Hmm, strange. It doesn't hang for me and AFAICS it shouldn't hang at all. I'm using 1.2.5. Which version of Open MPI are you using?
Hanging with 100% CPU utilization often means that your processes are
caught in a busy wait. You could try to set mpi_yield_when_idle:
> gentryx@hex ~ $ cat .openmpi/mca-params.conf
> mpi_yield_when_idle=1
But I don't think this should be necessary.
HTH
-Andreas
On 21:35 Mon 17 Mar , Giovani Faccin wrote:
> Hi there!
>
> I'm learning MPI, and got really puzzled... Please take a look at this very
> short code:
>
> #include <iostream>
> #include "mpicxx.h"
> using namespace std;
> int main(int argc, char *argv[])
> {
> MPI::Init();
>
> for (unsigned long t = 0; t < 10000000; t++)
> {
> //If we are process 0:
> if ( MPI::COMM_WORLD.Get_rank() == 0 )
> {
> MPI::Status mpi_status;
> unsigned long d = 0;
> unsigned long d2 = 0;
> MPI::COMM_WORLD.Recv(&d, 1, MPI::UNSIGNED_LONG, MPI::ANY_SOURCE,
> MPI::ANY_TAG, mpi_status );
> MPI::COMM_WORLD.Recv(&d2, 1, MPI::UNSIGNED_LONG, MPI::ANY_SOURCE,
> MPI::ANY_TAG, mpi_status );
> cout << "Time = " << t << "; Node 0 received: " << d << " and "
> << d2 << endl;
> }
> //Else:
> else
> {
> unsigned long d = MPI::COMM_WORLD.Get_rank();
> MPI::COMM_WORLD.Send( &d, 1, MPI::UNSIGNED_LONG, 0, 0);
> };
> };
> MPI::Finalize();
> }
>
> Ok, so what I'm trying to do is to make a gather operation using point to
> point communication. In my real application instead of sending an unsigned
> long I'd be calling an object's send and receive methods, which in turn would
> call their inner object's similar methods and so on until all data is
> syncronized. I'm using this loop because the number of objects to be sent to
> process rank 0 varies depending on the sender.
>
> When running this test with 3 processes on a dual core, oversubscribed node,
> I get this output:
> (skipped previous output)
> Time = 5873; Node 0 received: 1 and 2
> Time = 5874; Node 0 received: 1 and 2
> Time = 5875; Node 0 received: 1 and 2
> Time = 5876; Node 0 received: 1 and 2
>
> and then the application hangs, with processor usage at 100%. The exact time
> when this condition occurs varies on each run, but it usually happens quite
> fast.
>
> What would I have to modify, in this simple example, so that the application
> works as expected? Must I always use Gather, instead of point to point, to
> make a syncronization like this?
>
> Thank you very much!
>
> Giovani
>
>
>
>
>
>
>
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Andreas Schäfer
Cluster and Metacomputing Working Group
Friedrich-Schiller-Universität Jena, Germany
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