Michael Barabanov wrote:
>
> Yorck von Collani ([EMAIL PROTECTED]) wrote:
> > Hello,
> >
> > I'm new on these list so I do not know if these theme has ever been
> > discussed:
> >
> > I have measured the periodicity of a RT - Task with an oscilloscope and
> > and have sean a jittering. A stabile periodicity is very important
> > for me.
> >
> > Any idea ?
>
> Look at the measurement example from V1.1 of RTL. It shows a possible
> technique for reducing jitter down to a couple of microseconds.
Not a chance, again a pure illusion. With that one you can reduce the
average jitter but not the maximum. In fact while you are running that
task start adding load to the computer and you'll see that the average
changes immediatly. Typically: ping -f ..... (a cpu killer), a looping
"ls -aR /" and a looping kernel compilation while doing something under
X.
I've tested that same thing with: RTLinux 1.1, my variant of RTL for
2.0.35 and my latest Linux RTAI, both under UP and dual SMP. I could
take the chance to advertise my work in marginal terms but the rock
solid fact is that the maximums jitters are almost the same on all the
three. The least maximum jitter I got was on an 300 Celeron overclocked
at 450. Why? It has the lowest cache (128K) clocked at the CPU rate.
As pointed out by others you have just a few other choices to reduce
jitter: go hardware or use DSPs.
However if you need no jitter on a PC that has the most powerfull
Pentium cpu, and are willing to loose a lot of performances, an idea
pointed to me by Tomasz Motyleski can be the right one: interrupt at a
time before the needed one, according to the max jitter you have
measured, keeping the interrupts disabled, and keep reading the cpu
clock (TSC) till the very instant you are waiting for. In that way the
jitter is less than 1 us. After Tomasz's suggestion I tried it under
RTAI, it works nicely but sucks a lot.
Ciao, Paolo.
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