[ntp:questions] Re: time

Richard B. Gilbert rgilbert88 at comcast.net
Wed Nov 30 03:57:57 UTC 2005


Bjorn Gabrielsson wrote:

>"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
>
>  
>
>>K wrote:
>>    
>>
>
>  
>
>>>I know about NTP v3 and v4 and use it on some systems.  I need a time
>>>accuracy of < 1 ms.
>>>
>>>I know that the linux os is compiled for something like 1 ms precision
>>>      
>>>
>
>?
>  
>
>>So it can be done.   I think that the Motorola GPS has a great deal to
>>do with it.  
>>    
>>
>
>The motorola should carry you down towards 1us, but K was only asking
>for a thousand times less accuracy... :-)
>
>  
>
>>clock would be much less stable.  10 ms is about the best accuracy I
>>would expect from a client synchronized to a server that synchronized
>>over the internet.
>>    
>>
>
>Even with a DSL-line - if its not overloaded - sub 1ms is easily
>attained with good internet servers. Looking at a machine with four
>external servers, I get see,
>
>/usr/sbin/ntpq  -c pe 
>
>     remote           refid      st t when poll reach   delay   offset  jitter
>==============================================================================
> LOCAL(0)        73.78.73.84      5 l   25   64  377    0.000    0.000   0.001
> xxx             .PPS.            1 u  219  256  377   11.476   -0.235   0.779
> xxx             .PPS.            1 u  102  256  377   17.818   -0.140  75.660
> xxx             .PPS.            1 u  217  256  377   21.575    0.203   0.235
> xxx             .PPS.            1 u  192  256  377   20.889    0.385   0.142
>
>This machine usually have two GPS-refclocks and they show no
>systematic offsets wrt the Internet servers.
>
>--
>
>    Björn
>  
>
Either your internet connection, your internet servers, or both are far 
better than mine.  I have broadband cable.  My NTP server, Sunblok, uses 
five internet servers in addition to the Motorola GPS reference clock.  
The internet servers show systematic offsets with respect to GPS of +1 
to +5 ms.  There are also seemingly random spikes in the time received 
from the internet servers of +/- 5 to 20 ms.  When the GPS is not 
running, ntpd tends to synchronize with a different server every few 
minutes which does nothing for the stability of the clock.




More information about the questions mailing list