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

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