[ntp:questions] chrony as a server

William Unruh unruh at invalid.ca
Sat Feb 21 18:16:56 UTC 2015


On 2015-02-21, Rob <nomail at example.com> wrote:
> William Unruh <unruh at invalid.ca> wrote:
>> On 2015-02-19, Rob <nomail at example.com> wrote:
>>> Miroslav Lichvar <mlichvar at redhat.com> wrote:
>>>> On Thu, Feb 19, 2015 at 12:48:46PM +0000, Rob wrote:
>>>>> I am still finding out what sensor is best to use, we do have a room
>>>>> temperature sensor that has .1C resolution and is readable via snmp,
>>>>> and there are the usual sensors for board- and inlet air temperature.
>>>>> (and of course CPU temperature)
>>>>> 
>>>>> It does not matter if it is only a course indication, the room temperature
>>>>> varies over a -10 .. 50C range (don't ask...) and a 1C resolution is not
>>>>> bad relative to that.
>>>>
>>>> In my tests using a sensor with 1C resolution it was barely useful
>>>> with NTP sources and 1024s polling interval. If the sensitivity is
>>>> around 0.1 ppm per degree, 1C resolution means the compensation
>>>> jumping the frequency in 0.1ppm steps. That's a lot, especially if you
>>>> compare it to the tracking skew with a refclock.
>>>
>>> Ok but of course we are using PPS and a 16 second polling interval.
>>> (or maybe the PPS refclock polls even faster although it displays 4 as
>>> the poll interval indicator)
>>
>> The shm refclock will get one pulse per second, and then average the
>> offsets over a 16 sec period after getting rid of the outliers.
>
> I am not using the SHM refclock in those systems.

What are you using? Are you on ntpd or chrony? As I recall ntpd in its
pps refclock also collect say 16 outputs of PPS, finds the median, thros
away the 40% greatest outliers and reports the resultant median to ntpd
as the current offset. That is the same kind of filter chrony uses in
its shm driver.



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