[ntp:questions] Re: program to test actual resolution of Linux?
Ulrich.Windl at RZ.Uni-Regensburg.DE
Wed Sep 10 15:00:04 UTC 2003
"David L. Mills" <mills at udel.edu> writes:
> For whatever good or bad come come of it, the actual ntpd precision is
> calculated by calling get_systime(), which reads the system clock with
> the highest resolution syscall available. So, what ntpd calls precision
> is actually the time to cycle through the kernel and read the clock, not
> the inherent resolution of the clock counter, which can be much less
> than the precision. For instance, a Sun Blade 1000 has a clock counter
> resolution of a bit more than a nanosecond, but a precision of about 400
> ns. But, the actual resolution when reading the Blade clock is rounded
> up to the next nanosecond.
for practical use: Does it make any difference? The only way to
observe the system clock is by making system calls. If you see just
nanoseconds or multiples of it, you may assume the clock has no better
resolution. Did I think wrong?
> While the procedure reads the clock several times, it is a bit more
> tricky than first apparent. Some kernels (nanokernel/microkernel at
> least from here) require Lamport's happens-before relation be rigorously
> defended, so if the clock counter is really gross, like old Sun 500
> microseconds, and the clock is read several times during that clock
> tick, the time advances by une unit (nano or micro) for each read.
For my hardware this has never been a problem, because you cannot
execute two system calls within the same nanosecond. Even for a
microsecond you need a really fast machine. Still, the
time-compression is only temporary, right? I mean the amount is
returned to the user but the system clock doesn't get that amount
> Unless detected and avoided, this could mislead the results. Time is so
> fickle and intricate.
> Ulrich Windl wrote:
> > Drk,
> > call gettimeofday() twice and subtract. If you won't believe in the
> > method, describe what you are expecting as result.
> > Regards,
> > Ulrich
> > P.S: If you are lazy:
> > ntpq> rl
> > status=06f4 leap_none, sync_ntp, 15 events, event_peer/strat_chg,
> > version="ntpd 4.1.1 at 1.786 Tue Feb 4 16:39:46 UTC 2003 (1)",
> > processor="i586", system="Linux2.4.19-4GB", leap=00, stratum=2,
> > precision=-18, rootdelay=44.226, rootdispersion=275.741, peer=23695,
> > refid=lanczos.maths.tcd.ie,
> > reftime=c30861df.02caab8a Tue, Sep 9 2003 16:27:43.010, poll=7,
> > clock=c308624c.95697f1f Tue, Sep 9 2003 16:29:32.583, state=4,
> > offset=23.011, frequency=-15.395, jitter=15.000, stability=25.487
> > Precision means 2^-18 seconds...
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