TRUE realtime priority

[Roland Smith]

On Mon, Oct 06, 2008 at 10:21:01PM +0200, Wojciech Puchar wrote:
> is it possible on FreeBSD

No, I think.
 
> i run asterisk with realtime priority. it works perfectly no matter how 
> much CPU is loaded by other non-telephony tasks.
> 
> but with lots of VM pressure it starts to so... like like tha..that...
> 
> what causes it to behave like that and how to fix it.

Well, basically you are the only one who can answer that. And that's not
a paradox or an attempt at humor. You should investigate. Maybe
interrupts aren't processed fast enough (hardware sharing an
interrupt?), or memory or kernel resources are low.

> for example when lots of spam comes to server and lots of resource hungry 
> spamassassin processes are spawned our calls starts to be crappy.
> 
> CPU load for asterisk rarely exceed few percent!

Yes, but FreeBSD isn't a _hard_ real-time OS (see below).
 
> i think having separate computer just for this is stupid, i would do this 
> having no other choice, but can it be done without this.
> 
> realtime priority is realtime priority anyway - it should work.

It does depend what you mean by real-time. Usually real-time systems are
devided into the "soft" and "hard" categories. See the Wikipedia article
on real-time computing [http://en.wikipedia.org/wiki/Real-time_computing]
and operating systems [http://en.wikipedia.org/wiki/Real-time_operating_system].

Most hard real-time systems are embedded systems with a specific
function (say, ECU, FADEC, ABS, digital music player). I don't think
there are general use OS's which would classify as hard real-time
(AFAIK, RTLinux runs Linux as a low-priority task on a real-time
core). Most of them support soft real-time, as in "we'll try to get
these tasks done before a specific deadline, but no promises."

Roland
-- 
R.F.Smith                                   http://www.xs4all.nl/~rsmith/
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