[racket] Strange time function results

I'm definitely not the expert on this, but I just quickly subtracted the gc
time from the cpu time and the times look pretty consistent:

> (- 7176 4537)
2639
> (- 5600 2949)
2651
> (- 4040 1388)
2652

Also, I would guess that cpu time adds the time of all cores or threads or
whatever, so I assume there's some things happening in parallel? Just a
guess.

On Wed, May 16, 2012 at 2:22 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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