[racket] Strange time function results

Thanks,

I didn't realize cpu time includes gc time. and yes its dual core

On Wed, May 16, 2012 at 4:31 PM, Eli Barzilay <eli at barzilay.org> wrote:
> A few minutes ago, Harry Spier 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.
>> [...]
>>
>> 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
>
> They're not that far off -- if you subtract the gc time.  In general,
> for such timing it's best to run the code several times, possibly with
> calling `collect-garbage' between calls.
>
>
>> and why is CPU time in general more than real-time?
>
> (This looks weird, maybe you're using multiple cores?)
>
> --
>          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                    http://barzilay.org/                   Maze is Life!