Timing Functions

[jekwtw]

I'm putting together a script to gather run-time stats for some functions I'm working with, and I'm having a terrible time.  My strategy is to evaluate a function a number of times and compute the difference between the elapsed CPU time before and after the repeated calls.

> timeNReps :: (a -> b) -> a -> Int -> FilePath -> IO ()
> timeNReps func arg reps fileName =
>             do t0 <- System.CPUTime.getCPUTime
>                  runNReps func arg reps
>                  t1 <- System.CPUTime.getCPUTime
>                  appendFile fileName ((showMS (t1 - t0)) ++ "\n")
>    where
>    showMS n = show (n `quot` 1000000000)

showMS just converts the pico-second result into milli-seconds and stringifies it.

runNReps is an IO program (do sequence) that is intended to call the function and tail-call itself a given number of times:

> runNReps :: (Int -> a) -> Int -> Int -> IO ()
> runNReps f x todo
>             | todo > 0 = do let junk = (f x)
>                                    runNReps f x (todo - 1)
>             | otherwise = return (())

Apparently runNReps doesn't apply f to x at all!  I've called my test function with a suitable argument from top level (within ghci) and it takes ~20 sec. wall time to return; when I evaluate "runNReps test arg 1" it returns immediately.  When I use this within my timing script I get timing output that indicates that calls for all args between 1 and 50 take about the same (very small) amount of time, but I know, both from theory and experiments in Scheme versions, that my test function's complexity is exponential in its arg.

I'm using GHC 6.0.1 under Mandrake 9.1 on a 1.8 GHz Pentium box with 256MB RAM.

Any idea where I'm going wrong?

 -- Bill Wood
    bill.wood at acm.org
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