Re: Hard and soft real time with Ada

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sun, 16 May 2010 21:19:00 -0700 (PDT), zeta_no wrote:

> 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?

Depends on the effective priorities.
 
> 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.

No, in general.

However some Ada vendors provide task scheduling independent on the OS.
I.e. the Ada program has just one OS thread which internally runs all the
tasks. If Ada thread is never preemted when it does something that will be
real-time.

Then there exist bare board Ada distributions, i.e. in effect you trow your
kernel away, and replace with another mini OS, e.g. INTEGRITY.

> 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?

That is what an real-time OS is about. You have to be able to plan all OS
activities, in particular by assigning them right priorities etc.

> Maybe a stripped OS is not that resource
> consuming to be worried about?

I think you could start with giving your Ada program the highest possible
priority.

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

Yes, if fairly implemented by the compiler vendor. But it cannot do things,
the OS cannot or forbids.

> 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.

Hmm, I doubt USB can be considered real-time. But I see no obvious reason
why a computer vision application could not tolerate some jitter (say
100�s). I guess you just do not need "very hard" real-time.

> 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.

I didn't use RT Linux, I did VxWorks. Certainly you can implement
communication with the USB camera in Ada. Then you will be in full control
of what is going on.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de