Re: rtping differences between master and slaves

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

On Fri, 2018-11-23 at 12:49 +0100, Jan Kiszka wrote:
> 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.

Thank Jan for quick answer.

Yes, I understood your explanation, that perfectly clarify rtping
between A and B.

Instead, about the constant ping duration when master pings a slave: is
due to the fact that on master ICMP requests are synchronized with the
cycle?

Thanks in advance, regards

Mauro

> 
> Jan
>