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

David Lord snews at lordynet.org
Wed Apr 20 19:28:37 UTC 2011

unruh wrote:
> 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.)

My logs have eg:
Reference ID    : me6000g
Stratum         : 2
Ref time (UTC)  : Sat Jan  9 07:12:02 2010
System time     : 0.000001 seconds slow of NTP time
Frequency       : 0.125 ppm fast
Residual freq   : -0.010 ppm
Skew            : 0.223 ppm
Root delay      : 0.001297 seconds
Root dispersion : 0.007492 seconds

I've not yet checked all of the systems that were tried
with chrony but these servers are all in use so it's not
easy to just install chrony and expect it to work.


>>> 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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