| Re: [chrony-dev] SW/HW timestamping on Linux |
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On Tue, Nov 15, 2016 at 09:39:40PM -0800, Denny Page wrote:
>
> While I can get my head around a differential of 250ns resulting from the switch, I’m finding it very difficult to believe the almost 2500ns differential that appears when hardware time stamping is enabled.
>
> Any thoughts on this?
Is the port to the switch identical to the one connected to the third
server? It would be interesting to see if the offset changes when the
ports are swapped.
I'd trust HW timestamping. The 2.2us offset doesn't seem unrealistic.
There is a reason why there are switches with support for PTP. You
have exceptionally stable measurements with SW timestamping, but that
doesn't mean the asymmetry in delay and processing has to be the same
between the two ports.
To get a better perspective, I'd suggest to compare offsets between SW
and HW timestamping at the same time. It is possible to run two chronyd
instances, one using SW timestamping, the other HW timestamping. They
need to be configured to use a different cmdport (e.g. 323 and 324)
and pidfile, and they both need all their sources to have the noselect
option, so they don't try to control the clock and interfere with each
other. You can then run "chronyc -p 323 -h ::1" and "chronyc -p 324 -h
::1" to compare the offsets. You may need to synchronize the clock
first, so the offsets are small enough to see the difference between
them.
--
Miroslav Lichvar
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