Multi threading Raytracer - c

Question

I am making a raytracer, im trying to use pthread to divide the rendering. i noticed that isnt helping with the speed because the function pthread_join is to slow, if i use a loop to make the 'await' is way faster and works almost every time fine. But i cant use that because the time of rendering changes with the scene.
Is there a way to check if a thread is finished, on a more efficient way. This is the code.
`
int threats(t_file *c) //this function creates the threads
{
int i;
int err;
pthread_t th[THREADS];
i = 0;
printf("1\n");
c->thread = -1;
mlx_clear_window(c->mlx_ptr, c->win_ptr);
while (i < THREADS)
{
err = pthread_create(&th[i], 0, (void *)paint_scene, (void *)c);
if (err)
return parse_error("Thread Error: CAN NOT CREATE THREAD");
i++;
}
// while (i-- >= 0)
// pthread_join(th[i], 0);
//my await function xd
while (i < 200000000)
i++;
mlx_put_image_to_window(c->mlx_ptr, c->win_ptr, c->img.mlx_img, 0, 0);
c->thread = 0;
return 1;
}
void paint_scene(void *a)
{
int y;
int x;
t_ray ray;
int color;
t_file *c;
c = (t_file *)a;
color = 0;
c->thread++;
y = (c->thread * (c->win_heigth / THREADS));
printf("y:%d,hilo%d\n", y, c->thread);
while (y < (c->thread + 1) * (c->win_heigth / THREADS))
{
x = 0;
while (x < c->win_width)
{
ray = generate_ray(x, y, *c);
color = get_intersections(&ray, c);
if (c->ligth)
color = shading(&ray, color, c);
my_mlx_pixel_put(&c->img, x, y, color);
x++;
}
//ft_printf("\rLoading%d: %d%%", c->thread, y / (c->win_heigth / 100));
y++;
}
pthread_exit(0);
}
`

Answer 1

You have a concurrency problem here in your thread function:
c->thread++;
y = (c->thread * (c->win_heigth / THREADS));
printf("y:%d,hilo%d\n", y, c->thread);
while (y < (c->thread + 1) * (c->win_heigth / THREADS))
{
....
}
c->thread is shared between all threads, and based on likely thread timings and current face of the moon, I can make an educated guess and say that the first thread is calculating the whole image. When starting up, the first thread might see c->thread == -1, but later (if thread startup is faster than the while loop) other thread increase the value until the first thread sees c->thread == THREADS-1
To fix this, each call to create_thread must pass a pointer to a unique parameter object that holds that threads id. So remove the thread member from t_file. It probably serves no purpose there. And create a type of struct that holds the parameters to the thread function:
struct thread_param
{
unsigned int thread;
file_t *c;
}
You use it like this when starting threads:
struct thread_param params[THREADS];
while (i < THREADS)
{
params[i].thread = i;
params[i].c = c;
err = pthread_create(&th[i], 0, (void *)paint_scene, (void *)&(params[i]));
if (err)
return parse_error("Thread Error: CAN NOT CREATE THREAD");
i++;
}
And you access the data in your thread function:
void paint_scene(void *a)
{
struct thread_param *param = (struct thread_param *)a;
unsigned int thread = param->thread;
t_file *c = param->c;
/*
in the rest of the code you remove `c->thread++`
and replace `c->thread` with `thread`
*/
....
}

Answer 2

If you have atomic data types (C11, #ifndef __STDC_NO_ATOMICS__) then implement a global counter and wait until it hits zero (if decreasing) or the amount of threads (if increasing).
e.g.
#include <stdatomic.h>
atomic_int num_jobs;
void* thread_func(void*)
{
//the work to do in the thread function
//before exit decrease counter
--num_jobs;
pthread_exit(0);
}
int main()
{
num_jobs = THREADS; // same as your number of threads
create_threads(THREADS); // number of threads = THREADS
while (num_jobs) { // loop while threads running
//sleep for a while
}
join_threads(); // join threads for cleanup
return 0;
}
Otherwise classic lock mechanics,
e.g.
#include <pthread.h>
pthread_spinlock_t lock;
int num_jobs;
// called by main
int numJobs()
{
pthread_spin_lock(&lock);
int res = num_jobs;
pthread_spin_unlock(&lock);
return res;
}
// called by thread_func
void decNumJobs()
{
pthread_spin_lock(&lock);
--num_jobs;
pthread_spin_unlock(&lock);
}
int main()
{
pthread_spin_init(&lock, PTHREAD_PROCESS_PRIVATE);
// the other stuff as before
pthread_spin_destroy(&lock);
return 0;
}
Another alternative would be with pthread_cond_wait and pthread_cond_signal (mainly to avoid the sleep in the while loop, continue after receiving the signal and not based on a fixed amount of time).
e.g.
#include <pthread.h>
int num_jobs;
pthread_cond_t cond;
pthread_mutex_t lock;
void decNumJobs()
{
pthread_mutex_lock(&lock);
if (--num_jobs == 0)
pthread_cond_signal(&cond);
pthread_mutex_unlock(&lock);
}
void* thread_func(void*)
{
//the work to do in the thread function
//before exit decrease counter
decNumJobs();
pthread_exit(0);
}
int main()
{
num_jobs = THREADS;
pthread_cond_init(&cond, NULL);
pthread_mutex_init(&lock, NULL);
pthread_mutex_lock(&lock);
create_threads(THREADS);
pthread_cond_wait(&cond, &lock);
pthread_mutex_unlock(&lock);
join_threads();
pthread_cond_destroy(&cond);
pthread_mutex_destroy(&lock);
return 0;
}
Note: For the sake of simplicity, there is no error checking nor handling. Reading the documentation of the pthread_* functions (return values, interrupted wait, etc) is strongly advised.