[ntp:questions] Trimble Resolution T
Chris Albertson
albertson.chris at gmail.com
Mon Oct 17 18:41:03 UTC 2011
On Mon, Oct 17, 2011 at 10:45 AM, unruh <unruh at physics.ubc.ca> wrote:
> On 2011-10-17, Chris Albertson <albertson.chris at gmail.com> wrote:
> > 2011/10/17 Miguel Gon?alves <mail at miguelgoncalves.com>
> >
> >> what are the best (meaning well supported) refclocks today to
> >> set up a new server?
> >
> >
> > Likely the GPS with the best PPS is the new Oncore. But for NTP you
> don't
> > need 5 nanosecond accuracy. From NTP's point of view 5 nS is not better
> > than 50 nS.
>
> In fact it is really hard for a computer to keep to 1us accuracy because
> of the delays in interrupt processing. The computer has to recognize the
> interrupt, deliver the notification that an interrupt has occured to the
> interrupt processing driver, the driver has to then ask the operating
> system for the local time to timestamp the interrupt. All that takes
> time, and it is hard to keep that under 1us.
The interrupt processor in the kernel is very simple and fast. I think less
then a dozen lines of code are executed and none of them go off and make a
secondary request or do anything that requires much time. Once you get into
the interrupt handler the only thing it does is (1) load a hardware counter
to memory and (2) set a bit to indicate #1 is done. Nothing is logged to
disk and no secondary OS calls are made. The logging happens in a user
level routine that notices the set bit then reads the stored copy of the
counter
But still you are right, this works at the uS level. I think getting to the
handler is the problem. Sometimes the OS has to disable interrupts. And
then always the state has to be saved. In other words I think the
"bottle neck" is interrupt latency, not the processing that occurs inside
the handler.
Chris Albertson
Redondo Beach, California
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