Fri, 11 Aug 2000 Wilken Boie wrote:
> Hello RT community,
>
> I'm _not_ going to ask the question again:
> 'Is it possible to do TCP/IP from an RTL module?'
> though mine probably is a dummies one as well.
>
> I tried to follow the advice given many times: trigger a user space
> helper (I'm using /dev/rtf0). My helper thread blocks on reading
> a byte written by a RTAI module ervery 20ms and then sends some data.
>
> My aim is to send a udp broadcast as closely bound to that trigger as
> possible. Jitter would be well acceptable up to 1+ ms. I assumed to
> achieve that by setting the helper thread to SCHED_RR with a high
> priority (e.g. 99).
>
> But I find this construct working quite precise only most of the time:
> The reception of single triggers (as measured by timestamps from user
> space, with the RT activity known to be precise) is delayed by up to
> 15 ms. These runaways are not really constant but seem to happen in
> some repetitive patterns, the most obvious with a ~1.2sec period.
Indeed, this is to be expected from the soft real time scheduling provided
by SCHED_RR or SCHED_FIFO. (BTW, you should usually use SCHED_FIFO for this
kind of stuff, unless you have multiple RT threads that will use lots of CPU
time.) As opposed to RTL, it's not *hard* real time.
However, it is possible to get worst case latencies in the ms range with Ingo
Molnar's lowlatency patch. I think some people have succeded in applying
some versions of lowlatency and RTL together, which allows the great
combination of kernel drivers with �s timing and user space code with ms
timing. Have a look at:
http://www.gardena.net/benno/linux/audio/
http://www.crosswinds.net/~linuxmusic/lowlatency.html
Another alternative (which should provide significantly lower latencies) would
be the new hard RT signals from RTL. (Still beta, AFAIK.) I believe RTAI has
had a similar feature for some time.
David Olofson
Programmer
Reologica Instruments AB
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