[ntp:questions] serialPPS+gpsd+ntpd large offset & jitter

unruh unruh at wormhole.physics.ubc.ca
Wed Jan 12 17:57:02 UTC 2011


On 2011-01-12, Hal Murray <hal-usenet at ip-64-139-1-69.sjc.megapath.net> wrote:
> Even if you have a perfect source of UTC, it's going to be hard to
> keep a typical Linux server within 5 microseconds of that clock.
>
> I think you need to reconsider the whole system.  Where
> did that 5 microseconds come from?  What happens if/when
> the clocks are off?
>
> Can they be off occasionally, or does something nasty
> (factory melts down) happen if they ever get out of sync?
>
> What is the temperature like where the machines are located?
>
> One of the major sources of clock inaccuracy on typical PCs
> and servers is the minor change in frequency of the system
> clock because the temperature of the crystal changed.
>
> There are typically two sources of temperature change.
> One is the environment, the temperature typically has
> a sawtooth pattern as the air-conditioner turns on/off.
> The other is internal heating.  The internal temperature
> goes up when the CPU does work.
>

And ntpd is terrible at handling temperature drifts. It takes a long
time for it to correct for the changed drift due to  temp changes.
chrony for example is much better at this ( responds more raplidly to
correct such drifts).




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