[Haskell-beginners] Calling a foreign function: superlinear comlexity
David McBride
dmcbride at neondsl.com
Mon Apr 11 03:53:50 CEST 2011
The problem isn't the list or the library library. It is the use of
the function sum, which is lazy and when you try to sum a very large
list, it makes a thunk for every single element in the list before
finally evaluating them all backwards to get the sum.
main = do
let n = 40000000
let lst = map sin [1..n]
print $ sum' lst
where
sum' = foldl1' (+)
This is linear, repeatable and quite a bit faster.
On Sun, Apr 10, 2011 at 7:26 PM, Serguei Son <serguei.son at gmail.com> wrote:
> 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?
>
>
>
>
>
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