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SubjectRe: An important fact about real time computing
yodaiken@fsmlabs.com wrote:
>
> On Wed, Jul 19, 2000 at 03:47:01PM -0400, David Mansfield wrote:
> > David Balazic wrote:
> > >
> > > The first sentence my professor maid on the real time computing
> > > course was :
> > >
> > > "A common misconception : Real-time computing is fast computing."
>
> This is the standard lecture, but don't take it too far. You can control
> a robot with deterministic latecnies under 50 usecs, but not with
> deterministic latencies of 10 minutes.

I thought that the point of RT sched. was that in a soft real time
environment, given an expected amount of faults and a certain sys.
load you can expect a certain failure rate, and in a hard real time
environment given an expected number of faults and a sys. load you
can guarantee that if you add another task to the current set of tasks
it will (under most circumstances) succeed. If the granularity of the
time used to set deadlines isn't precise enough for you to achieve a
task (as per your example) that's one thing, but I thought that RT
scheduling
is about deadlines and whether or not they can be met, which has nothing
to
do with fast computing.

>
> > To summarize, I agree that the 'audio people' have a real-time
> > requirement. I claim that the kernel already has real-time
> > requirements. I also claim that any real-time system is only a
> > probability curve of making the deadline, and that the 'audio people'
> > only want to shift this curve to make the deadlines they are looking for
> > fall into the probability of failure they find acceptible.
>
> You understate the difficulties by using this form of argument. I'd
> like my 1992 V4 Ford Ranger Pickup to fly at Mach 3 at 35,000 feet.
> The truck already leaves the ground for short periods
> if I drive over bumps. So it already has an aircraft requirement and
> I only want to shift the flight curve to a level I find acceptable.
> Should be simple, no?
>
> --
> ---------------------------------------------------------
> Victor Yodaiken
> FSMLabs: www.fsmlabs.com www.rtlinux.com
> FSMLabs is a servicemark and a service of
> VJY Associates L.L.C, New Mexico.
>
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