At 05:05 PM 23-04-99 +0000, you wrote:
>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.
>
Hi Paolo:
On a SMP machine, could one CPU do that while the other is running?
Regards
_______________________________________________________
Pierre Cloutier
Tel: (450)-659-9186
Fax: (450)-659-0014
POSEIDON CONTROLS INC
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