[racket] Strange time function results

DrRacket is doing a lot of work that can disrupt your attempts to time
things. To really isolate what is going on with some specific piece of
code, you are best off running the timings in 'racket' from the
command-line.

Robby

On Wed, May 16, 2012 at 3:22 PM, Harry Spier <vasishtha.spier at gmail.com> wrote:
> I've just started using the "time" function to check the timings of
> some functions in my program.   I'm testing the timings of two
> functions, each of these functions takes a list of lists and
> transforms it into another list of lists.  The size of each list of
> lists is in the order of  about 1000 x 1000.
>
> Each time I used it in DRacket I got very different results.  So I did
> this test.
> I put three calls to the two functions right after each other in the
> program as follows.
>
> . . .
> (define page-as-binary-matrix  (time (image-file->binary-matrix
> path-to-image-of-page)))
> (define black-pixel-matrix (time (create-matrix-of-black-pixel-ranges
> page-as-binary-matrix)))
>
> (define page-as-binary-matrix1  (time (image-file->binary-matrix
> path-to-image-of-page)))
> (define black-pixel-matrix1 (time (create-matrix-of-black-pixel-ranges
> page-as-binary-matrix)))
>
> (define page-as-binary-matrix2  (time (image-file->binary-matrix
> path-to-image-of-page)))
> (define black-pixel-matrix2 (time (create-matrix-of-black-pixel-ranges
> page-as-binary-matrix)))
>
>
> and got these results
>
> cpu time: 7176 real time: 6620 gc time: 4537
> cpu time: 2262 real time: 2289 gc time: 437
>
> cpu time: 5600 real time: 5257 gc time: 2949
> cpu time: 3728 real time: 3516 gc time: 1748
>
> cpu time: 4040 real time: 3673 gc time: 1388
> cpu time: 2418 real time: 2311 gc time: 422
>
> Its in DrRacket on a dual core pentium under windows.
>
> Why the widely divergent timings and why is CPU time in general more
> than real-time?
>
> Thanks,
> Harry
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