Silly question about IORefs and MVars

[Nicolas Oury]

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