Um no. First, it doesn't "scan all memory". It scans the root and fans out. You can do read/write barriers and such to avoid blocking GC. It doesn't matter what

Um no. First, it doesn't "scan all memory". It scans the root and fans out. You can do read/write barriers and such to avoid blocking GC. It doesn't matter what value registers have in them. What matters is what is on the stack and what is reachable from said stack. If you have control of malloc, you can be far more clever than what you are describing. Ruby had a crummy implementation of GC, but you'll notice, for example, that most JVM's do "scan all memory" without forcing all active threads to be blocked at once.