Ordered construct

The ordered construct is used to guarantee the code run in iteration order of loop. Check the following example:

#include <stdio.h>
#include <omp.h>

int main(void)
{    
    #pragma omp parallel for
    for (int i = 1; i <= 8; i++)
    {
        printf("Thread %d is executing iteration %d \n", omp_get_thread_num(), i);
    }

    return 0;
}

Build and run it on my 24-core machine:

# gcc -fopenmp parallel.c
# ./a.out
Thread 0 is executing iteration 1
Thread 6 is executing iteration 7
Thread 7 is executing iteration 8
Thread 5 is executing iteration 6
Thread 4 is executing iteration 5
Thread 2 is executing iteration 3
Thread 3 is executing iteration 4
Thread 1 is executing iteration 2

We can see the execution order of every iteration is random. Modify the for-loop code as following:

......
#pragma omp parallel for ordered
for (int i = 1; i <= 8; i++)
{
    #pragma omp ordered
    printf("Thread %d is executing iteration %d \n", omp_get_thread_num(), i);
}
......

Build and run the program again:

# gcc -fopenmp parallel.c
# ./a.out
Thread 0 is executing iteration 1
Thread 1 is executing iteration 2
Thread 2 is executing iteration 3
Thread 3 is executing iteration 4
Thread 4 is executing iteration 5
Thread 5 is executing iteration 6
Thread 6 is executing iteration 7
Thread 7 is executing iteration 8

This time the program outputs in iteration sequence.

Please notice for the code outside of the ordered construct, it still runs in concurrency. Modify the for-loop:

......
#pragma omp parallel for ordered
for (int i = 1; i <= 8; i++)
{
    printf("Thread %d is executing iteration %d in no-order\n", omp_get_thread_num(), i);
    #pragma omp ordered
    printf("Thread %d is executing iteration %d \n", omp_get_thread_num(), i);
}
......

Build and run it again:

# gcc -fopenmp parallel.c
# ./a.out
Thread 0 is executing iteration 1 in no-order
Thread 1 is executing iteration 2 in no-order
Thread 5 is executing iteration 6 in no-order
Thread 7 is executing iteration 8 in no-order
Thread 4 is executing iteration 5 in no-order
Thread 0 is executing iteration 1
Thread 1 is executing iteration 2
Thread 3 is executing iteration 4 in no-order
Thread 6 is executing iteration 7 in no-order
Thread 2 is executing iteration 3 in no-order
Thread 2 is executing iteration 3
Thread 3 is executing iteration 4
Thread 4 is executing iteration 5
Thread 5 is executing iteration 6
Thread 6 is executing iteration 7
Thread 7 is executing iteration 8