Question

I have a total of 8 messages being passed on 4 nodes using MPI. I noticed that there were two messages whose arrays did not provide meaningful results. I have copied an excerpt of the code below? These are some related questions I had based on the code/results below:

1. Does the MPI_Isend also require a wait? I am not sure if there is a deadlock. I also tried just passing these two variables from one node to the other, and the array values were still NULL.
2. Will MPI_SendRecv improve the efficiency of the code as suggested here Non Blocking communication in MPI and MPI Wait Issue. Not all information is passed correctly? If so, how/why? Would also appreciate some pointers on setting that up.

Thanks!

Source Code:

if ((my_rank) == 0)
{
MPI_Irecv(A, Rows, MPI_DOUBLE, my_rank+1, MPI_ANY_TAG, MPI_COMM_WORLD, &request[6]);
MPI_Wait(&request[6], &status[6]);
}


if ((my_rank) == 1)
{
MPI_Isend(AA, Rows, MPI_DOUBLE, my_rank-1, 0, MPI_COMM_WORLD, &request[6]);
}


if ((my_rank) == 2)
{
MPI_Isend(B, Rows, MPI_DOUBLE, my_rank+1, 0, MPI_COMM_WORLD, &request[7]);
}

if ((my_rank) == 3)
{
MPI_Irecv(BB, Rows, MPI_DOUBLE, my_rank-1, MPI_ANY_TAG, MPI_COMM_WORLD, &request[7]);
MPI_Wait(&request[7], &status[7]);
}

Answer 1

Yes, All non-blocking calls (MPI_Isend, MPI_Irecv etc) require a matching MPI_Wait. The call is not guaranteed to complete until MPI_Wait is called. You should not change the contents of the buffer until after MPI_Wait returns.

https://computing.llnl.gov/tutorials/mpi/

To use SendRecv, same task has to send a message and wait to receive a message. That pattern doesnt hold true for your code.