Reference counted ones, by carefully controlling when the reference count for objects are released you can make sure that the rendering thread never has to do any of the expensive ones, and instead all (non-trivial) reference releases are shunted off into a different thread for deallocation. In the general case you don't care, and in the specific case you have a fine enough control you can get around the problem of unknown gc-pauses. It's slower - it is a dynamic language afterall - but it's consistent and real-time is more about being consistent.