CODEWITHSUNDEEP
cmultithreadingperformanceopenmpmandelbrot
martin49
martin49

Reputation: 182

Code execution slower with OpenMP

I am trying to speed up the execution of the following code with OpenMP. The code is for calculating a mandelbrot and output it to canvas.

The code works fine single threaded, but I want to use OpenMP to make it faster. I tried all sorts of combination of private and shared variables but nothing seems to work so far. The code always runs a little slower with OpenMP than without it (50 000 iterations - 2s slower).

I am using Ubuntu 16.04 and compiling with GCC.

void calculate_mandelbrot(GLubyte *canvas, GLubyte *color_buffer, uint32_t w, uint32_t h, mandelbrot_f x0, mandelbrot_f x1, mandelbrot_f y0, mandelbrot_f y1, uint32_t max_iter) {
mandelbrot_f dx = (x1 - x0) / w;
mandelbrot_f dy = (y1 - y0) / h;
uint16_t esc_time;
int i, j;
mandelbrot_f x, y;

//timer start
clock_t begin = clock();

#pragma omp parallel for private(i,j,x,y, esc_time) shared(canvas, color_buffer)
for(i = 0; i < w; ++i) {
    x = x0 + i * dx;
    for(j = 0; j < h; ++j) {
        y = y1 - j * dy;
        esc_time = escape_time(x, y, max_iter);

        canvas[ GET_R(i, j, w) ] = color_buffer[esc_time * 3];
        canvas[ GET_G(i, j, w) ] = color_buffer[esc_time * 3 + 1];
        canvas[ GET_B(i, j, w) ] = color_buffer[esc_time * 3 + 2];

      }
}

//time calculation
clock_t end = clock();
double time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
printf("%f\n",time_spent );
}

escape_time function which the code uses:

inline uint16_t escape_time(mandelbrot_f x0, mandelbrot_f y0, uint32_t max_iter) {
mandelbrot_f x = 0.0;
mandelbrot_f y = 0.0;
mandelbrot_f xtemp;
uint16_t iteration = 0;
while((x*x + y*y < 4) && (iteration < max_iter)) {
    xtemp = x*x - y*y + x0;
    y = 2*x*y + y0;
    x = xtemp;
    iteration++;
}
return iteration;

}

The code is from this repository https://github.com/hortont424/mandelbrot

Upvotes: 0

Views: 254

Answers (2)

martin49
martin49

Reputation: 182

As it was suggested my problem was caused by using the clock() function, which measures CPU time. Using omp_get_wtime() instead solved my problem.

Upvotes: 0

dreamcrash
dreamcrash

Reputation: 51463

First, like hinted in the comment, use omp_get_wtime() instead of clock() (it will give you the number of clock ticks accumulated across all threads) measure the time. Second, If I recall correctly, this algorithm have load balancing problems, so try to use a dynamic scheduling:

//timer start
double begin = omp_get_wtime();

#pragma omg parallel for private(j,x,y, esc_time) schedule(dynamic, 1)
for(i = 0; i < w; ++i) {
    x = x0 + i * dx;
    for(j = 0; j < h; ++j) {
        y = y1 - j * dy;
        esc_time = escape_time(x, y, max_iter);

        canvas[ GET_R(i, j, w) ] = color_buffer[esc_time * 3];
        canvas[ GET_G(i, j, w) ] = color_buffer[esc_time * 3 + 1];
        canvas[ GET_B(i, j, w) ] = color_buffer[esc_time * 3 + 2];

      }
}

//time calculation
double end = omp_get_wtime();
double time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
printf("%f\n",time_spent );

Upvotes: 1

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