From owner-freebsd-arch@FreeBSD.ORG  Thu Mar  1 00:44:04 2012
Return-Path: <owner-freebsd-arch@FreeBSD.ORG>
Delivered-To: arch@FreeBSD.org
Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52])
	by hub.freebsd.org (Postfix) with ESMTP id 61B8F106564A
	for <arch@FreeBSD.org>; Thu,  1 Mar 2012 00:44:04 +0000 (UTC)
	(envelope-from luigi@onelab2.iet.unipi.it)
Received: from onelab2.iet.unipi.it (onelab2.iet.unipi.it [131.114.59.238])
	by mx1.freebsd.org (Postfix) with ESMTP id 228918FC0A
	for <arch@FreeBSD.org>; Thu,  1 Mar 2012 00:44:03 +0000 (UTC)
Received: by onelab2.iet.unipi.it (Postfix, from userid 275)
	id 0B3937300A; Thu,  1 Mar 2012 02:02:19 +0100 (CET)
Date: Thu, 1 Mar 2012 02:02:19 +0100
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Bruce Evans <brde@optusnet.com.au>
Message-ID: <20120301010219.GA14508@onelab2.iet.unipi.it>
References: <20120229194042.GA10921@onelab2.iet.unipi.it>
	<20120301071145.O879@besplex.bde.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20120301071145.O879@besplex.bde.org>
User-Agent: Mutt/1.4.2.3i
Cc: arch@FreeBSD.org
Subject: Re: select/poll/usleep precision on FreeBSD vs Linux vs OSX
X-BeenThere: freebsd-arch@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussion related to FreeBSD architecture <freebsd-arch.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arch>
List-Post: <mailto:freebsd-arch@freebsd.org>
List-Help: <mailto:freebsd-arch-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 01 Mar 2012 00:44:04 -0000

On Thu, Mar 01, 2012 at 11:33:46AM +1100, Bruce Evans wrote:
> On Wed, 29 Feb 2012, Luigi Rizzo wrote:
> 
> >I have always been annoyed by the fact that FreeBSD rounds timeouts
> >in select/usleep/poll in very conservative ways, so i decided to
> >try how other systems behave in this respect. Attached is a simple
> >program that you should be able to compile and run on various OS
> >and see what happens.
> 
> Many are broken, indeed.
> 
> The simple program isn't attached.

attachment stripped by the mailing list, retrying to put it inline
(and comments on a followup email)


----



/*
 * test minimum select time
 *
 *	./prog usec [method [duration]]
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/time.h>
#include <poll.h>

enum { M_SELECT =0 , M_POLL, M_USLEEP };
static const char *names[] = { "select", "poll", "usleep" };
int
main(int argc, char *argv[])
{
	struct timeval ta, tb;
	int usec = 1, total = 0, method = M_SELECT, count = 0;

	if (argc > 1)
		usec = atoi(argv[1]);
	if (usec <= 0)
		usec = 1;
	else if (usec > 500000)
		usec = 500000;
	if (argc > 2) {
		if (!strcmp(argv[2], "poll"))
			method = M_POLL;
		else if (!strcmp(argv[2], "usleep"))
			method = M_USLEEP;
	}
	if (argc > 3)
		total = atoi(argv[3]);
	if (total < 1)
		total = 1;
	else if (total > 10)
		total = 10;
	fprintf(stderr, "testing %s for %dus over %ds\n",
		names[method], usec, total);

	gettimeofday(&ta, NULL);
	for (;;) {
		if (method == M_SELECT) {
			struct timeval to = { 0, usec };
			select(0, NULL, NULL, NULL, &to);
		} else if (method == M_POLL) {
			poll(NULL, 0, usec/1000);
		} else {
			usleep(usec);
		}
		count++;
		gettimeofday(&tb, NULL);
		timersub(&tb, &ta, &tb);
		if (tb.tv_sec > total)
			break;
	}
	fprintf(stderr, "%dus actually took %dus\n",
		usec, (int)(tb.tv_sec * 1000000 + tb.tv_usec) / count );
	return 0;
}
-----