Mandelbrot optimization in openmp
Well i have to paralellisize the mandelbrot program in C. I think i have done it well and i cant get better times. My question if someone has an idea to improve the code, ive been thinking perhaps in nested parallel regions between the outer and insider for...
Also i have doubts if its more elegant or recommended to put all the pragmas in a single line or to write separate pragmas ( one for omp parallel and shared and private variables and a conditional, and another pragma with omp for and schedule dynamic).
Ive the doubt if constants can be used as private variables because i think is cleaner to have constants instead of defined variables.
Also i have written a conditional ( if numcpu >1) it has no sense to use parallel region and make a normal sequential execution.
Finally as i have read the dynamic chunk it depends on hardware and your system configuration... so i have left it as a constant, so it can be easily changed.
Also i adapt the number of threads to the number of processors available..
int main(int argc, char *argv[])
{
omp_set_dynamic(1);
int xactual, yactual;
//each iteration, it calculates: newz = oldz*oldz + p, where p is the current pixel, and oldz stars at the origin
double pr, pi; //real and imaginary part of the pixel p
double newRe, newIm, oldRe, oldIm; //real and imaginary parts of new and old z
double zoom = 1, moveX = -0.5, moveY = 0; //you can change these to zoom and change position
pixel_t *pixels = malloc(sizeof(pixel_t)*IMAGEHEIGHT*IMAGEWIDTH);
clock_t begin, end;
double time_spent;
begin=clock();
int numcpu;
numcpu = omp_get_num_procs();
//FILE * fp;
printf("El número de procesadores que utilizaremos es: %d", numcpu);
omp_set_num_threads(numcpu);
#pragma omp parallel shared(pixels, moveX, moveY, zoom) private(xactual, yactual, pr, pi, newRe, newIm) (if numcpu>1)
{
//int xactual=0;
// int yactual=0;
#pragma omp for schedule(dynamic, CHUNK)
//loop through every pixel
for(yactual = 0; yactual < IMAGEHEIGHT; yactual++)
for(xactual = 0; xactual < IMAGEWIDTH; xactual++)
{
//calculate the initial real and imaginary part of z, based on the pixel location and zoom and position values
pr = 1.5 * (xactual - IMAGEWIDTH / 2) / (0.5 * zoom * IMAGEWIDTH) + moveX;
pi = (yactual - IMAGEHEIGHT / 2) / (0.5 * zoom * IMAGEHEIGHT) + moveY;
newRe = newIm = oldRe = oldIm = 0; //these should start at 0,0
//"i" will represent the number of iterations
int i;
//start the iteration process
for(i = 0; i < ITERATIONS; i++)
{
//remember value of previous iteration
oldRe = newRe;
oldIm = newIm;
//the actual iteration, the real and imaginary part are calculated
newRe = oldRe * oldRe - oldIm * oldIm + pr;
newIm = 2 * oldRe * oldIm + pi;
//if the point is outside the circle with radius 2: stop
if((newRe * newRe + newIm * newIm) > 4) break;
}
// color(i % 256, 255, 255 * (i < maxIterations));
if(i == ITERATIONS)
{
//color(0, 0, 0); // black
pixels[yactual*IMAGEWIDTH+xactual][0] = 0;
pixels[yactual*IMAGEWIDTH+xactual][1] = 0;
pixels[yactual*IMAGEWIDTH+xactual][2] = 0;
}
else
{
double z = sqrt(newRe * newRe + newIm * newIm);
int brightness = 256 * log2(1.75 + i - log2(log2(z))) / log2((double)ITERATIONS);
//color(brightness, brightness, 255)
pixels[yactual*IMAGEWIDTH+xactual][0] = brightness;
pixels[yactual*IMAGEWIDTH+xactual][1] = brightness;
pixels[yactual*IMAGEWIDTH+xactual][2] = 255;
}
}
} //end of parallel region
end= clock();
time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
fprintf(stderr, "Elapsed time: %.2lf seconds.n", time_spent);
You could extend the implementation to leverage SIMD extensions. As far as I know the latest OpenMP standard includes vector constructs. Check out this article that describes the new capabilities.
This whitepaper explains how SSE3 can be used when calculating the Mandelbrot set.
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