Re: [chrony-dev] [GIT] chrony/chrony.git branch, master, updated. 2.4-9-g6cd5583

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Thu, Jun 30, 2016 at 08:32:16AM -0700, Bill Unruh wrote:
> One really weird effect I found was that I have one machine connected to a
> Sure GPS, which seems to have quite a large fluctuation. (or for which the
> interrupt processing seems to have a large fluctuation-- impossible to figure
> out which of course.) but a few times this week, for hours at a time ( 3-6
> hrs) the GPS time seems to be much smoother (the fluctuations in slope are
> much less) and at the same time, the delay times to the other machines are
> smaller, and no longer bimodal. (the delay time seems to have two clumps
> aroung 16us and 24us-- see for example boson in the graphs-- the lower graph
> is the offset(blue)  and the delay(pink) Note that the transition to bimodal
> is at the same time as the GPS slope becomes much noisier (info is the gps
> driven one-- using the serial port interrupt with the Linux serial port driver. )
> The only thing I could imagine is that there is something messing with the
> interupts which affects both the serial port interrupts and the network card
> interrups at the same time.  Anyone seen anything like this befoe?

Was there a CPU intensive task running on the server in those
intervals? CPU needs more time to wake up from deep power-saving
states, so this might have an effect on accuracy of timestamping of
both NIC and PPS interrupts. On some machines, I can see offset of NTP
sources jump by few tens of microseconds when I start a program that
fully loads the CPU.

-- 
Miroslav Lichvar

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