David,
In Mirsolav's test, the frequency was computed at 172 PPM, which is well
within the capabilities of the algorithm. On the other hand, even if the
intrinsic hardware frequency error is more than 500 PPM, the frequency
will be set to 500 PPM and the daemon will continue normally, but will
not be able to reduce the residual time offset to zero. Mirsolav's
experience is far different; apparently, the computed frequency was not
installed and the daemon continued at that frequency error and exceed
the step threshold every twenty minutes. This apparently occurred
whether or not the kernel was enabled and whether or not the simulator
was involved.
Again I stress no such behavior occurs with Solaris or FreeBSD, so
something might be affected by Linux adjtime functionality. It might
help to eyeball the ntp_loopfilter.c and the direct_freq() routine.
There might be some angst with the Linux semantics.
Dave
David Woolley wrote:
David L. Mills wrote:
I don't think that is right. The adjtime() call can be in principle
anything, accoridng to the Solaris and FreeBSD man pages, but the
rate of adjustment is fixed at 500 PPM in the Unix implementation. If
the Linux argument is limited to 500 microseconds, Linux is
essentially unusable with NTP. I would be surprised if this were the
case.
I think what he is really saying is that he is not using the kernel
discipline and ntpd is tweaking the clock every second, but he has
broken hardware, which requires a correction of more than 500ppm, and,
as he is describing it, adjtime has a residual correction to apply
before the next tweak, or more likely ntpd is limiting it to 500ppm.
As to Linux, I would guess most users of ntpd are using Linux.
Miroslav: ntpd requires an uncorrected clock that is good to
significantly better than 500ppm. You can probably get away with
450ppm, but the transient response will be compromised.
A good quality PC should be within about 10ppm. A cheap one should be
within about 50ppm. > 500ppm is broken. You can use tickadj to
compensate in steps of 100ppm, but a machine with that error is likely
to have other problems; the crystal may be barely disciplining the
oscillator.
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