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From: dennison@telepath.com
Subject: Re: Tasking and preemption
Date: 1998/12/11
Message-ID: <74rr7n$4sp$1@nnrp1.dejanews.com>#1/1
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References: <36714AA1.96BD1AC5@systems.at>
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Date: 1998-12-11T00:00:00+00:00
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In article <36714AA1.96BD1AC5@systems.at>,
  Thomas Handler <thandler@systems.at> wrote:
> Hi to everyone!
>
> I'm trying to get into Ada and have now passed the tasking mechanism
> provided by Ada95.
> My problem is that my understanding of preempting seems not to match
> with that of my system ;-)
>
> I'm using GNAT 3.09p on Linux with libpthreads and when doing a simple
> program with two tasks where each task body is simply puttnig a string
> in a loop to the screen the tasks seem to have a real cooperative
> behavior, i.e. Task B starts working after Task A has finished
> (independent of the length of the loop).
> Building a delay into the loop has the desired effect.

Feel fortunate its working that way. Text_IO is typically not task-safe. You
shouldn't be trying to write to the same file (the screen counts as a file)
from two different concurrently-running tasks.


> So my question is: In the RM preemption is based on calling blocking
> statements (like delay), as far as I understand there is no definition
> of thread supported preemption. Is this true (i.e. I have to take care
> in my program to make calls to blocking statements) or am I terribly
> wrong?

There are multitudes of different scheduling policies that your system could
be using. The only predfined one is FIFO_Within_Priorities (D.2.2). Under
this policy a running task could concieveably hog the CPU forever unless it
performs a rendezvous, accept, or a delay. Check out Annex D of the LRM. It
has a very thourough treatment of this subject.

--
T.E.D.

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