[ntp:questions] What level of timesynch error is typical onWinXP?

David L. Mills mills at udel.edu
Mon Oct 25 17:31:57 UTC 2010


Miroslav,

No, it not expected, unless you are referring to broadcast mode when 
started with +-100 ms initial offset. That has been corrected as per 
your bug report.

For record, a hold timer is started when the first update is received 
after startup and ends when the residual offset is less than 0.5 ms or 
after a timeout of 600 s. During the hold interval the PLL loop time 
constant is set very low and the frequency discipline is disabled. With 
this arrangement, the offset typical converges within 600 s, even with 
initial offsets up to +-100 ms, and much less if the initial offset is 
in the 10-50 ms range. If you see different behavior, either with 
client/server or broadcast modes, please report.

Note that, if the initial frequency error is significant, there may 
still be a surge correction. If the frequency file is not present at 
startup, the frequency will be measured, typically within +-1 PPM, 
within 600 s, following with the above scheme will be in effect. Under 
worst case conditions, there still could be a wobble following startup 
not exceeding 1 ms. If somebody finds an extraordinarily unlikely y set 
of circumstances leading to, say 2 ms, I'm not going to lose sleep over 
that.

Dave

The Miroslav Lichvar wrote:

>On Fri, Oct 22, 2010 at 11:39:47AM +0100, David J Taylor wrote:
>  
>
>>Thanks, Dave.  I may be missing something here, but it seems to me
>>that 4.2.7p58 still takes a number of hours to reach the accuracy
>>limits where thermal effects dominate.  It's that which matters to
>>me, rather than something in the first few minutes.  I agree the
>>graphs would not show such short time-scale initial disturbances.
>>    
>>
>
>Did the clock frequency change before you started the new version?
>
>I played with the latest ntp-dev a bit and there indeed is a
>improvement on start, mainly when the initial offset is around
>0.01-0.05s. But the frequency error has to be very small to make a
>difference, see these plots:
>
>http://mlichvar.fedorapeople.org/tmp/ntp_start_offset.png
>http://mlichvar.fedorapeople.org/tmp/ntp_start_freq.png
>
>Also, I've noticed when ntpd is started without driftfile and the
>initial offset is over 0.05 second, the overshoot can easily reach 100
>percent, is this expected?
>
>  
>




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