Silly question about IORefs and MVars

Hello, I have a silly question about IORefs and MVars. When do we have to use MVars if a var is accessed by multiple threads. In fact, I wonder why IORefs updates aren't safe : it seems that preemptive scheduling takes place during memory allocation and I can't see where there could be an allocation (and so a switch) in a read or a write of an IORef. Related question : how less performant is a MVar comparated to simple ref. best regards, Nicolas Oury