Only one thread created with MPI & no speed-up with OpenMP - linux

Question

I actually have two questions but it seems they may be connected:
1) I've tried to run basic MPI example:
#include <mpi.h>
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char* argv[])
{
int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
printf("I am %d from %d\n", rank, size);
MPI_Finalize();
return 0;
}
It has to output something like:
I am 0 from 2
I am 1 from 2
Although I'm getting the following:
$ mpicc mpi_hello.c -o hello
$ mpirun -np 4 ./hello
I am 0 from 1
I am 0 from 1
I am 0 from 1
I am 0 from 1
$ mpirun -np 2 ./hello
I am 0 from 1
I am 0 from 1
Is it somehow connected to thread definition in Linux? I'm running it on Ubuntu 16.04.
2) My OpenMP program:
#include <omp.h>
#include <math.h>
#include <time.h>
#include <iostream>
#include <stdio.h>
const int N = 10000;
int matrix[N][N];
int main()
{
#pragma omp parallel num_threads(2)
#pragma omp for
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
matrix[i][j] = 1+i;
clock_t t;
t = clock();
#pragma omp parallel num_threads(2)
#pragma omp for
for (int i = 0; i < N; i++)
{
matrix[i][i] = 0;
for (int j = 0; j< N; j++)
if (j != i)
matrix[i][i] += sin(cos(log(matrix[i][j] + matrix[j][i])));
}
t = clock() - t;
std::cout << "It took " << ((float)t)/CLOCKS_PER_SEC << " sec" << std::endl;
return 0;
}
It works correctly and uses 2 threads. However, it loads 2 processors (~100% CPU) and takes the same time (~34 seconds) as the similiar consequtive one (loads 1 processor ~50% CPU). I know that OpenMP may need some time to start, but how can it result in the same duration of programs?

Answer 1

Answering to the MPI part of the question.
Do you have MPICH installed? If is that so, try to compile and run like this:
$ mpicc.mpich mpi_hello.c -o hello
$ mpirun.mpich -np 4 ./hello
I am 0 from 4
I am 1 from 4
I am 2 from 4
I am 3 from 4
It should work.