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    SubjectRe: slow nanosleep?
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    Eric Dumazet <eric.dumazet@gmail.com> wrote on 2010/09/08 09:56:40:
    >
    > 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 */

    That makes litte difference:
    root@localhost ~ # ./nanosleep
    nanosleep
    req:0 :0
    tv_res:0 :112

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