I am learning MPI with C and I wrote a code based on the one presented in this link: http://condor.cc.ku.edu/~grobe/docs/intro-MPI-C.shtml.
In this code a vector containing 1e8 values are summed. However, I am observing that when using more processes the run time is getting bigger. The code is given bellow:
/*
Based on the code presented at http://condor.cc.ku.edu/~grobe/docs/intro-MPI-C.shtml
Code which splits a vector and send information to other processes.
In case of main vector does not split equally to all processes, the leftover is passed to process id 1.
Process id 0 is the root process. Therefore it does not count while passing information.
Each process will calculate the partial sum of vector values and send it back to root process, which will calculate the total sum.
Since the processes are independent, the printing order will be different at each run.
compile as: mpicc -o vector_sum vector_send.c -lm
run as: time mpirun -n x vector_sum
x = number of splits desired + root process. For example: if * = 3, the vector will be splited in two.
*/
#include<stdio.h>
#include<mpi.h>
#include<math.h>
#define vec_len 100000000
double vec1[vec_len];
double vec2[vec_len];
int main(int argc, char* argv[]){
// defining program variables
int i;
double sum, partial_sum;
// defining parallel step variables
int my_id, num_proc, ierr, an_id, root_process; // id of process and total number of processes
int num_2_send, num_2_recv, start_point, vec_size, rows_per_proc, leftover;
ierr = MPI_Init(&argc, &argv);
root_process = 0;
ierr = MPI_Comm_size(MPI_COMM_WORLD, &num_proc);
ierr = MPI_Comm_rank(MPI_COMM_WORLD, &my_id);
if(my_id == root_process){
// Root process: Define vector size, how to split vector and send information to workers
vec_size = 1e8; // size of main vector
for(i = 0; i < vec_size; i++){
//vec1[i] = pow(-1.0,i+2)/(2.0*(i+1)-1.0); // defining main vector... Correct answer for total sum = 0.78539816339
vec1[i] = pow(i,2)+1.0; // defining main vector...
//printf("Main vector position %d: %f\n", i, vec1[i]); // uncomment if youwhish to print the main vector
}
rows_per_proc = vec_size / (num_proc - 1); // average values per process: using (num_proc - 1) because proc 0 does not count as a worker.
rows_per_proc = floor(rows_per_proc); // getting the maximum integer possible.
leftover = vec_size - (num_proc - 1)*rows_per_proc; // counting the leftover.
// spliting and sending the values
for(an_id = 1; an_id < num_proc; an_id++){
if(an_id == 1){ // worker id 1 will have more values if there is any leftover.
num_2_send = rows_per_proc + leftover; // counting the amount of data to be sent.
start_point = (an_id - 1)*num_2_send; // defining initial position in the main vector (data will be sent from here)
}
else{
num_2_send = rows_per_proc;
start_point = (an_id - 1)*num_2_send + leftover; // starting point for other processes if there is leftover.
}
ierr = MPI_Send(&num_2_send, 1, MPI_INT, an_id, 1234, MPI_COMM_WORLD); // sending the information of how many data is going to workers.
ierr = MPI_Send(&vec1[start_point], num_2_send, MPI_DOUBLE, an_id, 1234, MPI_COMM_WORLD); // sending pieces of the main vector.
}
sum = 0;
for(an_id = 1; an_id < num_proc; an_id++){
ierr = MPI_Recv(&partial_sum, 1, MPI_DOUBLE, an_id, 4321, MPI_COMM_WORLD, MPI_STATUS_IGNORE); // recieving partial sum.
sum = sum + partial_sum;
}
printf("Total sum = %f.\n", sum);
}
else{
// Workers:define which operation will be carried out by each one
ierr = MPI_Recv(&num_2_recv, 1, MPI_INT, root_process, 1234, MPI_COMM_WORLD, MPI_STATUS_IGNORE); // recieving the information of how many data worker must expect.
ierr = MPI_Recv(&vec2, num_2_recv, MPI_DOUBLE, root_process, 1234, MPI_COMM_WORLD, MPI_STATUS_IGNORE); // recieving main vector pieces.
partial_sum = 0;
for(i=0; i < num_2_recv; i++){
//printf("Position %d from worker id %d: %d\n", i, my_id, vec2[i]); // uncomment if youwhish to print position, id and value of splitted vector
partial_sum = partial_sum + vec2[i];
}
printf("Partial sum of %d: %f\n",my_id, partial_sum);
ierr = MPI_Send(&partial_sum, 1, MPI_DOUBLE, root_process, 4321, MPI_COMM_WORLD); // sending partial sum to root process.
}
ierr = MPI_Finalize();
}
Obs.: Compile as
mpicc -o vector_sum vector_send.c -lm
and run as:
time mpirun -n x vector_sum
with x = 2 and 5. You will see that with x=5 it takes more time to run.
Did I do something wrong? I did not expected it to be slower, since the summation of each chunk is independent. Or it is a matter of how the program is sending the information for each process? It seems to me that the loops for sending the information for each process is the responsible for this longer time.
