[ntp:questions] What level of timesynch error is typical onWinXP?

David L. Mills mills at udel.edu
Thu Nov 4 02:47:33 UTC 2010


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.
>
> _______________________________________________
> questions mailing list
> questions at lists.ntp.org
> http://lists.ntp.org/listinfo/questions





More information about the questions mailing list