Re: rtping differences between master and slaves

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

On Fri, 2018-11-23 at 13:55 +0100, Jan Kiszka wrote:
> On 23.11.18 13:49, Mauro Salvini wrote:
> > 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?
> 
> I don't recall the details, but chances are high that, because the
> master drives 
> the TDMA cycle, its rtping loop happens to remain in sync with that
> cycle.

Ok, thank you very much Jan.

Regards.

Mauro