[etherlab-users] Jitter + Timed Out datagrams

Christian Gieseler chg at alpenjodel.de
Fri Feb 27 11:32:55 CET 2009

Hello Hobin,

thank you for your reply, i followed your advice and seperated the power source, furthermore i used another switch but it does not change a thing. There are still large jitter peaks that are periodic.

The thing is that while looking for the source of the jitter i connected the master directly to the slaves and then i get all these messages about the timed out frames. Of course i still want to know what causes the jitter, but at the moment i would really like to know why i get loads of Messages about timed aut datagrams, even in when the master is in idle mode. Does someone have an advice for me?

Best Regards


> Hello Christian,
> I experienced similar problem. In my case, it was the switch that caused
> the
> problem. When there were many slaves, sometimes the current to the switch
> went down, and it caused datagram time-out. After all, the power source
> had
> to be isolated from other slaves. It took my team a lot of days to point
> the
> cause. Why don't you check it?
> Hobin
> On Fri, Feb 27, 2009 at 1:34 AM, Christian Gieseler
> <chg at alpenjodel.de>wrote:
> > Hello together,
> >
> > I was trying to do some benchmarking with my master and came across a
> > behavior that i couldn´t explain. Usually my master has a jitter of
> some
> > microseconds, but every approximatly 750 cycles i have a peak in the
> jitter
> > that is about ten times higher. First i tried to switch of any
> unnecessary
> > services and daemons that could cause the problems. But the result was
> the
> > same. Then i tried to play a bit with the cycle time, made it faster,
> than
> > made it slower, but still i have the same behaviour independent from the
> > cycle time. So i thought that it can´t be linux that is causing the
> problems
> > and must have something to do with the master or my application. But in
> fact
> > it did not change a thing even when i strip of most of my application an
> > just have the basic example running.
> >
> > The last thing to be changed was the switch that is between the master
> and
> > the slaves, and my hope was that the switch was the thing causing the
> > trouble. But in fact i the get really a lot of messages about timed out
> > Datagrams. If i put the switch in again ist works again, wich is not
> what i
> > expact since the latence should be lower without the switch. I don´t
> want to
> > set up the datagram timout since 500 micro seconds seem to bei quite a
> lot
> > form me at 1KHz and as one would expect that this is enouph a ist is ok
> witz
> > the switch.
> >
> > There is no difference if i use the stable 1.4 Version, oder the
> > development branch 1.5 (Release 1667). The master runs on a debian with
> > kernel 2.6.24, has an intel Pro100 nic on a pentium m without XServer. 
> I
> > use --enable-cycles when configuring. Skipping this does not change a
> thing.
> >
> > Can anybody give me some hints what causes the fequentlich returning
> high
> > jitter, and even more important, what to do against the timed out
> datagrams.
> >
> > Best Regards
> >
> > Christian

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