Re: Tasking and preemption

[Thomas Handler <th@umundum.vol.at>]

dennison@telepath.com wrote:
> 
> 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.
OK, it was not a quite good example using Text_IO but it was just for a
test ;-)

> 
> > 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.
I have read this annex several times in LRM and AARM and with your above
answer it seems that I got itz the right way.
So this does actually mean that having a task that does heavy
computations has to be written 
a.) in a way that ensures that a blocking statement will be executed
from time to time
b.) this task runs on lower priority and the other tasks execute a
blocking statement (will this work ?)
c.) use a different (implementation dependant) scheduling policy (BTW:
which task dispatching policies are defined for GNAT?)

I have to admit that I have not used threads ever before on Unix due to
the fact that I have gained some experiences with them on a propietary
system developed by my former employer. After a look on the PThread
documentation I decided not to use them with C/C++ since the latter of
the combination itself is already troublesome :-)
But it seems strange to me since my understanding of threads always was
that they are preempting (that's why you a thread-safe library will help
a lot ;-) and GNAT uses libpthread. So I'm a bit surprised at the
moment.

Ciao,

Thomas Handler

> 
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