Suppose that I have a code block that attempts to calculate the first 15 numbers in the Fibonacci sequence and distributes each unique number among 3 processes (MPI_Send) using a for loop as shown in the code block below.

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

    int rank, size, recieve_data1, recieve_data2;
    MPI_Init(NULL, NULL);
    MPI_Status status;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    printf("Available ranks are: %d \n \n", rank); // first rank rollcall
    fflush(stdout);

    int num1 = 1; int num2 = 1;
    int RecieveNum; int SumNum;

    for
        (int n = 0; n < 16; n  ) {
        if
            (rank == 0) {

            // perform the fibb sequence algorithim
            SumNum = num1   num2;
            num1 = num2;
            num2 = SumNum;

            // define the sorting algorithim
            int DeliverTo = (n % 3)   1;

            // send calculated result
            MPI_Send(&SumNum, 1, MPI_INT, DeliverTo, 1, MPI_COMM_WORLD);
        }
        else {
            // recieve the element integer
            MPI_Recv(&RecieveNum, 1, MPI_INT, 0, 1, MPI_COMM_WORLD, &status);

            // print and flush the buffer
            printf("I am process rank %d, and I recieved the number %d. \n", rank, RecieveNum);
            fflush(stdout);
        }
    }
    printf("Available ranks are: %d \n \n", rank); // second rank roll call
    fflush(stdout);

/*

more code that I run...

*/

    MPI_Finalize();
    return 0;
}

Before the first for loop is called, processes 0,1,2,3 all respond to the printf(Available ranks are: %d \n \n", rank); on line 25. However, after executing the first for loop and using a second printf, only process 0 responds. I was expecting all 4 processes 0 - 3 to respond again after the execution of the for loop. To solve this problem, I isolated this section of code and attempted to debug for several hours with no sucess. This particular issue proves to be problematic, as I have additional code (not shown here for the sake of being concise), that will access the numbers generated from this sequence.

Finally, I am running the code by building the solution, running the VS terminal as an administrator, and typing mpiexec -n 4 my_file_name.exe. No build errors or complication mistakes occurred. From what I can see (correct me if I'm wrong), all processes hang after completing the for loop, but I am unsure why or how to fix it.

After searching the website, I did not see anything that answered this question (from my point of view). I am a bit of an MPI (and Stack Overflow) newbie, so any code pointers are also welcomed. Thanks

CodePudding user response：

You have process zero compute who to send to. And then everyone does a receive. That means that all processes that are not the computed receiver will hang.

This scenario where you send to a dynamically computed receiver is not easy to do in MPI. You either need to

• send a message to all other processes "no, I have nothing for you", or
• send a message to all, and all-but-one process ignores the data, or
• use one sided operations where you MPI_Put the data on the computer receiver.