Parallelizing with OpenMP - how? - parallel-processing

Question

I want to parallelize an OpenMP raytracing algorithm that contains two for loops.
Is there anything more I can do than just setting omp_set_num_threads(omp_get_max_threads()) and putting #pragma omp parallel for in front of the first for loop?
So far I've reached a 2.13-times faster algorithm.
Code:
start = omp_get_wtime();
#pragma omp parallel for
for (int i = 0; i < (viewport.xvmax - viewport.xvmin); i++)
{
for (int j = 0; j < (viewport.yvmax - viewport.yvmin); j++)
{
int intersection_object = -1; // none
int reflected_intersection_object = -1; // none
double current_lambda = 0x7fefffffffffffff; // maximum positive double
double current_reflected_lambda = 0x7fefffffffffffff; // maximum positive double
RAY ray, shadow_ray, reflected_ray;
PIXEL pixel;
SPHERE_INTERSECTION intersection, current_intersection, shadow_ray_intersection, reflected_ray_intersection, current_reflected_intersection;
double red, green, blue;
double theta, reflected_theta;
bool bShadow = false;
pixel.i = i;
pixel.j = j;
// 1. compute ray:
compute_ray(&ray, &view_point, &viewport, &pixel, &camera_frame, focal_distance);
// 2. check if ray hits an object:
for (int k = 0; k < NSPHERES; k++)
{
if (sphere_intersection(&ray, &sphere[k], &intersection))
{
// there is an intersection between ray and object
// 1. Izracunanaj normalu...
intersection_normal(&sphere[k], &intersection, &ray);
// 2. ako je lambda presjecista manji od trenutacnog:
if (intersection.lambda_in < current_lambda)
{
current_lambda = intersection.lambda_in;
intersection_object = k;
copy_intersection_struct(&current_intersection, &intersection);
}
// izracunaj current lambda current_lambda =
// oznaci koji je trenutacni object : intersection_object =
// kopiraj strukturu presjeka : copy_intersection_struct();
}
}
// Compute the color of the pixel:
if (intersection_object > -1)
{
compute_shadow_ray(&shadow_ray, &intersection, &light);
theta = dotproduct(&(shadow_ray.direction), &(intersection.normal));
for (int l = 0; l<NSPHERES; l++)
{
if (l != intersection_object)
{
if (sphere_intersection(&shadow_ray, &sphere[l], &shadow_ray_intersection) && (theta>0.0))
bShadow = true;
}
}
if (bShadow)
{ // if in shadow, add only ambiental light to the surface color
red = shadow(sphere[intersection_object].ka_rgb[CRED], ambi_light_intensity);
green = shadow(sphere[intersection_object].ka_rgb[CGREEN], ambi_light_intensity);
blue = shadow(sphere[intersection_object].ka_rgb[CBLUE], ambi_light_intensity);
}
else
{
// the intersection is not in shadow:
red = blinnphong_shading(&current_intersection, &light, &view_point,
sphere[intersection_object].kd_rgb[CRED], sphere[intersection_object].ks_rgb[CRED], sphere[intersection_object].ka_rgb[CRED], sphere[intersection_object].shininess,
light_intensity, ambi_light_intensity);
green = blinnphong_shading(&current_intersection, &light, &view_point,
sphere[intersection_object].kd_rgb[CGREEN], sphere[intersection_object].ks_rgb[CGREEN], sphere[intersection_object].ka_rgb[CGREEN], sphere[intersection_object].shininess,
light_intensity, ambi_light_intensity);
blue = blinnphong_shading(&current_intersection, &light, &view_point,
sphere[intersection_object].kd_rgb[CBLUE], sphere[intersection_object].ks_rgb[CBLUE], sphere[intersection_object].ka_rgb[CBLUE], sphere[intersection_object].shininess,
light_intensity, ambi_light_intensity);
}
tabelaPixlov[i][j].red = red;
tabelaPixlov[i][j].green = green;
tabelaPixlov[i][j].blue = blue;
glColor3f(tabelaPixlov[i][j].red, tabelaPixlov[i][j].green, tabelaPixlov[i][j].blue);
intersection_object = -1;
bShadow = false;
}
else
{
// draw the pixel with the background color
tabelaPixlov[i][j].red = 0;
tabelaPixlov[i][j].green = 0;
tabelaPixlov[i][j].blue = 0;
intersection_object = -1;
bShadow = false;
}
current_lambda = 0x7fefffffffffffff;
current_reflected_lambda = 0x7fefffffffffffff;
}
}
//glFlush();
stop = omp_get_wtime();
for (int i = 0; i < (viewport.xvmax - viewport.xvmin); i++)
{
for (int j = 0; j < (viewport.yvmax - viewport.yvmin); j++)
{
glColor3f(tabelaPixlov[i][j].red, tabelaPixlov[i][j].green, tabelaPixlov[i][j].blue);
glBegin(GL_POINTS);
glVertex2i(i, j);
glEnd();
}
}
printf("%f\n št niti:%d\n", stop - start, omp_get_max_threads());
glutSwapBuffers();
}

Answer 1

With ray tracing you should use schedule(dynamic). Besides that I would suggest fusing the loop
#pragma omp parallel for schedule(dynamic) {
for(int n=0; n<((viewport.xvmax - viewport.xvmin)*(viewport.yvmax - viewport.yvmin); n++) {
int i = n/(viewport.yvmax - viewport.yvmin);
int j = n%(viewport.yvmax - viewport.yvmin)
//...
}
Also, why are you setting the number of threads? Just use the default which should be set to the number of logical cores. If you have Hyper Threading ray tracing is one of the algorithms that will benefit from Hyper Threading so you don't want to set the number of threads to the number of physical cores.
In addition to using MIMD with OpenMP I would suggest looking into using SIMD for ray tracing. See Ingo Wald's PhD thesis for an example on how to do this http://www.sci.utah.edu/~wald/PhD/. Basically you shoot four (eight) rays in one SSE (AVX) register and then go down the ray tree for each ray in parallel. However, if one ray finishes you hold it and wait until all four are finished (this is similar to what is done on the GPU). There have been many papers written since which have more advanced tricks based on this idea.