[ntp:questions] Loopstats jitter field mostly zero?

unruh unruh at invalid.ca
Tue Nov 27 22:54:17 UTC 2012


On 2012-11-27, gabs <gsricalde at gmail.com> wrote:
> On Sunday, November 25, 2012 3:56:28 AM UTC+8, David Taylor wrote:
>> I've just started the statistics collection on my Linux/RPi NTP server, 
>> and when plotting the jitter results I found that most of the values 
>> were zero.  I don't see that on my Windows stratum-1 systems.  Is there 
>> a limit in the calculation of the jitter value recorded (to 9 decimal 
>> places) in the loopstats file.
>> 
>> I really can't imagine that such a low-powered server is /that/ good - 
>> better than a nanosecond.
>> -- 
>> Cheers,
>> David
>> Web: http://www.satsignal.eu
>>
>
> The displayed jitter in loopstats or the peer list can be different from the
> actual jitter of the source. This case is the reverse: low actual jitter,
> 15 us displayed jitter.

Well, no, it is NOT low actual jitter. As you can see, from the
peerstats file, the actual offsets are not zero. and are not all the
same. That is what you would need for no jitter. Also to have exactly
the same jitter for many times in a row would also be pretty
unbelievable. 


>
> root at OpenWrt:/tmp# tail /tmp/ntp/loops
> 56257 76888.554 -0.000000080 6.553 0.000015259 0.000053 3
> 56257 76896.554 -0.000000098 6.553 0.000015259 0.000052 3
> 56257 76904.554 -0.000000071 6.553 0.000015259 0.000051 3
> 56257 76912.554 -0.000000061 6.553 0.000015259 0.000049 3
> 56257 76920.554 -0.000000091 6.553 0.000015259 0.000049 3
> 56257 76928.554 -0.000000008 6.553 0.000015259 0.000046 3
> 56257 76936.554 -0.000000048 6.553 0.000015259 0.000043 3
> 56257 76944.554 -0.000000014 6.553 0.000015259 0.000041 3
> 56257 76952.554 0.000000039 6.553 0.000015259 0.000038 3
> 56257 76960.554 0.000000016 6.553 0.000015259 0.000036 3
>
> This is normal for this platform since ntpd seems to use the precision as the
> lower bound for reported jitter. The precision is low as it takes 12 us to read
> the clock and convert to NTP timestamp format.

What kind of platform is that?

>
> For the RPi, I have no idea but I'll be getting one before the year ends :)



More information about the questions mailing list