linux-kernel - slow nanosleep?

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Date:	Wed, 8 Sep 2010 09:45:12 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: slow nanosleep?


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.

    Jocke

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