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C MPI - spawning multiple threads in batches

Discussion in 'C' started by Tamil_Rasigan, May 20, 2012.

  1. Tamil_Rasigan

    Tamil_Rasigan New Member

    May 20, 2012
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    I have a basic question about MPI programming in C. Essentially what I want is that there is a master process that spawns a specific number of child processes, collects some information from all of them (waits until all of the children finish), calculates some metric, based on this metric it decides if it has to spawn more threads... it keeps doing this until the metric meets some specific condition.

    Thanks for the help.

    Courtesy :In the example below , lets say, "for some lame reason", I want the master process to sum the contents of the array twice. I.e in the first iteration, the master process starts the slave processes which compute the sum of the arrays, once they are done and the master process returns the value, I would like to invoke the master process to reinvoke another set of threads to do the computation again. Why would the code below not work? I added a while loop around the master process process which spawns the slave processes.

    #include <stdio.h>
      #include <mpi.h>
      #define max_rows 100000
      #define send_data_tag 2001
      #define return_data_tag 2002
      int array[max_rows];
      int array2[max_rows];
    main(int argc, char **argv) 
       long int sum, partial_sum,number_of_times;
      MPI_Status status;
      int my_id, root_process, ierr, i, num_rows, num_procs,
         an_id, num_rows_to_receive, avg_rows_per_process, 
         sender, num_rows_received, start_row, end_row, num_rows_to_send;
      /* Now replicte this process to create parallel processes.
       * From this point on, every process executes a seperate copy
       * of this program */
      ierr = MPI_Init(&argc, &argv);
      root_process = 0;
      /* find out MY process ID, and how many processes were started. */
      ierr = MPI_Comm_rank(MPI_COMM_WORLD, &my_id);
      ierr = MPI_Comm_size(MPI_COMM_WORLD, &num_procs);
      if(my_id == root_process) {
         /* I must be the root process, so I will query the user
          * to determine how many numbers to sum. */
         //printf("please enter the number of numbers to sum: ");
         //scanf("%i", &num_rows);
          while (number_of_times<2)
         if(num_rows > max_rows) {
            printf("Too many numbers.\n");
         avg_rows_per_process = num_rows / num_procs;
         /* initialize an array */
         for(i = 0; i < num_rows; i++) {
            array[i] = i + 1;
         /* distribute a portion of the bector to each child process */
         for(an_id = 1; an_id < num_procs; an_id++) {
            start_row = an_id*avg_rows_per_process + 1;
            end_row   = (an_id + 1)*avg_rows_per_process;
            if((num_rows - end_row) < avg_rows_per_process)
               end_row = num_rows - 1;
            num_rows_to_send = end_row - start_row + 1;
            ierr = MPI_Send( &num_rows_to_send, 1 , MPI_INT,
                  an_id, send_data_tag, MPI_COMM_WORLD);
            ierr = MPI_Send( &array[start_row], num_rows_to_send, MPI_INT,
                  an_id, send_data_tag, MPI_COMM_WORLD);
         /* and calculate the sum of the values in the segment assigned
          * to the root process */
         sum = 0;
         for(i = 0; i < avg_rows_per_process + 1; i++) {
            sum += array[i];   
         printf("sum %i calculated by root process\n", sum);
         /* and, finally, I collet the partial sums from the slave processes, 
          * print them, and add them to the grand sum, and print it */
         for(an_id = 1; an_id < num_procs; an_id++) {
            ierr = MPI_Recv( &partial_sum, 1, MPI_LONG, MPI_ANY_SOURCE,
                  return_data_tag, MPI_COMM_WORLD, &status);
            sender = status.MPI_SOURCE;
            printf("Partial sum %i returned from process %i\n", partial_sum, sender);
            sum += partial_sum;
         printf("The grand total is: %i\n", sum);
      else {
         /* I must be a slave process, so I must receive my array segment,
          * storing it in a "local" array, array1. */
         ierr = MPI_Recv( &num_rows_to_receive, 1, MPI_INT, 
               root_process, send_data_tag, MPI_COMM_WORLD, &status);
         ierr = MPI_Recv( &array2, num_rows_to_receive, MPI_INT, 
               root_process, send_data_tag, MPI_COMM_WORLD, &status);
         num_rows_received = num_rows_to_receive;
         /* Calculate the sum of my portion of the array */
         partial_sum = 0;
         for(i = 0; i < num_rows_received; i++) {
            partial_sum += array2[i];
         /* and finally, send my partial sum to hte root process */
         ierr = MPI_Send( &partial_sum, 1, MPI_LONG, root_process, 
               return_data_tag, MPI_COMM_WORLD);
      ierr = MPI_Finalize();

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