Re: [offtopic] Re: 2.2.2: 2 thumbs up from lm

Arvind Sankar (
Fri, 26 Feb 1999 12:32:55 -0500

On Fri, Feb 26, 1999 at 08:06:01AM -0500, Richard B. Johnson wrote:
> On Fri, 26 Feb 1999, Neil Conway wrote:
> >
> > But how can you promise the customer that your system will respond to
> > events while they are current if you DON'T control the latency?
> I have a CPU which does nothing but poll a port. I want to detect
> when the sun comes up which happens once a day. There is a photo
> controller relay connected to that port. I need to know this time
> within one second.
> Therefore I need to poll this port at least twice per second.
> Twice comes from Shannon since this has become a sampled system.

Talk about overkill... btw, I think you might be referring to Nyquist's thm?
Or is it just called the Nyquist frequency, but the theorem was by Shannon?

> Do we care how much time it takes to poll this port as long as
> the specified conditions are met? This is the latency and, in
> fact we don't care as long as the required specifications are
> met. If the latency is 1/2 second, we can't guarantee that the
> port is polled twice per second. However, if the latency is
> one microfortnight less than 1/2 second we can.

and this is exactly what is meant by controlling latency. I find it difficult
to put any other interpretation on that phrase other than by seriously
distorting the English language.

Your system in this case guarantees latency < 1/2 second. Hence it is a
`realtime system', according to the most common definition, which is that a
realtime system is one that gives guaranteed upper bounds on response time to
whatever events are relevant to the system.

A guarantee, is, of course, with some probability bounds, which should be part
of the system requirement specifications.

An operating system on its own cannot be classed as realtime, without making
assumptions about the hardware it is going to run on.

eg. if somebody has a PC with a suspend-to-disk mode, it would be difficult to
guarantee anything about response time to signals on ports if he started
hitting the suspend button.

-- arvind

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