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

David Woolley david at ex.djwhome.demon.invalid
Wed Nov 3 22:08:41 UTC 2010


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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