[Haskell-beginners] Calling a foreign function: superlinear comlexity

10 Apr 2011

      I call GSL's gsl_ran_ugaussian function in the following way (using
bindings-gsl):

module Main where

import Bindings.Gsl.RandomNumberGeneration
import Bindings.Gsl.RandomNumberDistributions
import Foreign
import Control.Monad
import Data.List

main = do
        let n = 100000
	p <- peek p'gsl_rng_mt19937
	rng <- c'gsl_rng_alloc p
	lst <- replicateM n $ c'gsl_rng_uniform rng
	print $ sum lst	

As I increase n from 10^4 to 10^5 to 10^6 execution time grows superlinearly.

To forestall the answer that the reason is the overhead of List, 
this code scales approximately linearly:

module Main where

import Foreign
import Control.Monad
import Data.List

main = do
        let n = 100000
	let lst = map sin [1..n]
	print $ sum lst	

Another interesting observation: when I wrap the sin function 
of math.h with signature CDouble -> IO CDouble calling it 
repeatedly scales superlinearly, whereas when I wrap it as a pure 
function calling it repeatedly scales linearly.

What is the reason for this performance and how can 
I make the first code scale linearly in execution time?

[Haskell-beginners] Calling a foreign function: superlinear comlexity

Serguei Son