mpi_send will be blocked when it sends the second message - mpi

Question

The node00 can send the information to node01 whose rank is 1 sucessfully, but it will be blocked on the second send to the node01. Why did it happen? Thanks a lot. I think there is no deadlock in the code.
Given the MPI programming, there are 5 nodes
.....
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Get_processor_name(processor_name, &namelen);
MPI_Status status;
int buff;
if(rank ==0)
{
buff=123;
for(int i=1;i<size;i++){
MPI_Send(&buff, 1, MPI_INT, i, tag, MPI_COMM_WORLD); //succeed
MPI_Send(&buff, 1, MPI_INT, i, tag, MPI_COMM_WORLD); //blocked
}
}
else
{
MPI_Recv(&buff, 1, MPI_INT, 0, tag, MPI_COMM_WORLD, &status);//succeed
MPI_Recv(&buff, 1, MPI_INT, 0, tag, MPI_COMM_WORLD, &status);//blocked
}
............

Answer 1

Why don't you use MPI_Isend and MPI_Irecv.These are non blocking calls.

Answer 2

#include<stdio.h>
#include<math.h>
#include<mpi.h>
#define tag 777
int rank;
int size;
int main(int argc,char *argv[])
{
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
//MPI_Get_processor_name(processor_name, &namelen);
MPI_Status status;
int buff;
int i;
if(rank ==0)
{ printf("hai");
buff=123;
for(i=1;i<size;i++){
MPI_Send(&buff, 1, MPI_INT, i, tag, MPI_COMM_WORLD); //succeed
MPI_Send(&buff, 1, MPI_INT, i, tag, MPI_COMM_WORLD); //blocked
}
}
else
{
printf("hello");
MPI_Recv(&buff, 1, MPI_INT, 0, tag, MPI_COMM_WORLD, &status);//succeed
MPI_Recv(&buff, 1, MPI_INT, 0, tag, MPI_COMM_WORLD, &status);//blocked
}
//MPI_Finalize(); //finalize MPI operations
return 0;
}

Answer 3

You should use mpi_finialize, like this:
int main(int argc, char *argv[])
{
int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
int buff, tag = 99;
MPI_Status status;
if (rank == 0)
{
buff = 123;
for (int i = 1; i < size; i++)
{
MPI_Send(&buff, 1, MPI_INT, i, tag, MPI_COMM_WORLD); //succeed
MPI_Send(&buff, 1, MPI_INT, i, tag, MPI_COMM_WORLD); //blocked
}
}
else
{
MPI_Recv(&buff, 1, MPI_INT, 0, tag, MPI_COMM_WORLD, &status); //succeed
printf("%d\n", buff);
MPI_Recv(&buff, 1, MPI_INT, 0, tag, MPI_COMM_WORLD, &status); //blocked
printf("%d\n", buff);
}
MPI_Finalize();
return 0;
}