On Mon, Apr 11, 2011 at 8:09 AM, Serguei Son
Consider two versions of sin wrapped: foreign import ccall "math.h sin" c_sin_m :: CDouble -> IO CDouble
Marking this call as unsafe (i.e. foreign import ccall unsafe "math.h sin") can improve performance dramatically. If the FFI call is quick, then I believe this is the recommended approach. If you really need the imported function to be thread safe, then perhaps you should move more of the calculation into C to decrease the granularity of FFI calls. It is remarkably easy to get the meanings of safe and unsafe confused, and I can't even see the word "unsafe" in the current FFI user's guide! http://www.haskell.org/ghc/docs/7.0.3/html/users_guide/ffi-ghc.html Anthony
and foreign import ccall "math.h sin" c_sin :: CDouble -> CDouble
One can invoke them so:
mapM c_sin_m [1..n] mapM (return . c_sin) [1..n]
On my computer with n = 10^7 the first version never finishes, whereas the second one calculates the result within seconds.
To give you my context, I need to call a random variable generator multiple times, so that it must return IO a.
Any explanation for this behavior?
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