rtping differences between master and slaves

[Mauro Salvini <mauro.salvini@smigroup.net>]

Hi all,

I'm trying RTNet (an old version: 0.9.13) on Xenomai (yet another old
version, 2.5.6: unluckily I cannot move to newer versions).

My setup has 3 identical nodes on isolated RT network: cycle time of
5000us, master with slot offset 0, slave A with slot offset 200us and
slave B with slot offset 400us; network is configured using rtnet
utility script.

Using rtping I observe two different behaviors on slaves and master.

For example, pinging slave A from B:

...
64 bytes from 10.0.43.91: icmp_seq=1 time=7777.7 us
64 bytes from 10.0.43.91: icmp_seq=2 time=7846.3 us
64 bytes from 10.0.43.91: icmp_seq=3 time=7966.6 us
64 bytes from 10.0.43.91: icmp_seq=4 time=8096.6 us
...
64 bytes from 10.0.43.91: icmp_seq=15 time=9461.7 us
64 bytes from 10.0.43.91: icmp_seq=16 time=9604.2 us
64 bytes from 10.0.43.91: icmp_seq=17 time=4719.5 us
64 bytes from 10.0.43.91: icmp_seq=18 time=4844.7 us
64 bytes from 10.0.43.91: icmp_seq=19 time=4968.3 us
64 bytes from 10.0.43.91: icmp_seq=20 time=5124.4 us
...
64 bytes from 10.0.43.91: icmp_seq=53 time=9215.4 us
64 bytes from 10.0.43.91: icmp_seq=54 time=9331.5 us
64 bytes from 10.0.43.91: icmp_seq=55 time=9461.0 us
64 bytes from 10.0.43.91: icmp_seq=56 time=9585.3 us
64 bytes from 10.0.43.91: icmp_seq=57 time=4712.5 us
64 bytes from 10.0.43.91: icmp_seq=58 time=4847.0 us
64 bytes from 10.0.43.91: icmp_seq=59 time=4967.2 us
...

so time varies between a minimum equal to time distance between two
slots and a maximum equal to this time plus cycle duration. This can
makes sense supposing that Linux timer used to generate ping request in
rtping.c slowly drifts from real-time timer that governs tx slot, so
the duration changes between requests depending on when request happens
into the TDMA cycle. Same behavior if I ping A from B or if I ping
master from A or B.

On master I instead observe this (pinging slave A or B equally):

...
64 bytes from 10.0.43.89: icmp_seq=1 time=2434.1 us
64 bytes from 10.0.43.89: icmp_seq=2 time=2443.1 us
64 bytes from 10.0.43.89: icmp_seq=3 time=2438.7 us
64 bytes from 10.0.43.89: icmp_seq=4 time=2450.0 us
64 bytes from 10.0.43.89: icmp_seq=5 time=2447.9 us
64 bytes from 10.0.43.89: icmp_seq=6 time=2450.6 us
64 bytes from 10.0.43.89: icmp_seq=7 time=2428.5 us
64 bytes from 10.0.43.89: icmp_seq=8 time=2442.8 us
64 bytes from 10.0.43.89: icmp_seq=9 time=2442.3 us
...

ping duration is constant into a rtping execution (changes between
different executions).

So I'm puzzling about this difference and I wonder if this is normal or
if there could be problems.

Thanks in advance, regards

Mauro