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

[Denny Page <dennypage@xxxxxx>]

> On Nov 16, 2017, at 10:32, Bill Unruh <unruh@xxxxxxxxxxxxxx> 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.


Hmm… I can’t speak to parallel port, but I can’t see that it would be much different that any other APIC based interrupt. I’ve experimented with several serial interface chips, Intel and AMD with power management disabled, and the 6-7us seems pretty consistent. I would love to lower this, and am really open to suggestion if you have ideas. Yes, I know that MSI should be lower, and while I do have serial cards that support MSI, none of the Linux drivers support it for the chips I have. I haven’t been motivated yet to rewrite the drivers for MSI. :)


> 
>> can get better than this using a gpio and spinning on it, but I haven’t seen anyone do this
> 
> Well, that is unclear, and hugely expensive in terms of taking up the time of
> the cpu.

It’s not completely free running. You run a loop that includes a pause which allows you to trade off between the duty cycle of the CPU and resolution of the pulse. 

Denny


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