Re: tasksing and TCP sockets

["Rolf" <rolf.ebert_nospam_@gmx.net>]

Stephen Leake wrote:
> "Rolf" <rolf.ebert_nospam_@gmx.net> writes:
>
> > In a stripped down version I can succesfully connect to the server
> > (using GNAT.Sockets) and either read the sensor data *or* write actor
> > data. The program is currently split in two tasks. One tasks contains
> > the application code and the actor communication (using
> > type_id'output(channel)). The second tasks gets started during program
> > initialisation. It reads the data from the socket via
> > type_id'input(channel) and provides the data to the application through
> > a protected object.
> >
> > The problem that I face is that the second task seems to starve the
> > first one. As soon as it is active, the application does not generate
> > debug output anymore.
>
> You need to set the priorities correctly,

Does that mean e.g. to set the main task to 10 and the second task to
5?

> and you need to ensure that
> there are no busy waits; the second task should be _suspended_ waiting
> on socket completions, not polling for them.

How would I do that? I have no explicit busy wait, I use

  Var := Sensor_Cmd_T'Input (Channel);

If possible, I'd like to keep that easy way of data input and avoid
explicit conversion and read commands.

> > Some background info: gcc-3.4.4 on Windows XP.
>
> That may be a problem. I find the commercially supported GNAT to be
> less broken.

This is a hobby project.

> > Java for the world simulation.
>
> Yuck. I assume that's not by choice?

The world simulation already exists.  I won't recode it in Ada.  (and I
kind of like Java, too)

> > The simulator sends about 20 packages à 11bytes every 100ms. The
> > application sends typically 2-3 packages per second.
>
> That's certainly "reasonable rates"
>

That is almost nothing!

> > I have delay statements in both main loops (varied between 0.0 and
> > 0.2 seconds).
>
> You should _not_ need delay statements. Redesign to be waiting on
> socket completions.

How?

> Or, if you are designing to a cyclic execution, you can poll on the
> sockets, but just once per cycle. You can use
> GNAT.Sockets.Control_Socket for that.
>

I shall check that.

> > My questions:
> >  - is that a reasonable tasking approach?
>
> In general, yes. But the details matter a lot.
>
> >  - I don't know much about sockets (or better I don't remember
> > much). Does a waiting read (in the second task) block a write in the
> > first task?
>
> Not in Windows with GNAT 5.03a. gcc-3.4.4 on Cygwin may be different.

There is no real Ada compiler on Cygwin.  I use the Windows
(mingw-based) gcc-3.4.4. I shall see if GNAT GPL 2005 gives different
results.

> > - how can I interrupt the waiting read task to force activity in the
> > main task?
>
> Depends on how it is waiting. If is in a GNAT.Sockets.Receive_Socket,
> you can't (short of shutting down the socket). So periodic polling
> might be better.

I suppose it waits in the Sensor_Cmd_T'Input (Channel) routine, but I
am not sure.

> > - what other hints can you give me?
>
> Get an AdaCore support contract.
> 
In a professional environment certainly, but not here ...

    Rolf