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SubjectRe: slow nanosleep?
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Le mercredi 08 septembre 2010 à 09:45 +0200, Joakim Tjernlund a écrit :
> Hi Thomas
>
> while playing with nanosleep I noticed that it is slow
> compared to select. This little test program shows what
> the effect:
> #include <time.h>
> #include <sys/time.h>
> #include <stdio.h>
> #define NANO_SLEEP 1
> main()
> {
> struct timespec req, rem;
> struct timeval tv1, tv2, tv_res;
> int res;
>
> rem.tv_sec = 0;
> rem.tv_nsec = 0;
>
> req.tv_sec = 0;
> req.tv_nsec = 0;
>
> tv2.tv_sec = req.tv_sec;
> tv2.tv_usec = req.tv_nsec/1000;
>
> gettimeofday(&tv1, NULL);
> #ifdef NANO_SLEEP
> res = nanosleep(&req, &rem);
> #else
> res = select(0, NULL,NULL,NULL, &tv2);
> #endif
> gettimeofday(&tv2, NULL);
> timersub(&tv2, &tv1, &tv_res);
> #ifdef NANO_SLEEP
> printf("nanosleep\n");
> #else
> printf("selectsleep\n");
> #endif
> printf("req:%d :%d\n", (int)req.tv_sec, (int)req.tv_nsec/1000);
> printf("tv_res:%d :%d\n", (int)tv_res.tv_sec, (int)tv_res.tv_usec);
> }
> root@localhost ~ # ./nanosleep
> nanosleep
> req:0 :0
> tv_res:0 :119
> root@localhost ~ # ./selectsleep
> selectsleep
> req:0 :0
> tv_res:0 :36
>
>
> Isn't nanosleep to slow here? The min time is about 120 us compared
> to select which is 36 us. I would expect nanosleep to be better than
> select.
>
> Kernel 2.6.35 with HIGH_RES timers on Powerpc(MPC8321, 266 MHz)
> x86 shows the same effect.
>

You need :

#define PR_SET_TIMERSLACK 29

prctl(PR_SET_TIMERSLACK, 1); /* 1 nsec resolution, please */


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