On Tuesday 02 August 2011, 17:58:59, Ovidiu Deac wrote:
I'm trying to do some performance evaluation and I'm stuck with the fact that Haskell's lazy evaluation - which make my sort extremely fast :)
Well, *I* can do nothing even faster ;)
I read this page: http://www.haskell.org/haskellwiki/Performance/Strictness but I didn't get it so I'm asking here: How do I make the function 'measure' to actually force the evaluation of (f p)?
You have to make the getting of the second time value depend on (f p) - and in such a way that f p is completely evaluated. One way would be to print the result between the gettings of time, but for certain tasks, the printing may take a substantial amount of time relative to the function you want to time, so other dependencies would have to be introduced. Also, printing values may not be desirable, so you can introduce artificial (data) dependencies with seq, pseq and such. For sorting lists, evaluating the last element or the length forces complete evaluation, so [see below]
Thanks, ovidiu
See the code below: ------------------------------------ module Main where import Prelude import Data.List import Data.Time.Clock import System.Random
quickSort [] = [] quickSort (x:xs) = (quickSort small) ++ [x] ++ (quickSort big) where small = [p | p <- xs, p <= x] big = [p | p <- xs, p > x]
randomlist :: Int -> StdGen -> [Int] randomlist n = take n . unfoldr (Just . random)
len = 10 ^ 10
measure f p = do t1 <- getCurrentTime let sorted = f p
len = length sorted
-- t2 <- getCurrentTime t2 <- len `seq` getCurrentTime
-- we made the action to get the second time depend on len, so the list has to be sorted before the action can be performed.
let diff = diffUTCTime t2 t1 return diff
However, for timing such functions, wall-clock time is not good, better is measuring CPU-time, so use System.CPUTime.getCPUTime instead of getCurrentTime (needs some adaption of other code).
main = do seed <- newStdGen let rs = randomlist len seed
putStrLn $ "Sorting " ++ (show len) ++ " elements..."
t <- measure quickSort rs
putStrLn $ "Time elapsed: " ++ (show t)