[LinuxPPS] Interrupt problems?

[Remco den Besten]

I always have a few (0 - 15) microseconds jitter, and I run my ntpd also
on a web/file/etc-server.

Here I have ntpd locked onto GPS-PPS and get 'the time' from DCF:
ntpq -p
     remote           refid      st t when poll reach   delay   offset 
jitter
==============================================================================
+SHM(0)          .DCF.            0 l   41   64  377    0.000    0.659 
0.202
oPPS(1)          .GPPS.           0 l   15   64  377    0.000   -0.081 
0.012
*snip*

Depending upon the temperature gradient, load of the server and traffic 
intensity
the offset 'jitters' between +/- 200 usec = +/- 0.1 msec. With DCF only I 
have
+/- 2.5 msec, so with my PPS is '10 times better'.
And.... I configured the kernel with 100 Hz (make menuconfig and the 
processor menu)) ticks.


> I am still seeing some jitter, though with the Atom refclock handling PPS
> (rather than using NMEA for both) it's now a fraction of a millisecond
> rather than several milliseconds.
>
> This seems to be partly related to machine loading, ntp just loses 
> stability
> and all the poll intervals drop. (I run the PPS setup on my fileserver, I
> can't justify having yet another machine solely for NTP.)
>
>
> I'm guessing it's due to either missed clock interrupts or simply delays 
> in
> processing the PPS interrupts when another interrupt routine is tying up 
> the
> CPUs.
>
> I also suspect some bug in NTP or the kernel, as the present setup keeps
> showing 'reach' other than 377 even on peers on the same local subnet, but
> the ethernet interfaces involved on all three machines show zero errors.
>
> Does anyone have any general machine setup tweaks that could reduce or
> eliminate the remaining jitter?
> The present system (on an Athlon64 dual core CPU) is running at 1000Hz 
> with
> irqbalance loaded.
>
>
> Thanks,
> Robert.
>
>
>
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