Matrix Multiplication OpenMP Counter-Intuitive Results - matrix

Question

I am currently porting some code over to OpenMP at my place of work. One of the tasks I am doing is figuring out how to speed up matrix multiplication for one of our applications.
The matrices are stored in row-major format, so A[i*cols +j] gives the A_i_j element of the matrix A.
The code looks like this (uncommenting the pragma parallelises the code):
#include <omp.h>
#include <iostream>
#include <iomanip>
#include <stdio.h>
#define NUM_THREADS 8
#define size 500
#define num_iter 10
int main (int argc, char *argv[])
{
// omp_set_num_threads(NUM_THREADS);
int *A = new int [size*size];
int *B = new int [size*size];
int *C = new int [size*size];
for (int i=0; i<size; i++)
{
for (int j=0; j<size; j++)
{
A[i*size+j] = j*1;
B[i*size+j] = i*j+2;
C[i*size+j] = 0;
}
}
double total_time = 0;
double start = 0;
for (int t=0; t<num_iter; t++)
{
start = omp_get_wtime();
int i, k;
// #pragma omp parallel for num_threads(10) private(i, k) collapse(2) schedule(dynamic)
for (int j=0; j<size; j++)
{
for (i=0; i<size; i++)
{
for (k=0; k<size; k++)
{
C[i*size+j] += A[i*size+k] * B[k*size+j];
}
}
}
total_time += omp_get_wtime() - start;
}
std::setprecision(5);
std::cout << total_time/num_iter << std::endl;
delete[] A;
delete[] B;
delete[] C;
return 0;
}
What is confusing me is the following: why is dynamic scheduling faster than static scheduling for this task? Timing the runs and taking an average shows that static scheduling is slower, which to me is a bit counterintuitive since each thread is doing the same amount of work.
Also, am I correctly speeding up my matrix multiplication code?

Answer 1

Parallel matrix multiplication is non-trivial (have you even considered cache-blocking?). Your best bet is likely to be to use a BLAS Library for this, rather than writing it yourself. (Remember, "The best code is the code I do not have to write").
Wikipedia: Basic Linear Algebra Subprograms points to many implementations, a lot of which (including Intel Math Kernel Library) have free licenses.