Reminded me of a game engine of mine. Years ago I was experimenting with a time travel game concept. It had a virtual machine with green threads whose state could be suspended and serialized into a single binary blob at any time. I basically repurposed the GC's reachability analysis to find all objects reachable from the roots (VM's stack and global variables). IIRC it had also some sort of topological sort when serializing so that most references could be restored in one pass. It also serialized the stack itself and the current instruction pointer. "Manual persistence" they mention is actually quite straightforward to do if you're in full control of the VM. Additionally, it also serialized the physics engine's state as well, so that the forces applied were fully restored (important in a game). The concept was about being able to return to any point in time and have N versions of yourself working together to solve a problem. NPCs and objects had running scripts attached and that's why I needed the whole thing. It also allowed to have proper game saves out of the box. Oh, those were the fun times, and now I'm writing boring enterprise stuff :)