply to older versions of ruby and a the information in them wasn't always consistent. (As things stand I'm getting the impression that it's mostly reference counting.)

Might anyone with some unrstanding of ruby开发者_JAVA技巧 1.9.2's internals be around to chime in, on whether ruby knew how to handle the trickier back references and circular references? (Ideally with a few details/good pointers on how it's actually implemented.)

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Mark-and-sweep GC, like almost every algorithm commonly labeled as garbage collection save reference counting, handles circular references just fine. This has nothing to do with the specific implementation. Regardless of the actual GC used by Ruby 1.9, it won't have trouble with cycles. Here's a sketch of the approach of mark-and-sweep collectors, but be assured that other collection schemes handle cyclic references just as well.

1. Mark all things known to be always reachable ("roots", basically everything that's directly in scope - global variables, local variables, etc.)
2. Mark all not-yet-marked objects referenced by marked objects
3. Repeat 2 until no references from marked to not-yet-marked objects remain
4. Enumerate all objects allocated, deallocates those not marked

You see, a circle of references that's reachable "from the outside" doesn't lead to infinite recursion (we don't visit a given object's references more than once) and a circle of references that isn't reachable isn't marked as reachable and thus freed (each element independently) after marking.