| Re: [chrony-users] GPS / Chrony NTP server config questions |
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On Thu, 16 Nov 2017, Rob Janssen wrote:
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.
How do you measure that latency? It does seem that the parallel port is a much
faster interrupt than the serial port, so it might have something to do with
the serial port hardware used. (in particular if the serial port is really a
phony serial->usb port I would expect large latency).
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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