[ntp:questions] NTPD can take 10 hours to achieve stability

unruh unruh at wormhole.physics.ubc.ca
Wed Apr 20 15:16:20 UTC 2011


On 2011-04-20, David Lord <snews at lordynet.org> wrote:
> unruh wrote:
>> On 2011-04-19, David Lord <snews at lordynet.org> wrote:
>>> unruh wrote:
>>>> On 2011-04-19, David Lord <snews at lordynet.org> wrote:
>
> .....
>
>>>>> When I first compared chrony with ntpd there was no contest
>>>>> but more recent experiments with chrony had periods of
>>>>> severe instability much worse than ntpd.
>>>> More recent means what? What version of chrony? (there was a bug found
>>>> in the past two weeks that was introduces a few months ago which did
>>>> result in instability) 
>>> More recent than  November 2009.
>>> Probably between Dec 2009 and Jan 2010 and p4x2400c.
>>>
>>> chrony 1.23
>> 
>> OK well before that change. 
>> 
>>>
>>>>> P4X2666 with chrony
>>>>> <http://www.lordynet.org.uk/mrtg/stats/>
>>>> Not at all sure what I am supposed to see. I have no idea what the graph
>>>> axes represent?  What is 1.1k 
>>> X-axis is in hours
>>> Y-axis is offset in us so 1.1k = 1100us
>> 
>> WOW. What kind of network are you attached to? Even on an ADSL link
>> through the phone company, I
>> was getting in the tens of usec (not ms) as the offsets of chrony.
>> (checked by a gps receiver attached to the local computer).
>
> I'm on ADSL-1 with 2 Mbit/s down and 288 kbit/s up. Latency
> is what BT delivers and has been as low as 12ms up to 50ms or
> more but mostly it's about 18ms to nearest sites (when exchange
> gets near capacity the latency jumps up by 15ms or more same as
> when interleave is enabled on ADSSL-2).
>
> Although the peaks of the graph are at about 1ms the stats
> have "System time" mostly being in the 10s of usec.

Not sure what you mean by that. What I mean is , what are the measured
offsets? (Of course if you have a GPS PPS that you could use as a
reference-- not a source of time-- that would make much clearer what is
going on.)

>
>> 
>> And what is it about the graph that makes you believe you are seeing
>> instability. 
>
> There is nothing in that session but later graphs now
> wrapped (as in loops around after a year) showed much
> worse offset variations than the systems using ntpd at
> which point I went back to using ntpd.

Certainly not my experience. And the lack of evidence makes it hard to
fix any problems if they are there. 





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