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 <[email protected]> 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 -- [email protected] > 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. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
