Re: tasksing and TCP sockets

[Stephen Leake <stephen_leake@acm.org>]

"Rolf" <rolf.ebert_nospam_@gmx.net> writes:

> Consider an application that communicates with its environment via
> sensors and actors. For testing purposes the low level access to the
> sensor and actor data gets replaced by a TCP socket communication whose
> other end simulates the world, i.e. the world simulation continuously
> writes sensor data to the socket and accepts commands as actor data.
>
>   +-----------------------+
>   |                       |
>   | application           |
>   |                       |
>   +-----------------------+
>   |    h/w abstraction    |
>   |  sensors  |  actors   |
>   +-----------------------+
>              |
>          TCP client
>              |
>              |
>              |
>   TCP server that simulates the world
>

Ok, this makes sense.

> 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, 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. And the simulator must
send data at a reasonable rate, not too fast.

> 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.

> Java for the world simulation. 

Yuck. I assume that's not by choice?

> The simulator sends about 20 packages � 11bytes every 100ms. The
> application sends typically 2-3 packages per second. 

That's certainly "reasonable rates"

> 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.

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.

> 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.

> - 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.

> - what other hints can you give me?

Get an AdaCore support contract.

-- 
-- Stephe