[ntp:questions] 500ppm - is it too small?

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Thu Aug 13 13:17:50 UTC 2009


"Ulrich Windl" <Ulrich.Windl at RZ.Uni-Regensburg.DE> wrote in message 
news:87y6poxfg8.fsf at pc9454.klinik.uni-regensburg.de...
> "David J Taylor"
> <david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid> writes:
>
>> I've recent been suggesting the Windows port of NTP as a program 
>> suitable for
>> an application where the timekeeping needed to be within a second or 
>> two.
>> Yes, NTP is overkill, but it has the advantages of multiple servers, 
>> best
>> server selection, adaptive poll rate, and memory of the clock drift 
>> etc.
>> However, on quite a few installations - at a guess between 1% and 5% - 
>> NTP has
>> failed because the click frequency error appears to be too great for 
>> NTP to
>> correct.
>
> I still say NTP is no technology to fix bad hardware or clocks. Those
> Windows people all seem not to care much about time, while the NTFS
> filesystem stores timestamps in nanoseconds AFAIK.

Why the insult?  Just because someone runs a particular OS doesn't mean 
they do or don't care about timekeeping.  Their OS may be forced on them 
by the applications they need to run.  NTFS timestamps are in 100 
nanosecond units, IIRC.

>>
>> Is there any feeling for changing the 500ppm limit, perhaps to 1000ppm 
>> or even
>> as much as 5000ppm (to pull a figure out of the hat)?  Or is 500ppm 
>> generally
>> believed to be the worst error which should be compensated?
>
> When increasing the PPM range, you must also decrease the polling
> interval. Do we really want that? I'd say no.

I agree.

> (Interestingly Windows "genuine" NTP client adjusts the clock once per
> week by default. Why not use that service?)

Again, are you trying to put down Windows?   It comes across like that. 
People are interested in NTP because it can provide better performance 
than the manufacturer-supplied service.

>> One possibility is that some of the problem PCs are portables, with 
>> some sort
>> of power-saving or even hibernation scheme.  I don't have direct 
>> visibility of
>> the type of PC.
>
> Well if someone runs ntpd on a machine and does a suspend to disk (or to
> RAM), and then after a few hours resumes execution, ntpd will be really
> confused about the time it missed. I think those machines should not run
> NTP. Maybe the solution Microsoft provides fits the needs of those.
>
> Regards,
> Ulrich

Ulrich,

So machine running other than Windows don't suspend?  In any case, it was 
more the clock-speed variation I was thinking of.

But I note that you think 500ppm is enough.

Thanks for your input.
David 




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