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From: zeta_no <olivier_henley@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Hard and soft real time with Ada
Date: Sun, 16 May 2010 21:19:00 -0700 (PDT)
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Date: 2010-05-16T21:19:00-07:00
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Hi to all,

My research about real time programming left me with some unanswered
questions.

Proposition 1: The problem with a general kernel, let say a genuine
Linux, is that even stripped off, hard real time can not be guaranteed
because we can't fully control the scheduling policy of the kernel.
Therefore, any loaded module in kernel or user space, will be given
some CPU time without our consent and, in time, be unpredictable.  To
grossly simplify, let say I want to build a small drone and I decide
to put a minimal Linux install bundled with CRON on the embedded
hardware.

Question 1: There is a possibility, that during my flight, the kernel
accept to service the CRON scheduler, in between sensor readings?  Am
I right?

Question 2: Does the use of Ada, with its real time capabilities
really helps to achieve real time if we know that ultimately, its the
kernel who decides what to service.  Does the real time promises are
based on the assumption that the designer are brilliant enough to
disable any OS loadable module not required by the embedded
application?  If so, this would be a problem for me, because I am far
from being a kernel literate who knows everything that is going on
inside a particular OS.  Maybe a stripped OS is not that resource
consuming to be worried about?

Question 3: Does the Real Time Annex helps with that? I mean trying to
make real time programming on a general purpose OS.

Question 4: Now, if the best option is to use a real time kernel, what
about the need for a USB camera and some specialized library like
OpenCV for making some computer vision on a multi-core architecture?
This kind of support on real time kernel looks almost inexistent, let
say in open source options like RTEMS or MarteOS.

Question 5: Does RTLinux would be my best bet, or should I try to use
a GigE camera and port OpenCV myself onto RTEMS.  I just want insight
on global issues here, example of typical decisions made in the design
of a general robotic embedded system.

That's it for now.

Thank you,

Olivier Henley