[ntp:questions] Second attempt at GPS-18 based NTP server

unruh unruh at invalid.ca
Thu Feb 2 16:55:42 UTC 2012

On 2012-02-02, David J Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>> On NetBSD-5 with ntpd 4.2.6p5 the default is 64s.
> I wonder what expertise they have to select polling for reference clocks? 
> BTW: Paul has FreeBSD.

The reason for longer polling is a) network load, and b) rate
If you are running ntp in order that your computer clock be ticking at
exactly 1s/s, then longer polls are better (longer lever arm) if your
computer clock does not change rate (eg due to internal temperature
changes due to use of the computer) If, you want least deviationo of the
clock from UTC shorter polls are better. Since the primary ntp noise
source, variable network delays, is not there, and since the primary
noise source is interrupt servicing delays, shorter is probably better.

>> I've just added the "minpoll 4 maxpoll 4" back to my ntp.conf
>> as it could reduce the 30 usec offset blips I get when logs are
>> rotated.

Not sure what would cause 30us blips. Are interrupts really being
switched off by something for those lengths of time?

> Agreed.
>> On my two pool servers I have named internet servers with
>> "minpoll 8 maxpoll 10 iburst" which gives reasonably fast
>> recovery from internet outages.
>> Temperature changes  here mean that on pcs that don't have
>> a PPS source "minpoll 10" just doesn't work and leads to
>> ntpd stepping the clock.
>> David
> As I have five stratum-1 servers here fed from four GPS receivers, I'm 
> hopeful that I won't need to reply on Internet servers.  The portable PC I 
> might take out and use Wi-Fi has the standard "minpoll 6 maxpoll 10 
> iburst" settings.

Since that portable is liable to be switched on and off, or moved on a
timescale of 2^10 sec (20 min) having a poll of 10 on a laptop,
especially with ntpd's sloooow response time, seems not terribly useful.
If your network can support it, a much shorter poll for laptops would
seem to be sensible. 

> Don't you link you other PCs to your stratum-1 server?  I do, with lines 
> like:
>   server  192.168.0.n  iburst  maxpoll 5  prefer # FreeBSD server
>   server  192.168.0.n  iburst  maxpoll 5 # Windows server
>   server  192.168.0.n  iburst  maxpoll 5 # Windows server
> Cheers,
> David 

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