Re: [chrony-users] Chrony offset and stability adjustments? (fwd)

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Wed, Jan 09, 2019 at 10:56:56AM +1300, Bryan Christianson wrote:
> > On 8/01/2019, at 10:31 PM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> > 
> > In the estimate of the maximum error, it's important to include also
> > half of the root delay, so in your examples it would be about 250
> > microseconds.
> 
> OK - still less than 1ms as required by the OP.
> Maybe this maximum error estimate could also be shown in 'chronyc tracking'?

Yeah, that would probably make sense. It's included in the tracking
log.

> > Raspberry PIs have ethernet on USB, which adds some latency, and IIRC
> > the default interrupt coalescing is very large. You might want to try
> > setting a smaller rx-usecs value with ethtool -C, which should reduce
> > the root delay and give the clients a better estimate of the maximum
> > error.
> 
> I had a quick look at this, but the nic in the RPi doesn't seem to support the coalesce options of ethtool with Debian Stretch.

You are right. From the current source code of the driver it doesn't
look like there is any support for getting or setting the coalescing
parameters. There is no support for SW timestamping either. I probably
confused it with a different board. It may not even make sense to do
on a USB NIC.

I did a quick test with a RPi 3 I have here for non-NTP purposes and
the peer delay to a good server connected to the same switch is around
160 microseconds, so the maximum error due to timestamping would be
about 80 microseconds. Not an ideal HW for timekeeping, but certainly
good enough for sub-millisecond accuracy.

-- 
Miroslav Lichvar

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