[ntp:questions] Bug 2341 - ntpd fails to keep up with clock drift at poll>7

unruh unruh at invalid.ca
Tue Dec 3 05:35:14 UTC 2013

On 2013-12-03, David Lord <snews at lordynet.org> wrote:
> unruh wrote:
>> On 2013-12-02, David Lord <snews at lordynet.org> wrote:
>>> Martin Burnicki wrote:
>>> My server with Sure gps/pps has offset below 3 us other than when
>>> nightly cron jobs give a couple of 35 us spikes. From loop_summary
>>> over 7 days, typical range for rms offset is 3.9-6.1.
>>> Over 7 days my four pool servers have following rms offset ranges
>>> ntp0=784-1646, ntp1=405-837, ntp2=310-434, ntp3=586-1270 but there
>>> were numerous reboots and ntpd restarts over that period.
>>> I want to try using a stable external system clock source, TCXO,
>>> OCXO or rubidium laser.
>> A sure gps IS a stable external system clock source. What more do you
>> want?
> ? it is a PPS source not a system clock source.
> I'd prefer the 35 us blips to go away which would require the PC
> to be idle 24/7 or a use of a stable external system clock.

??? That 35us blip is quite possibly there either because of delays in
reading the clock, which would also be delayed with an external clock. 
Ie, I would need to see some evidence that an external clock would be
any more accurate in getting the time into the system, than would the

>> And those offsets for your pool servers really seem very high. 
> did you miss that:
>> >> were numerous reboots and ntpd restarts over that period.

No I meant that delays I have seen getting time from stratum 1 servers
were in the tens of  microsecond not hundreds or thousands of microsecond regime. Now it may be that the
pool servers you are using are really just horrible as clocks
(milliseconds I call just horrible). But I certainly would not expect
network delays to give that kind of errors. 

Also, I would assume that you are not taking the behaviour during
startup transients into account. 

> but peak offsets are still 300-500 us and I'd prefer if they
> were even lower, again requiring a more stable system clock.

Why would they be lower. Those are probably the fluctuations in teh
remote clocks, and have nothing to do with the fluctuations in your
local clock. 

> What do you think the local offsets should be for a pool server?

It depends on the pool server. Some simply use nmea and claim to be
stratum 1. As I say, the offsets from a reliable pps driven stratum 1
from across the country are in the 10s of microsecond range for me.
(even though delays were in the 10s of ms range)


> David

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