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

[John Lato]

>
> From: Gregory Collins <greg at gregorycollins.net>
>
> On Mon, Apr 11, 2011 at 3:55 PM, Serguei Son <serguei.son at gmail.com>
> wrote:
> > So if I must use a safe function returning IO a,
> > there is no way to improve its performance? To give you
> > a benchmark, calling gsl_ran_ugaussian a million times
> > in pure C takes only a second or two on my system.
>
> In the C version, are you also producing a linked list containing all
> of the values? Because that's what mapM does. Your test is mostly
> measuring the cost of allocating and filling ~3 million machine words
> on the heap. Try mapM_ instead.
>

I've done some rough tests of a few versions, and I believe Gregory's
analysis is correct.  These are for calculating a sum of sines of 1..10^7
with ghc-7.0.3.

version              execution time
IO, safe          - 4.08s
Pure, safe     - 3.07s
IO, unsafe      - 2.77s
Pure, unsafe  - 1.95s

For the IO versions, I used the following:

main = foldM (\ !acc n -> fmap (acc +) $ c_sin_io n) [1..10^7] >>= print

(this can possibly be improved; it was the first thing I thought of)

The most important factor (as often the case with Haskell) is to pay
attention to the space properties of your algorithm.  Instead of using mapM,
which forces the spine of the list, it's much better to interleave the fold
step with the other IO.

Note that the "acc" parameter is strict, this is necessary to avoid building
a thunk.

John L.
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