[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

David Lord snews at lordynet.org
Thu Jun 20 21:42:02 UTC 2013

Paul G wrote:
> On Tuesday, June 4, 2013 3:44:19 PM UTC-4, E-Mail Sent to this address will be added to the BlackLists wrote:
>> Is interrupt coalescing disabled on the gigabit ethernet adapters?
> Not by me. I assume when ntpq reports symmetric delays proportional to interface speed that interrupts are not the issue for my question.  Perhaps that's a poor assumption.
> My question was -- given PPS clocks on the same network with ntpq offsets of:
> 0.021, 0.130, 0.064 and -0.046 -- any of which can be fudged away how do I determine the truth.  That is how much is (consistent) hardware latency of any sort versus algorithmic restriction.
> e.g. delay, offset and jitter from two sides of a gigE connection:
> 0.073   -0.008   0.002
> 0.064    0.011   0.006
> Likewise fastE:
> 1.226   -0.059   0.076
> 1.104    0.109   0.068

I don't really understand your question.

When I have two pps sourced systems they will each be << 10us offset
but depending on network delay at that time ntpq from a different
system will show offset of > 100us. Ntpd tries to compensate for
delays but best it can do is assume equal delays and maximum delay
equal to total rtt delay. The interrupt delay is another variable
of possibly few us unless there are queued interrupts.

Loop_summary and peer_summary will produce daily averages but even
then I see big differences (relating to temperature/load).


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