[Info-vax] Real time, was: Re: Volatile, was: Re: yet another sys$qiow question

[JF Mezei]

On 15-08-29 14:47, Simon Clubley wrote:

> It doesn't matter how fast Ethernet is; the root problem is that it
> doesn't have deterministic behaviour.

While the above is correct, one has to take a "big picture" approach.

Say for instance the military protocol is deterministic, but you can
only poll once per second.

The ethernet is not deterministic, but the maximum jitter you could
expect is half a second. So the worse case scenario of ethernet, you are
still below the response time of 1 second which the military protocol
provides. (random values to make the point).

So if your requirement is precision to 1 second, and ethernet gives you
precision that is well below 1 second despite not being deterministic,
shouldn't it be considered acceptable ?

Also, while ethernet on single coax had to content with collisions,
modern switch based ethernet has pretty much eliminated that problem and
if you design a network that can handle the data load that your real
time app generates, then it will work fine in a very deterministic way.

It is the same for data collection. If you must receive data at certain
thoughput, you would calculate what is the maximum time the system can
be hung due to interrupt, and whether the buffers that receive data can
buffer all the incoming data during that time and the next read will get
it all.