'TLSF' sounds very interesting, I'll have to keep an eye on that project. What are the reference-counting options for though, given the absence of weak references? Under what circumstances is it acceptable to permit memory-leaks in case of cycles? Short-lived processes (i.e. arena-based memory management)?

When I say 'primitive GC', I mean Boehm, which is obviously decades behind the state of the art, but easy to get off the ground. GNU Guile uses it, for instance. Serious garbage collectors are tremendously complex, and are something less popular languages unfortunately tend to lack. I believe even D is miles behind Java.

Can't say I know much about Go's GC.

Edit Oops I misread, TLSF isn't a GC, it's an allocator. It sounds then like Nim doesn't have a proper real-time solution, does it? useRealtimeGC is actually a leaky reference-counting system?