c++ - Problems with MPI_Reduce -

first of all, new mpi. have program uses rectangle/midpoint rule calculate area of function equal pi. when use method involving sends , receives calculation gives value pi. however, when testing mpi_reduce achieve same thing pi 1 processor, using different numbers of processors value less pi. have code output local_sum each processor , global_sum mpi_reduce. local sums add global sum, isn't pi. can't figure out wrong. appreciated. here section of code relevant:

/* add areas calculated each process */ if (my_rank == 0) {     total = my_area;     (source = 1; source < p; source++) {         mpi_recv(&my_area, 1, mpi_double, source, tag,             mpi_comm_world, &status);         total = total + my_area;     } } else {       mpi_send(&my_area, 1, mpi_double, dest,         tag, mpi_comm_world); }  //*********test begin using mpi_reduce********* // print local sums on each process printf("local sum process %d - %f\n",    my_rank, my_area);  double global_sum;  // reduce of local sums global sum mpi_reduce( &my_area,        // send data &global_sum,     // receive data 1,               // number of elements in send buffer mpi_double,      // mpi datatype mpi_sum,         // mpi reduce operation 0,               // root process  mpi_comm_world); // mpi communicator //**********test end using mpi_reduce**********  /* print result */ if (my_rank == 0) {     printf("with n = %d rectangles, our estimate\n",         n);     printf("of area %f %f = %.15f\n",         a, b, total);     cout << "the area calculated using mpi_reduce " << global_sum << "." << endl; } 

the output of code single processor is:

enter limits of integration a, b, , number of bins n, separated spaces

0.0 1.0 10000000

local sum process 0 - 3.141593

with n = 10000000 rectangles, our estimate

of area 0.000000 1.000000 = 3.141592653473855

the area calculated using mpi_reduce 3.14159.

the output of code using (for example) 4 processors is:

local sum process 1 - 0.923739

local sum process 2 - 0.775058

local sum process 0 - 0.453312

local sum process 3 - 0.453312

with n = 10000000 rectangles, our estimate

of area 0.000000 1.000000 = 3.141591655190580

the area calculated using mpi_reduce 2.60542.