Im trying to modify some code that is involved in point-to-point communications. Process A has a one way mode of communication with Process B. A checks to see if its rank is zero and if so will send a command to B (MPI_Issend) about what kind of data is going to be coming next. After sending the command to B, A then issues an Issend, sending a block of data to B.

Process B sets up a number of request instances via MPI_Recv_init and then issues an MPI_Startall on the requests. B sits in a while (1) loop, where the basic processing is a switch statement based on the content of the command being sent from A. At the top of the loop, B sits at an MPI_Wait until a command comes in. Then at each case in the switch, B sits in a MPI_Waitall to make sure that all As have sent their data. B then processes the received data, issues an MPI_Startall on the receive requests instances, exits the switch statement and then issues an MPI_Start on the B command request so it can go back to waiting at the top of the loop.

In the original process A code, prior to sending out a command, A will issue an MPI_Wait to make sure that the command request instance is free. After which it sends out the command, followed by the data. So I’ve taken this infrastructure and have tried to add a new command from within a function called in A. The function is passed the command request instance to be used for the MPI_Wait. I check the status of this MPI_Wait, and all is good. I then issue my own MPI_Issend (have also tried MPI_Ssend) to process B. The status coming back from the send is good. At the end of this function I added in another MPI_Wait because this function sends several commands from within a loop. None of the commands are received by B at least not at the beginning. After process A goes through an outer loop a few times (each time calling my new function with the MPI calls in it), process B suddenly gets some of the commands for one pass through the function. After that it never comes back from the MPI_Wait at the end of the inner function.

What am I missing about how the MPI calls work?