[ntp:questions] ntp & system without a rtc

David Lord snews at lordynet.org
Wed May 15 19:46:39 UTC 2013


David Lord wrote:
> unruh wrote:
>> On 2013-05-14, David Woolley <david at ex.djwhome.demon.invalid> wrote:
>>> Richard B. Gilbert wrote:
>>>
>>>> NTPD is NOT designed for fast convergence.  From start up to get the 
>>>> minutes correct, NTPD will need about thirty minutes!  To get the 
>>>> best time you are going to get, you will need to wait for about ten 
>>>> hours!
>>>>
>>> Convergence to within 128ms should take a lot less than that.  With 
>>> iburst, I believe it should be less than one minute.
>>
>> Since ntpd steps ( changes the rate by an infinite slope) if the time is
>> out by 128 ms, yes, convergence IS faster than that. ( however it has to
>> make sure that the clock really is out by that much and that takes a few
>> cycles of clock querrying).
>>
>> For a ms accuracy your could expect a couple of hours. for 10
>> microsecond, about 10 hr.
>>>>
> 
> Hi
> 
> I posted a reply on May 11, but did not give the ntpq stats
> as they'd already been archived away.
> 
> After three reboots to replace kernel and userland taking
> around 10-15 minutes 'ntpq -p' with polling at 6 minutes gave
> the following:
> 
>    min  reach   offset   jitter
> 
>    -18    377   -0.001    0.004
>    -12    377    0.007    0.007

Ouch! I've checked with the actual times logged and there
was no poll logged at 19:30

19:24     -6    377   -0.001    0.006
19:30
19:36      0      0    0.000    0.000
19:42      6     17   -0.036  275.881

>     12    377    0.126    0.125
>     18    377    0.173    0.049
>     24    377    0.047    0.092
>     30    377    0.007    0.028
>     36    377   -0.001    0.007
>     42    377   -0.001    0.004
> 
> There was a well established ntp.drift file and my rc.conf starts
> ntpd with flags="-g -N -p /var/run/ntpd.pid"
> 
> NetBSD/6.1_RC3
> ntpd 4.2.6p5-o
> PPS from Sure demo board
> 
> 
> David



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