Date: Wed, 16 May 2001 18:35:42 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Matt Dillon <dillon@earth.backplane.com> Cc: Terry Lambert <tlambert2@mindspring.com>, dave <dleimbac@earthlink.net>, freebsd-questions@FreeBSD.ORG, arch@FreeBSD.ORG Subject: Re: Gettimeofday Again... Message-ID: <Pine.BSF.4.21.0105161759410.8173-100000@besplex.bde.org> In-Reply-To: <200105151737.f4FHbBL55271@earth.backplane.com>
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On Tue, 15 May 2001, Matt Dillon wrote:
> Also, using gettimeofday() is a ridiculous way to measure fine grained
> time billions of times in production code. I mean, sure, it works...
> but getitimer() is about 5 times faster.
This seems unlikely, since most of the overhead for both is in the syscall.
Actual testing shows that getitimer() is a whole 10% faster on a Celeron
5.5 * 95 MHz:
$ sysctl kern.timecounter.hardware
kern.timecounter.hardware: TSC
$ sysctl machdep.tsc_freq
machdep.tsc_freq: 522493830
$ cat z.c
#include <sys/types.h>
#include <sys/time.h>
#include <err.h>
#include <stdlib.h>
int
main(void)
{
struct timeval tv;
int i;
for (i = 0; i < 1000000; i++)
if (gettimeofday(&tv, NULL) != 0)
err(1, "gettimeofday");
exit(0);
}
$ cc -O -o z z.c
$ time ./z
2.04 real 0.47 user 1.57 sys
$ cat q.c
#include <sys/types.h>
#include <sys/time.h>
#include <err.h>
#include <stdlib.h>
int
main(void)
{
struct itimerval it;
int i;
it.it_interval.tv_sec = 0;
it.it_interval.tv_usec = 0;
it.it_value.tv_sec = 100;
it.it_value.tv_usec = 0;
if (setitimer(ITIMER_REAL, &it, NULL) != 0)
err(1, "setitimer");
for (i = 0; i < 1000000; i++)
if (getitimer(ITIMER_REAL, &it) != 0)
err(1, "getitimer");
exit(0);
}
$ cc -O -o q q.c
$ time ./q
1.84 real 0.46 user 1.37 sys
This is for my version of -current, which has a few pessimizations in
microtime(). Plain -current would be a few nsec faster.
OTOH, gettimeofday() may be significantly slower than getitimer() if the
hardware timecounter is slow:
# sysctl -w kern.timecounter.hardware=i8254
$ sysctl kern.timecounter.hardware
kern.timecounter.hardware: i8254
$ time ./z
5.06 real 0.44 user 4.60 sys
Bruce
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