[Haskell-cafe] measuring time differences between functions
daniel.is.fischer at web.de
Wed Jun 17 15:07:36 EDT 2009
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?
> main = do
> xs <- readCSV "dataconvert/lineitem.tbl" '|'
> start <- getCurrentTime
> let pnp = projection  xs
> let snp = selection (\x -> (x!!0) > (Int 17000)) pnp
> let sortnp = sort  [ASC] snp
> end <- getCurrentTime
> putStrLn $ show (end `diffUTCTime` start)
> start2 <- getCurrentTime
> let pp = pProjection  xs
> let sp = pSelection (\x -> (x!!0) > (Int 17000)) pp
> let sortp = pSort  [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
To measure the time needed for the computations, you must force them to be carried out
between the two calls to getCurrentTime.
print $ last snp
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).
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