linux-kernel - Re: slow nanosleep?

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Date:	Wed, 8 Sep 2010 10:04:50 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: slow nanosleep?

Eric Dumazet <eric.dumazet@...il.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@...alhost ~ # ./nanosleep
> > nanosleep
> > req:0 :0
> > tv_res:0 :119
> > root@...alhost ~ # ./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@...alhost ~ # ./nanosleep
nanosleep
req:0 :0
tv_res:0 :112

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