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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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