[Barrelfish-users] [Barrelfish] Messages in Barrelfish

Zeus Gómez Marmolejo zeus.gomez at bsc.es
Mon Jul 18 03:10:41 CEST 2011

Hi Andrew,

Thank you very much for your response. Now everything is much clearer. What
was happening was exactly that.

So I tried the two variants. First, in the single-threaded one, I used the
continuation closure associated to the message to trigger the callback when
the message is sent. Just like the idctest example you were telling. In that
case it's working perfectly. So this may solve my problem, although things
get a bit more complicated.

I would better try the multi-threaded version, so I created one extra thread
per core just dispatching messages from the default waitset, as you
suggested. However, in this case the problem doesn't get solved. The
transmit queue fills and there is no more space after 32 messages. I'm
wondering if I'm waiting in the correct waitset. You said there is an
incoming waitset? Is it different from the default one?

The "dispatcher" thread looks like this:

static int dispatch_thread(void *p)
    while (1) event_dispatch(get_default_waitset());

    return 0;

Thank you again for your help!


El 17 de julio de 2011 19:54, Andrew Baumann
<Andrew.Baumann at microsoft.com>escribió:

> Hi Zeus,
> > I made a very simple example where core 0 is sending n messages to core 1
> > using the UMP backend. Core 1 is just waiting and dispatching messages.
> >
> > So the problem is that I can not manage to send more than 31 messages to
> > core 1. The message 32 is sent but core 1 doesn't receive it. The error I
> get is:
> >
> > Failure: (       flounder) Cannot queue message for transmit: queue is
> full
> >
> > In the meantime all other 31 messages have been dispatched and received
> > by core 1 but the sending error keeps on, so no more messages can be
> sent.
> > Message 32 seems to be on queue but never dispatched.
> Are you processing the incoming event queue on core 0, by dispatching the
> waitset associated with the channel?
> If your program is single-threaded, then a loop like while (1)
> send_message(); won't work, because you also need to dispatch events on the
> incoming waitset in order for the UMP channel implementation to see the acks
> which tell it that the receiver has consumed the messages. The TX_BUSY error
> you're seeing is the channel implementation telling you (on core 0) that
> there is no space in the outgoing message queue.
> The correct thing to do in this case depends on your program, but assuming
> you want to use the single-threaded state-machine model, the right thing to
> do is to register a callback for the ability to send further messages (e.g.
> with b->register_send()), and then return to your event processing loop
> which should free up space in the outgoing message queue and then run your
> callback. You might want to have a look at the idctest program in the tree
> for some examples of how to use callbacks and continuations for sending.
> If this all sounds very painful and low-level, that's because it is :) Have
> you considered using a higher-level abstraction like THC?
> Cheers,
> Andrew

Zeus Gómez Marmolejo
Barcelona Supercomputing Center
PhD student
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