[Haskell-cafe] Foreign function performance: monadic vs pure

[Anthony Cowley]

On Mon, Apr 11, 2011 at 8:09 AM, Serguei Son <serguei.son at gmail.com> wrote:
> 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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