On 23.11.18 11:51, Mauro Salvini via Xenomai wrote:
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.
As you are running RTmac/TDMA: The latency is affected by when during some cycle
you issue the ICMP request. This is not synchronized with the cycle, so you will
see random shifts there already. Furthermore, if station A has a time slot
before station B and A issues the ping, B may have a change to reply in the same
cycle. If you change roles, this is surely not possible, thus you get different
round trip times.
Jan
--
Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux
