This is going back to the fact that you, as a developer, are the best
placed to know exactly when asynchronous progress is needed for your
algorithm, so from that perspective you can provide that progress in the
most timely manner. One way to force MPI to do progress, is to spawn
another thread (using pthread_create as an example), dedicated to providing
a means to the MPI library to execute progress. This communication thread
can use a non-blocking ANY_SOURCE receive, using a tag that will never
match any other message. This way, you can safely cancel this pending
request upon completion of your app. You can then rely on this thread to
call MPI_Test when you know you need guaranteed progress, and the rest of
the time you can park it on some synchronization primitive
(mutex/condition).
George.
On Tue, Dec 31, 2019 at 5:12 PM Martín Morales <
martineduardomora...@hotmail.com> wrote:
> Hi George, thank you very much for your answer. Can you please explain me
> a little more about "If you need to guarantee progress you might either
> have your own thread calling MPI functions (such as MPI_Test)". Regards
>
> Martín
>
> ------------------------------
> *De:* George Bosilca <bosi...@icl.utk.edu>
> *Enviado:* martes, 31 de diciembre de 2019 13:47
> *Para:* Open MPI Users <users@lists.open-mpi.org>
> *Cc:* Martín Morales <martineduardomora...@hotmail.com>
> *Asunto:* Re: [OMPI users] Non-blocking send issue
>
> Martin,
>
> The MPI standard does not mandate progress outside MPI calls, thus
> implementations are free to provide, or not, asynchronous progress. Calling
> MPI_Test provides the MPI implementation with an opportunity to progress
> it's internal communication queues. However, an implementation could try a
> best effort to limit the time it spent in MPI_Test* and to provide the
> application with more time for computation, even when this might limit its
> own internal progress. Thus, as a non-blocking collective is composed of a
> potentially large number of point-to-point communications, it might require
> a significant number of MPI_Test to reach completion.
>
> If you need to guarantee progress you might either have your own thread
> calling MPI functions (such as MPI_Test) or you can use the asynchronous
> progress some MPI libraries provide. For this last option read the
> documentation of your MPI implementation to see how to enable asynchronous
> progress.
>
> George.
>
>
> On Mon, Dec 30, 2019 at 2:31 PM Martín Morales via users <
> users@lists.open-mpi.org> wrote:
>
> Hello all!
> Im with OMPI 4.0.1 and I have a strange behaviour (or at least,
> unexpected) with some non-blocking sending calls: MPI_Isend and MPI_Ibcast.
> I really need asyncronous sending so I dont use MPI_Wait after the send
> call (MPI_Isend or MPI_Ibcast); insted of this I check "on demand" with
> MPI_Test to verify if sending its or not complete. Test Im doing it sends
> just an int value. Here some code (with MPI_Ibcast):
>
> ***SENDER***
>
> //Note that It use an intercommunicator
> MPI_Ibcast(&send_some_int_data, 1, MPI_INT, MPI_ROOT, mpi_intercomm,
> &request_sender);
> //MPI_Wait(&request_sender, MPI_STATUS_IGNORE); <-- I dont want this
>
>
> ***RECEIVER***
>
> MPI_Ibcast(&recv_some_int_data, 1, MPI_INT, 0, parentcomm,
> &request_receiver);
> MPI_Wait(&request_receiver, MPI_STATUS_IGNORE);
>
> ***TEST RECEPTION (same sender instance program)***
>
> void test_reception() {
>
> int request_complete;
>
> MPI_Test(&request_sender, &request_complete, MPI_STATUS_IGNORE);
>
> if (request_complete) {
> ...
> } else {
> ...
> }
>
> }
>
> But when I invoke this test function after some time has elapsed since I
> sent, the request isnt complete and i have to invoque this test function
> again and againg... x (variable) times, until it finally its completed. Its
> just an int it was sended, just that (all on a local machine); has no sense
> such delay. The request should be completed on the first function test
> invocation.
>
> If, instead of this, I uncomment the unwanted MPI_Wait (i.e. doing it like
> a synchronous request), it completes immediately, like expected.
> If I send with MPI_Isend I get the same behaviour.
>
> I dont understand whats is going on. Any help will be very appreciated.
>
> Regards.
>
> Martín
>
>
