[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Sat Mar 17 21:21:49 UTC 2012


On 3/17/2012 3:20 PM, David J Taylor wrote:
> "Ron Frazier (NTP)" <timekeepingntplist at c3energy.com> wrote in message 
> news:4F64D793.9010708 at c3energy.com...
> []
>> Hi David,
>>
>> I'm not sure what will happen if you simultaneously prefer and 
>> noselect the local server.  Assuming the local stratum 1 server is 
>> the most stable time source, you'll get a much better picture of what 
>> the internet servers are doing relative to it if you allow it to be 
>> selectable as well as being preferred.  When you graph it, if the 
>> local server is the active clock, all the lines for the internet 
>> servers will be gathered around and relative to the local server.  
>> When I tried to do things the other way around, with an internet 
>> server preferred, the graph looked awful because there was so much 
>> variation.  Also, if your local server starts reporting time that 
>> looks too far from the internet servers, regardless of who's fault it 
>> is, ntp will clock hop over to the internet servers.
>>
>> I don't THINK your internet servers will ever poll above their 
>> default minpoll value of 6, or 64 seconds.
>>
>> I realize you don't have a gps attached to this pc, but the iburst 
>> lines reminded me of something.  I read somewhere that having iburst 
>> on internet server lines, if a local gps is attached, could prevent 
>> the PC from synchronizing to the gps before it synchronizes to the 
>> internet. On my pc with the gps attached, I don't use the iburst 
>> command.
>>
>> Sincerely,
>>
>> Ron
>
> Ron,
>
> The local stratum-1 server shows without a tally code against it in 
> the ntpq -p output, so it's being recorded in the peerstats, but not 
> used for syncing.  The noselect msut override the prefer.
>
> After about three hours running, the Internet servers are all at 512 
> seconds poll interval.  The averaged jitter has been below 1 
> millisecond for the last couple of hours.  The offset is reporting 
> between -0.7 and -1.8 milliseconds, and the frequency is stabilising 
> very nicely (because of the long poll interval).  I'll leave this 
> running overnight and tomorrow to see how it handles temperature 
> changes and any Internet access changes, and to get a few more points 
> on the graph.
>

Come to think of it, my comment about the polling interval not 
increasing may only apply to a local refclock, not a local server.

> One caveat is that I am using the most recent NTP (ntpd 4.2.7p263 from 
> Dave Hart's download page), and that with Windows-8, it may be using 
> the new precision time system call.  From my own tests, this is 
> similar, on earlier versions of NTP, to setting the environment 
> variable NTPD_USE_INTERP_DANGEROUS, thus forcing the NTP time 
> interpolation to be used.
>

Can you elaborate more about what NTPD_USE_INTERP_DANGEROUS does?

Sincerely,

Ron


> The configuration I have is:
>
> - cable modem (with built-in router, but working as a bridge by 
> putting my own router in a device in the DMZ).
>
> - Samknows network monitor (modified WRT54GL router)
>
> - WRT54GL router running DD-WRT firmware
>
> - Netgear 8-port consumer 1 Gb/s switch (G5108)
>
> - wired connection to ~2 year old laptop PC
>
> I only mention this to show that (a) it's not a direct connection and 
> (b) there's no wireless involved.  My aim here is simply to see what 
> performance may be had with just Internet servers.  The PC is only 
> running NTP and monitoring software - no user programs and no 
> interactive work, so it is a best-case scenario.
>
> Cheers,
> David
>


-- 

(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
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Ron Frazier
timekeepingdude AT c3energy.com



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