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From: Robert Dewar <robert_dewar@my-dejanews.com>
Subject: Re: delay until and GNAT - expand
Date: 1999/05/11
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In article <rracine.13.0007AF82@draper.com>,
  rracine@draper.com (Roger Racine) wrote:

> Note that I said, "given a good real-time operating system".
> That does  not include any form of Unix (with the possible
> exception of LynxOS and any  other real-time versions; I have
> not looked at their documentation recently).

I do not believe that the estimate of one microsecond latency
for a delay is anywhere near achievable for any of the typical
operating systems or executives that are used today for typical
real time systems. Roger, do some measurements if you want to
challenge this. Most certainly Lynx, which I know well cannot
begin to achieve this performance. Modern RISC machines have
a lot of state to save and restore, and of course context
switches typically involve cache switch overs which waste
even more time.

Certainly in all measurements we have done on GNAT, the time
spent in pthreads calls completely overwhelms all the time
in the GNAT runtime itself.

Probably the best case is to run VxWorks on a really fast
machine, I will see if I can get some measurements for this.


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