Path: utzoo!attcan!uunet!xavax!jat
From: jat@xavax.com (John Tamplin)
Newsgroups: comp.realtime
Subject: Re: My IBM rep says that the RS/6000/w AIX is realtime, is this true?
Message-ID: <1990Sep5.191913.20896@xavax.com>
Date: 5 Sep 90 19:19:13 GMT
Organization: Xavax
Lines: 34

In article <4350@cica.cica.indiana.edu>, ssw@cica.cica.indiana.edu
(Steve Wallace) writes:
> Our IBM rep is telling us that the RS/6000 running AIX will do
> realtime applications.  Specifically, the RS/6000 will be able to
> time user response time with a given accuracy.  The local IBM
> tech type says just adjusting the priority on the process will
> guaranty a maximum latency between an interrupt (keypress) and
> the process waking up and recording the time.
> 
> Does anyone know if the RS/6000 & AIX have this capabilities, or
> is IBM bluffing?

I was a consultant to IBM AWD while they were developing AIXv3 and I
think I can answer this.  There are two classes of process priorities,
fixed and variable (normal Unix style).  The scheduling is done preemptively
with the highest priority runnable process getting the CPU.  Every so often,
variable priority processes are adjusted based on the amount of CPU they
have received, but are always lower priority than fixed priority processes.
At each fixed priority level, there is an instruction count associated with
that level.  Any process running at that level should run for no more than
that many instructions.  Interrupt levels and off-level interrupt processing
are treated as the highest priority level of those that are fixed.  Given
these constraints, AIXv3 is indeed a "soft" real-time system.  Soft, since
there is no guarantee of meeting a dead-line, but it is predictible if you
know what set of real-time processes are running.  Of course, if any device
drivers or real time processes violate the instruction count, all bets are
off.

The kernel is rebuilt from the ground up, and has very little to do with
stock SysV or BSD code.
-- 
John Tamplin						Xavax
jat@xavax.COM						2104 West Ferry Way
...!uunet!xavax!jat					Huntsville, AL 35801