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

David Mills mills at udel.edu
Thu May 7 19:46:45 UTC 2009


Guys,

The 500 PPM limit is not due to an arbitrary decision. It is the maximum 
that the Unix adjtime() syscall can do. I have no idea what the 
corresponding limit is in Windows. In any case it can be changed by the 
NTP_MAXFREQ define in the ntp_proto.h header file. However, note that a 
500 PPM frequency error amounts to over 43 s per day.



Dave

Richard B. Gilbert wrote:

>E-Mail Sent to this address will be added to the BlackLists wrote:
>  
>
>>Richard B. Gilbert wrote:
>>    
>>
>>>The specifications for NTPD say that it will correct
>>> errors less than or equal to 500 PPM.
>>>
>>>I beleive that hardware outside of this limit can
>>> properly be described as broken!
>>>      
>>>
>>Could this be mitigated by setting panic to 0?
>>
>> (Wasn't there some other bigtimestep option too?)
>>
>>
>>    
>>
>
>You'd have to ask Dave Mills.  I'd just scrap out the mother board or 
>the whole computer!  If it's software that's causing your pain it may be 
>unreasonable to ask you to scrap out Windows.
>
>It's not unreasonable to expect a computer clock to have a frequency 
>error of less than 50 Parts Per Million and NTPD will cope with up to 
>500 PPM.  Remember that 500 PPM works out to 43 seconds per day!  As I 
>said: "SERIOUSLY BROKEN"!!
>
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