Re: [chrony-users] GPS / Chrony NTP server config questions

[Rob Janssen <rob@xxxxxxxxxxxxxxxxx>]

Bill Unruh wrote:
> On Thu, 16 Nov 2017, Denny Page wrote:
>
> > Interrupt latency for character devices in Linux is 6-7 us, even with no other activity. You
>
> It has been a while but I ran tests on a system in which I changed the state
> of a parallel port pin and fed that into an interrupt grabbing the system time
> just before toggling the pin, and the time when the interrupt service routine
> got the interrupt. The latency was of the
> order of 1us, not 7. Now that was on an older machine with the 1us, not the ns
> clock. I have not tried this recently but would be surprized if the time was
> worse now.

I am using chrony with serial PPS on a number of different machines, and I see the 6-7 us figure
on HP Proliant systems while on Dell PowerEdge sytems it is more like 1us.
No idea where the difference comes from.

Plotting the offset from chronyc tracking normally yields values well within 200ns (with the occasional
peak to 600ns) for a Dell, while a HP is normally within 2us and peaks to 6us.

It apparenly varies with system design so it doesn't surprise me that people find different values.
(what does surprise me that it varies little between very different models from the same manufacturer,
yet it varies so widely between manufacturers)

Rob

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