Re: waittime function precision

So, there is a 10 ms overhead roughly on iOS but under load I guess that will vary.

> To: c400-l-Zwy7GipZuJhWk0Htik3J/[email protected]
> From: [email protected]
> Date: Wed, 16 Apr 2014 10:00:40 -0400
> Subject: Re: [C400-L] waittime function precision
> 
> On 4/16/2014 3:39 AM, Jevgeni Astanovski wrote:
> > I tried what you said (_WAIT_MPL_SET)
> > Results are pretty similar:
> > Delay = 10 ms, waittime: 18.480 ms 
> > Delay = 20 ms, waittime: 38.960 ms 
> > Delay = 30 ms, waittime: 59.432 ms 
> > Delay = 40 ms, waittime: 52.360 ms 
> > Delay = 50 ms, waittime: 69.720 ms 
> > Delay = 60 ms, waittime: 89.440 ms 
> > Delay = 70 ms, waittime: 69.824 ms 
> > Delay = 80 ms, waittime: 91.824 ms 
> > Delay = 90 ms, waittime: 119.128 ms
> > 
> > Actually I do not expect that waittime is somewhat precise in milliseconds range - at least I see it isn't.
> > If you tell her to wait for a minute, you probably do not care about +/- 20ms!
> > 
> > But is there something that works reasonably in a milliseconds range?
> 
> I normally think of usleep() but that is not precise on IBM i either.
> Here are some runs I did:
> 
> Delay = 10 ms, waittime: 20.024 ms
> Delay = 20 ms, waittime: 40.016 ms
> Delay = 30 ms, waittime: 60.024 ms
> Delay = 40 ms, waittime: 70.024 ms
> Delay = 50 ms, waittime: 71.920 ms
> Delay = 60 ms, waittime: 74.640 ms
> Delay = 70 ms, waittime: 100.024 ms
> Delay = 80 ms, waittime: 110.024 ms
> Delay = 90 ms, waittime: 120.024 ms
> 
> Delay = 10 ms, waittime: 10.072 ms
> Delay = 20 ms, waittime: 40.024 ms
> Delay = 30 ms, waittime: 38.376 ms
> Delay = 40 ms, waittime: 41.104 ms
> Delay = 50 ms, waittime: 80.024 ms
> Delay = 60 ms, waittime: 66.120 ms
> Delay = 70 ms, waittime: 72.312 ms
> Delay = 80 ms, waittime: 110.024 ms
> Delay = 90 ms, waittime: 120.032 ms
> 
> Delay = 10 ms, waittime: 12.648 ms
> Delay = 20 ms, waittime: 40.024 ms
> Delay = 30 ms, waittime: 31.712 ms
> Delay = 40 ms, waittime: 70.024 ms
> Delay = 50 ms, waittime: 57.080 ms
> Delay = 60 ms, waittime: 77.936 ms
> Delay = 70 ms, waittime: 74.416 ms
> Delay = 80 ms, waittime: 109.944 ms
> Delay = 90 ms, waittime: 120.016 ms
> 
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/time.h>
> #include <decimal.h>
> 
> int main (int argc, char *argv[])
> {
>   struct timeval tp1, tp2 ;
>   struct timezone tpz ;
>   decimal (9,3) Duration ;
>   decimal (19,6) StartTime, EndTime ;
>   int i ;
> 
>   useconds_t useconds ;
> 
>   for (i = 1; i < 10; i++)
>   {
>     gettimeofday(&tp1, &tpz) ;
>     StartTime = tp1.tv_usec ;
>     StartTime = tp1.tv_sec + (StartTime / 1000000D) ;
> 
>     useconds = i * 10000 ;  /* 10ms = 10 000us */
>     usleep(useconds) ;
> 
>     gettimeofday(&tp2, &tpz) ;
>     EndTime = tp2.tv_usec ;
>     EndTime = tp2.tv_sec + (EndTime / 1000000D) ;
>     Duration = (decimal (9,3)) ((EndTime - StartTime) * 1000D) ;
>     printf("Delay = %d ms, waittime: %D(9,3) ms\n", i * 10, Duration) ;
>   }
>   return(0);
> }
> 
> I tried it on Cygwin running on Windows 7.  Cygwin doesn't have decimal
> so I had to make some changes:
> 
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/time.h>
> 
> int main (int argc, char *argv[])
> {
>   struct timeval tp1, tp2 ;
>   struct timezone tpz ;
>   int Duration ;
>   int StartTime, EndTime ;
>   int i ;
> 
>   useconds_t useconds ;
> 
>   for (i = 1; i < 10; i++)
>   {
>     gettimeofday(&tp1, &tpz) ;
>     StartTime = tp1.tv_usec ;
> 
>     useconds = i * 10000 ;  /* 10ms = 10 000us */
>     usleep(useconds) ;
> 
>     gettimeofday(&tp2, &tpz) ;
>     EndTime = tp2.tv_usec ;
>     Duration = EndTime - StartTime ;
>     printf("Delay = %d ms, waittime: %d us\n", i * 10, Duration) ;
>   }
>   return(0);
> }
> 
> $ ./usleep
> Delay = 10 ms, waittime: 11001 us
> Delay = 20 ms, waittime: 20001 us
> Delay = 30 ms, waittime: 30002 us
> Delay = 40 ms, waittime: 40002 us
> Delay = 50 ms, waittime: 50003 us
> Delay = 60 ms, waittime: 60003 us
> Delay = 70 ms, waittime: 70004 us
> Delay = 80 ms, waittime: 80005 us
> Delay = 90 ms, waittime: 96006 us
> 
> $ ./usleep
> Delay = 10 ms, waittime: 15001 us
> Delay = 20 ms, waittime: 22001 us
> Delay = 30 ms, waittime: 30002 us
> Delay = 40 ms, waittime: 40002 us
> Delay = 50 ms, waittime: 50003 us
> Delay = 60 ms, waittime: 60004 us
> Delay = 70 ms, waittime: 70004 us
> Delay = 80 ms, waittime: 80004 us
> Delay = 90 ms, waittime: 90005 us
> 
> 
> I guess that we can't expect an operating system that is optimised for
> database operations to honour precise timings in the same way that an
> embedded processor or even Windows will.  :-(
> 
>   --buck
> -- 
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