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

David Lord snews at lordynet.org
Fri Feb 3 00:55:31 UTC 2012


unruh wrote:
> On 2012-02-02, David Lord <snews at lordynet.org> wrote:
>> unruh wrote:
>>> 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
>>> optimality.
>>> 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?
>> It's the load from log rotation and cron jobs running at
>> that time of night.
> 
> The load should not matter. The interrupt sevice routines take
> precidence. Now it might be that the interrupt routines require paging
> in, but I thought that IRQ servicing was kept in memory as much as
> possible. 
> IRQs have to be fast. 
> 
> And 30us is a HUGE time  (over 10,000 cpu instructions)

I've not monitored the temperature but my guess is that
the increased load increases the system temperature. I've
already observed that slower temperature variations during
the day cause an increased offset vs days when temperature
is fairly constant.

offset is mostly < 5u but there are two blips each night:
        usec
03:24    33
03:30     9
03:36    21
03:42    17
03:48     9


David



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