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

Richard B. Gilbert rgilbert88 at comcast.net
Thu Aug 13 16:56:00 UTC 2009


David J Taylor wrote:
> 
> "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.
> 

Do you disagree with the 500 PPM limit?  500 PPM works out to 43 seconds 
per day.  A clock that gains or loses that much time every day has to be 
considered seriously broken!  Plus or minus 50 PPM, or less, is typical.

I suppose that someone could change the limit to 1000 PPM or even 
10,000PPM.  Would that be the right thing to do?  Why?




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