Re: [Boost-users] [mpi] irecv / send problem

Subject: Re: [Boost-users] [mpi] irecv / send problem
From: Nick Collier (nick.collier_at_[hidden])
Date: 2009-06-15 10:03:18

Hmm, I think it should work. Thanks though. The example I posted was  
modified from:

http://ci-tutor.ncsa.illinois.edu/content.php?cid=1137

Namely,

/* deadlock avoided */
#include <stdio.h>
#include <mpi.h>

void main (int argc, char **argv) {

int myrank;
MPI_Request request;
MPI_Status status;
double a[100], b[100];

MPI_Init(&argc, &argv);  /* Initialize MPI */
MPI_Comm_rank(MPI_COMM_WORLD, &myrank); /* Get rank */
if( myrank == 0 ) {
   /* Post a receive, send a message, then wait */
   MPI_Irecv( b, 100, MPI_DOUBLE, 1, 19, MPI_COMM_WORLD, &request );
   MPI_Send( a, 100, MPI_DOUBLE, 1, 17, MPI_COMM_WORLD );
   MPI_Wait( &request, &status );
}
else if( myrank == 1 ) {
   /* Post a receive, send a message, then wait */
   MPI_Irecv( b, 100, MPI_DOUBLE, 0, 17, MPI_COMM_WORLD, &request );
   MPI_Send( a, 100, MPI_DOUBLE, 0, 19, MPI_COMM_WORLD );
   MPI_Wait( &request, &status );
}

MPI_Finalize();          /* Terminate MPI */

}

I tried again with one that more close matches the example:

class Item {
private:

        friend class boost::serialization::access;

        template<class Archive>
        void serialize(Archive& ar, const unsigned int version) {
                ar & val;
        }

public:
        int val;
        Item() : val(1) {
        }

};

struct Receipt {

        boost::mpi::request request;
        std::vector<Item> items;
};

int main(int argc, char **argv) {

        mpi::environment env(argc, argv);
        mpi::communicator world;
        Receipt receipt;

        vector<Item> msg(100000);

        int myRank = world.rank();
        if (myRank == 0) {
                receipt.request = world.irecv(1, 19, receipt.items);
                world.send(1, 17, msg);
                receipt.request.wait();
        } else if (myRank == 1) {
                receipt.request = world.irecv(0, 17, receipt.items);
                world.send(0, 19, msg);
                receipt.request.wait();
        }
        cout << "Done" << endl;
        return 0;
}

And I still get deadlock. Moreover, there is no deadlock if I irecv /  
send a very large array of doubles. I'd really appreciate it if  
someone else could try the example and see if it works.

Nick

On Jun 13, 2009, at 1:01 AM, Ryo IGARASHI wrote:

> Hi, Nick,
>
> # I am not a MPI expert.
>
> On Sat, Jun 13, 2009 at 4:16 AM, Nick  
> Collier<nick.collier_at_[hidden]> wrote:
>> I running into an issue where an irecv followed by a send results in
>> deadlock. A simple test case,
>
>> Run with mpirun -np 2, this never completes. It does complete with
>> vector<Item> msg(10) however.
>
> According to the MPI standard, MPI_irecv() finishes when MPI_wait()  
> is calledg,
> and MPI_send() never returns before receive completes.
> So the deadlock is of no surprise.
>
> I think you shouldn't rely on the behavior with small size object.
> It is the buffering mechanism in the MPI implementation
> that avoid the deadlock of the small size object case.
>
> See http://www.mpi-forum.org/docs/ for specification.
>
> Best regards,
> -- 
> Ryo IGARASHI, Ph.D.
> rigarash_at_[hidden]
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