OpenMP: Run 2 thread groups in parallel - c

Question

I want to have 2 threads groups run at the same time. For example, 2 threads are executing code block 1 and another 2 threads are executing another code segment. There was a stackoverflow question here OpenMP: Divide all the threads into different groups and I changed the code to see if it suits the logic I need in my code.
I have the below code with me.
#include <stdio.h>
#include <omp.h>
#define NUM_THREADS 1
int main(int argc, char **argv) {
omp_set_nested(1); /* make sure nested parallism is on */
int nprocs = omp_get_num_procs();
int nthreads1 = NUM_THREADS;
int nthreads2 = NUM_THREADS;
int t1[nthreads1];
for (int i=0; i<nthreads1; i++) {
t1[i] = 0;
}
#pragma omp parallel default(none) shared(nthreads1, nthreads2, t1) num_threads(2)
#pragma omp single
{
#pragma omp task // section 1
#pragma omp parallel for num_threads(nthreads1) shared(t1)
for (int i=0; i<nthreads1; i++) {
printf("Task 1: thread %d of the %d children of %d: handling iter %d\n",
omp_get_thread_num(), omp_get_team_size(2),
omp_get_ancestor_thread_num(1), i);
t1[i] = 1;
}
#pragma omp task // section 2
#pragma omp parallel for num_threads(nthreads2) shared(t1, t2)
for (int j=0; j<nthreads2; j++) {
while (!t1[j]) {
printf("Task 2: thread %d of the %d children of %d: handling iter %d\n",
omp_get_thread_num(), omp_get_team_size(2),
omp_get_ancestor_thread_num(1), j);
}
}
}
return 0;
}
To check if my code is running 2 thread groups at once, I set the thread count in each group to 1 and then I keep a boolean list that is initialized to 0.
In the first code segment, I set the boolean value to 1, and in the 2nd code segment, I check the boolean value to break out of the while loop. It seems like the above code is only run by 1 thread, because if the thread starts running the 2nd code block/section, then it keeps stuck inside the while loop because another thread is not setting the boolean value to 1.
How to run 2 thread groups in parallel?
UPDATE: My use case: I am writing a word count map-reduce program using OpenMP. I want one thread group 2 read files which adds read lines to a queue. I want another thread group to process lines from those queues and update the counts in a chained hash table. I already wrote the code to first do the reading to formulate the queues and then do the mapping to take data from queues and generate word counts -- but I want to change my program to have 2 thread groups to do reading and mapping in parallel -- at the same time. That's why I made this shortcode to check how I can implement 2 thread groups, running in parallel executing 2 different code segments.

Answer 1

It seems like the above could be solved using single directives with nowait and task directives. The below approach puts the tasks on to a queue and then threads pickup work from the queue. So ideally, 2 thread groups will be working on 2 different tasks, which is what is required in the question. Below is the code;
#include <stdio.h>
#include <omp.h>
#define NUM_THREADS 1
int main(int argc, char **argv) {
omp_set_nested(1); /* make sure nested parallism is on */
int nprocs = omp_get_num_procs();
int nthreads1 = NUM_THREADS;
int nthreads2 = NUM_THREADS;
int t1[nthreads1];
for (int i=0; i<nthreads1; i++) {
t1[i] = 0;
}
#pragma omp parallel default(none) shared(nthreads1, nthreads2, t1)
{
#pragma omp single nowait // section 1
for (int i=0; i<nthreads1; i++) {
#pragma omp task
{
printf("Task 1: thread %d of the %d children of %d: handling iter %d\n",
omp_get_thread_num(), omp_get_team_size(2),
omp_get_ancestor_thread_num(1), i);
t1[i] = 1;
}
}
#pragma omp single nowait // section 2
for (int j=0; j<nthreads2; j++) {
#pragma omp task
{
while (!t1[j]) {
printf("Task 2: thread %d of the %d children of %d: handling iter %d\n",
omp_get_thread_num(), omp_get_team_size(2),
omp_get_ancestor_thread_num(1), j);
}
}
}
}
return 0;
}
Also, you can simply use an if-else statement inside the #pragma omp construct to run 2 threadgroups in parallel
#include <stdio.h>
#include <omp.h>
#define NUM_THREADS 2
int main(int argc, char **argv) {
omp_set_nested(1); /* make sure nested parallism is on */
int nprocs = omp_get_num_procs();
int nthreads1 = NUM_THREADS/2;
int nthreads2 = NUM_THREADS/2;
int t1[nthreads1];
for (int i=0; i<nthreads1; i++) {
t1[i] = 0;
}
#pragma omp parallel default(none) shared(t1) num_threads(NUM_THREADS) {
int i = omp_get_thread_num(); // section 1
if (i<nthreads1) {
printf("Section 1: thread %d\n",i);
t1[i] = 1;
} else {
int j = i - nthreads1;
while (!t1[j]) {
printf("Section 2: thread %d, shift_value %d\n", i, j);
}
}
}
return 0;
}