cvs commit: src/sys/sys time.h src/sys/kern kern_time.c

[Robert Watson]

On Sun, 27 Nov 2005, Robert Watson wrote:

>  Add experimental low-precision clockid_t names corresponding to these
>  clocks, but implemented using cached timestamps in kernel rather than
>  a full time counter query.  This offers a minimum update rate of 1/HZ,
>  but in practice will often be more frequent due to the frequency of
>  time stamping in the kernel:
>
>  New clockid_t name              Approximates existing clockid_t
>
>  CLOCK_REALTIME_FAST             CLOCK_REALTIME
>  CLOCK_MONOTONIC_FAST            CLOCK_MONOTONIC
>  CLOCK_UPTIME_FAST               CLOCK_UPTIME
>
>  Add one additional new clockid_t, CLOCK_SECOND, which returns the
>  current second without performing a full time counter query or cache
>  lookup overhead to make sure the cached timestamp is stable.  This is
>  intended to support very low granularity consumers, such as time(3).

These changes are primarily intended to support the identification of 
time-associated bottlenecks and allow experimentation with application 
scoped reduction in quality of time use.  Attached is a small LD_PRELOAD 
library to allow applications to be switched to the _FAST variants for 
gettimeofday().  Build and install, then set:

   LD_PRELOAD=/usr/lib/libwrapper.so ; export LD_PRELOAD

This is not intended to be the last word in how this should be done -- 
more, to facilitate some experimentation by providing some framework for 
experimentation (i.e., a sample API and code to tweak applications).  I 
expect that the details will be subject to substantial change. :-)

Robert N M Watson
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# $FreeBSD$

LIB=		wrapper
SHLIB_MAJOR=	1
SRCS=		wrapper.c

NO_MAN=

.include <bsd.lib.mk>
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/*-
 * Copyright (c) 2005 Robert N. M. Watson
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $FreeBSD$
 */

/*
 * Small wrapper library to substitute implementations of gettimeofday(2) and
 * time(3) with lower resolution variations.  time(3) is unconditionally
 * degraded, since it will return a truncated time anyway.  gettimeofday(3)
 * checks the TIMEWRAPPER environmental variable, which can be set to either
 * "PRECISE" or "FAST".
 */

#include <sys/time.h>

#include <stdlib.h>
#include <string.h>

static int		timewrapper_initialized;
static clockid_t	timewrapper_clock;

/*
 * Select whatever system default is present for CLOCK_REALTIME as the
 * default if the environmental variable is unset, or set to an invalid
 * value.
 */
#define	TIMEWRAPPER_ENV		"TIMEWRAPPER"
#define	TIMEWRAPPER_DEFAULT	CLOCK_REALTIME
#define	TIMEWRAPPER_STRPRECISE	"PRECISE"
#define	TIMEWRAPPER_STRFAST	"FAST"

static void
timewrapper_initialize(void)
{
	const char *whichclock_env;

	whichclock_env = getenv(TIMEWRAPPER_ENV);
	if (whichclock_env == NULL) {
		timewrapper_clock = TIMEWRAPPER_DEFAULT;
		timewrapper_initialized = 1;
		return;
	}

	if (strcmp(whichclock_env, TIMEWRAPPER_STRPRECISE) == 0)
		timewrapper_clock = CLOCK_REALTIME_PRECISE;
	else if (strcmp(whichclock_env, TIMEWRAPPER_STRFAST) == 0)
		timewrapper_clock = CLOCK_REALTIME_FAST;
	else
		timewrapper_clock = TIMEWRAPPER_DEFAULT;
	timewrapper_initialized = 1;
}

int
gettimeofday(struct timeval *tp, struct timezone *tzp)
{
	struct timespec ts;

	if (!timewrapper_initialized)
		timewrapper_initialize();

	/*
	 * XXXRW: Not ideal, since returning EINVAL from gettimeofday() isn't
	 * expected.  Fall back to the gettimeofday() system call instead?
	 */
	if (clock_gettime(timewrapper_clock, &ts) < 0)
		return (-1);

	tp->tv_sec = ts.tv_sec;
	tp->tv_usec = ts.tv_nsec / 1000;

	return (0);
}

time_t
time(time_t *tloc)
{
	struct timespec ts;

	if (clock_gettime(CLOCK_SECOND, &ts) < 0) {
		if (tloc != NULL)
			*tloc = -1;
		return (-1);
	}

	if (tloc != NULL)
		*tloc = ts.tv_sec;

	return (ts.tv_sec);
}