[ntp:questions] PPS jitter in Linux

Chris Albertson albertson.chris at gmail.com
Tue Apr 3 01:13:09 UTC 2012

On Mon, Apr 2, 2012 at 4:28 PM, Mike S <mikes at flatsurface.com> wrote:
> 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).
> 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, jitter will shrink to +- 1-3
> us. I've played around with different cpufreq setting, thinking it might be
> related to the processor speed during an IRQ, but that seems to have minimal
> impact (performance vs. conservative vs. ondemand).

I think your CPU goes into some power saving mode when there is no
load and takes some number of micro seconds to wake up after an
interrupt.   Keeping a load on it prevents the sleep or power save
mode and hence the need to "wake up."


Chris Albertson
Redondo Beach, California

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