MPI in C parallelisation of custom function running in serial - c

Question

I am a baginner MPI user and I may made some mistake with my parallel code for my calculation.
I need to compute an iterative estimation on a large data set and I want to calculate it in parallel using MPI in C.
I made a standard (ANSI) C function ('myFunc') to estimate an element in the output dataset ('param_2') based on the input parameters ('param_1',param_3,'table_1','table_2','table_3') and the estimation of the previous iteration ('param_2'). The calculation could be done parallel if we partition the new estimation ('param_2') into chunks.
When I made some profiling on the code, I realised that the calculation started almost at the same time on each node (thread), but it is finished in a serial fashion, one after another (with a fixed time interval between them).
It looks like they are using some shared resources or something like that... I tried to eliminate all concurrency between the threads, but i am affraid I do not have enough experience in MPI to solve the problem.
I thought all MPI thread have its own 'copy' of the declared variables and using them independently from each other, so I do not understand why the threads wait for each other to finish the calculation when they have their own copy of the parameters...
Here is the simplefield version of the code:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <mpi.h>
#define X 131
#define Y 131
#define Z 150
#define MASTER 0
float table_1[31][8];
float table_2[31][4];
float table_3[31][2];
int main(int argc, char* argv[]) {
float *param_1;
float *param_2;
float param_3;
float *chunk;
int file_length = X*Y*Z;
float myFunc(int i, float *param_1, float *param_2, float param_3);
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &taskid);
MPI_Comm_size(MPI_COMM_WORLD, &numtasks);
chunk_size = ceil(file_length / numtasks);
/* Allocate memory for the input parameters */
param_1 = malloc(file_length*sizeof(float));
param_2 = malloc(file_length*sizeof(float));
if( taskid == MASTER) {
/* Read parameters from file (table_1, table_2, table_3, param_1) */
}
MPI_Bcast(table_1,31*8,MPI_FLOAT,MASTER,MPI_COMM_WORLD);
MPI_Bcast(table_2,31*4,MPI_FLOAT,MASTER,MPI_COMM_WORLD);
MPI_Bcast(table_3,31*2,MPI_FLOAT,MASTER,MPI_COMM_WORLD);
MPI_Bcast(param_1,file_length,MPI_FLOAT,MASTER,MPI_COMM_WORLD);
for(it = 0; it < 10; it++) {
for(i = 0; i < chunk_size; i++) {
chunk[i] = myFunc((taskid*chunk_size)+i, param_1, param_2, param_3);
}
MPI_Gather(chunk, chunk_size, MPI_FLOAT, param_2, chunk_size, MPI_FLOAT, MASTER, MPI_COMM_WORLD);
MPI_Bcast(param_2, file_length, MPI_FLOAT, MASTER, MPI_COMM_WORLD);
}
MPI_Finalize();
free(...);
return 0;
}
float myFunc(int i, float *param_1, float *param_2, float param_3) {
/* Using the global tables (table_1,table_2,table_3) and some localy declared variable */
/* No MPI function here, only Math functions */
}
If you have a solution, advise or a comment please be kind and share with me, I would be grateful, thank you!