Date: Fri, 21 Oct 2005 16:47:29 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Bruce Evans <bde@zeta.org.au> Cc: cvs-src@FreeBSD.org, src-committers@FreeBSD.org, Andre Oppermann <andre@FreeBSD.org>, cvs-all@FreeBSD.org Subject: Re: Timekeeping [Was: Re: cvs commit: src/usr.bin/vmstat vmstat.c src/usr.bin/w w.c] Message-ID: <30613.1129906049@critter.freebsd.dk> In-Reply-To: Your message of "Sat, 22 Oct 2005 00:23:03 %2B1000." <20051021230751.Q5110@delplex.bde.org>
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In message <20051021230751.Q5110@delplex.bde.org>, Bruce Evans writes:
>On Thu, 20 Oct 2005, Poul-Henning Kamp wrote:
>> ...
>> If people do stupid things like use hard steps (*settime*()) to
>> correct rate problems, then they get what they deserve, including
>> potentially backwards jumps in time, but the integral over time of
>> all steps apart from the first one amounts to a rate correction.
>
>Using *settime*() isn't stupid.
Using it more than once per boot is stupid, and abundantly makes
it clear that precise timekeeping is not what you are attempting.
>> In summary: CLOCK_MONOTONIC is our best estimate of how many SI
>> seconds the system have been runing [3].
>
>Actual testing shows that CLOCK_MONOTONIC, or possibly CLOCK_REALTIME
>less the boot time, gives a very bad estimate of how long the system has
>been running. The difference between these clocks was about 500 seconds
>on all systems tested:
>
>% sledge:
>% 1:03PM up 22:45, 1 user, load averages: 0.23, 0.08, 0.02
>% uptime 1 81900
>% uptime 2 82887
Works fine for me:
sledge phk> cat > a.c
#include <stdio.h>
#include <sys/time.h>
int
main(int argc __unused, char **argv __unused)
{
struct timeval boottime;
struct timespec up, utc;
size_t s;
s = sizeof boottime;
sysctlbyname("kern.boottime", &boottime, &s, NULL, 0);
printf("Boottime\t%ld.%06d\n",
boottime.tv_sec, boottime.tv_usec);
clock_gettime(CLOCK_MONOTONIC, &up);
printf("Uptime\t\t%ld.%09d\n", up.tv_sec, up.tv_nsec);
clock_gettime(CLOCK_REALTIME, &utc);
printf("UTC\t\t%ld.%09d\n", utc.tv_sec, utc.tv_nsec);
return(0);
}
^D
sledge phk> make a
cc -O2 -fno-strict-aliasing -pipe a.c -o a
sledge phk> ./a
Boottime 1129904420.816916
Uptime 1317.689747507
UTC 1129905738.506671500
sledge phk> bc
[...]
1129904420.816916+1317.689747507
1129905738.506663507
Mind you, there is no way the above cannot work because that is how
the math in the kernel works.
>Not given, and not true. After syncing with an accurate external clock
>by a step, we know the real time very accurately. Normally we sync
>soon after booting. Then we know the boot time very accurately (it
>is the current real time less CLOCK_MONOTONIC). Then if we resync
>with the external clock later using a step, we again know the real
>time very accurately, [...]
But have resigned ourselves to not caring about the actual length
of seconds and therefore, presumably, having no serious interest
in timekeeping quality.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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