[Haskell-cafe] measuring time differences between functions

[Daniel Fischer]

Am Mittwoch 17 Juni 2009 20:25:08 schrieb Nico Rolle:
> Hi everyone.
> I made parallel versions of my functions projection selection and sort
> and i wanted to test the difference in runtime.
> But he only printed 0.00 twice.
> Maybe hes just too fast?
> Or is it because of lazy evaluation?
> or is there a better way to mesure performance in execution time?
> regards
>
> main = do
>     xs <- readCSV "dataconvert/lineitem.tbl" '|'
>     start <- getCurrentTime
>     let pnp = projection [5] xs
>     let snp = selection (\x -> (x!!0) > (Int 17000)) pnp
>     let sortnp = sort [0] [ASC] snp
>     end <- getCurrentTime
>     putStrLn $ show (end `diffUTCTime` start)
>     start2 <- getCurrentTime
>     let pp = pProjection [5] xs
>     let sp = pSelection (\x -> (x!!0) > (Int 17000)) pp
>     let sortp = pSort [0] [ASC] sp
>     end2 <- getCurrentTime
>     putStrLn $ show (end2 `diffUTCTime` start2)
>     return snp

The let bindings do not cause any computation because of laziness, all they do is bind the 
names to a thunk that says how to calculate the value if it is needed. Regardless of how 
long the computation would take, binding the names to a thunk takes only a few 
nanoseconds.

To measure the time needed for the computations, you must force them to be carried out 
between the two calls to getCurrentTime.

Probably inserting

print $ last snp

before 
end <- getCurrentTime
(and likewise for the second) would be enough (if the sorting doesn't require the values 
in snp to be fully evaluated, you would have to do more forcing).