[Haskell-cafe] Why do "unsafe" foreign calls block other threads?

[Gregory Crosswhite]

 On 08/03/10 15:33, Evan Laforge wrote:
>>> Just think of "unsafe" in relation to "unsafeIndex" or something.
>>> It's faster, but you have to be sure the index is in bounds.
>> Yes, but the whole reason to use "unsafe" is to get higher performance
>> at the cost of safety.  If the result of calling an "unsafe" foreign
>> function is that you *lose* performance because the other threads have
>> to be halted first, then this seems to defeat the whole point of marking
>> a call as "unsafe" in the first place.
> That's why the function has to return soon and shouldn't do a lot of work.
But again, then what is the point of marking it "unsafe" if it means
that you have to pay a hefty cost of waiting for all the other threads
to halt?  Is the cost of halting all of the other threads really less
than the cost of setting up for a "safe" call?  Maybe it is, and that is
what I am missing here.  If it is not, though, then it seems to me that
marking a call as "unsafe" will *never* gain you performance in a
multi-threaded environment, so that there is never any point in using it
in such an environment.  (Though, of course, it could gain you
performance in a single-threaded environment.)

Cheers,
Greg