[time-nuts] NTP jitter with Linux

[Eric Williams]

Could the CPU be reducing its clock rate when it's not being loaded?  Just
a guess, most multi-core processors these days have power saving features
like that.
--
eric

On Wed, Apr 4, 2012 at 3:22 PM, Mike S <mikes at flatsurface.com> wrote:

> I asked this on an NTP list, got some guesses, but no knowledgeable
> responses.
>
> I've got a Trimble Thunderbolt PPS source for NTP, Linux 2.6.35, on a quad
> core CPU. PPS source is coming into a multiport serial card, which
> /proc/interrupts shows is sharing IRQ with some inactive USB ports (IRQ
> 17). It's a PCI-E card, so it would be using MSI interrupts. My
> understanding is that those aren't really "shared," in the traditional
> sense, but IDK. The kernel clocksource is TSC, which is claimed to be core
> invariant on my processor (AMD Athlon II 610e). Changing to HPET doesn't
> help.
>
> Running normally, I'll get about +- 20 us ptp of jitter (as reported by
> ntpq -p, and in loopstats). If I load up the CPU (load average >4 is
> swell), jitter will shrink to +- 1-2 us. I've played around with different
> cpufreq setting, thinking it might be related to the processor speed during
> an IRQ varying, but that seems to have minimal impact (performance vs.
> conservative vs. ondemand).
>
> I've also tried irqbalance, with no change in performance.
>
> So, running a process(es) which keep the CPU completely busy reduces the
> jitter. The busier, the better. Why? I'm guessing it has something to do
> with interrupt latency, but why does a busy CPU make it more consistent -
> I'd expect the opposite? The difference is very obvious.
>
> Is there something else I can do to keep the jitter low?
>
> Aside: Something which I believe was discussed here a few weeks ago -
> clocksource speeds changing between reboots. I patched the kernel to allow
> statically setting the TSC frequency ( http://old.nabble.com/-PATCH--**
> tsc_khz%3D-boot-option-to-**avoid-TSC-calibration-**
> variance-td23494975.html<http://old.nabble.com/-PATCH--tsc_khz%3D-boot-option-to-avoid-TSC-calibration-variance-td23494975.html>). This eliminates the semi-random, often 30-40 ppm change in frequency
> reported by NTP between reboots. After tweaking, it's now consistently < 1
> us, reboots be damned. This should be in the mainline kernel! This made no
> difference to the jitter mentioned above, although non was expected.
>
> ______________________________**_________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/**
> mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>
> and follow the instructions there.
>