Are all tasks that are created in worksharing loop constructs inside a parallel region sibling tasks in OpenMP?

Question

I have this simple self-contained example of a very rudimentary stencil application to work with OpenMP tasks and the dependence clause. At 2 steps one location of an array is added 3 values from another array, one from the corresponding location and its left and right neighbours. To avoid data races I have set up dependencies so that for every section on the second update its task can only be scheduled if the relevant tasks for the sections from the first update step are executed. I get the expected results but I am not sure if my assumptions are correct, because these tasks might be immediately executed by the encountering threads and not spawned. So my question is whether the tasks that are created in worksharing loops all sibling tasks and thus are the dependencies retained just like when the tasks are generated inside a single construct.

#include <iostream>
#include <omp.h>
#include <math.h>

typedef double value_type;
int main(int argc, char * argv[]){

    std::size_t size = 100000;
    std::size_t part_size = 25;

    std::size_t parts = ceil(float(size)/part_size);
    std::size_t num_threads = 4;

    value_type * A = (value_type *) malloc(sizeof(value_type)*size);
    value_type * B = (value_type *) malloc(sizeof(value_type)*size);
    value_type * C = (value_type *) malloc(sizeof(value_type)*size);


    for (int i = 0; i < size; ++i) {
        A[i] = 1;
        B[i] = 1;
        C[i] = 0;
    }


#pragma omp parallel num_threads(num_threads)
{

    #pragma omp for schedule(static)
        for(int part=0; part<parts; part++){
            std::size_t current_part = part * part_size;
            std::size_t left_part = part != 0 ? (part-1)*part_size : current_part;
            std::size_t right_part = part != parts-1 ? (part+1)*part_size : current_part;
            std::size_t start = current_part;
            std::size_t end = part == parts-1 ? size-1 : start+part_size;
            if(part==0) start = 1;

            #pragma omp task depend(in: A[current_part], A[left_part], A[right_part]) depend(out: B[current_part])
            {
                for(int i=start; i<end; i++){
                    B[i] += A[i] + A[i-1] + A[i+1];
                }
            }
        }
    #pragma omp for schedule(static)
        for(int part=0; part<parts; part++){
            int current_part = part * part_size;
            std::size_t left_part = part != 0 ? (part-1)*part_size : current_part;
            std::size_t right_part = part != parts-1 ? (part+1)*part_size : current_part;
            std::size_t start = current_part;
            std::size_t end = part == parts-1 ? size-1 : start+part_size;
            if(part==0) start = 1;
            #pragma omp task depend(in: B[current_part], B[left_part], B[right_part]) depend(out: C[current_part])
            {
                for(int i=start; i<end; i++){
                    C[i] += B[i] + B[i-1] + B[i+1];
                }
            }
        }
}



    value_type sum = 0;
    value_type max = -1000000000000;
    value_type min =  1000000000000;
    for(int i = 0; i < size; i++){
        sum+=C[i];
        if(C[i]<min) min = C[i];
        if(C[i]>max) max = C[i];
    }
    std::cout << "sum: " << sum << std::endl;
    std::cout << "min: " << min << std::endl;
    std::cout << "max: " << max << std::endl;
    std::cout << "avg: " << sum/(size) << std::endl;
    return 0;
}

Answer 1

In OpenMP specification you can find the corresponding definitions:

sibling tasks - Tasks that are child tasks of the same task region.

child task - A task is a child task of its generating task region. A child task region is not part of its generating task region.

task region - A region consisting of all code encountered during the execution of a task. COMMENT: A parallel region consists of one or more implicit task regions

In the description of parallel construct you can read that:

A set of implicit tasks, equal in number to the number of threads in the team, is generated by the encountering thread. The structured block of the parallel construct determines the code that will be executed in each implicit task.

This practically means that in the parallel region many task regions are generated and using #pragma omp for different task region will generate explicit tasks (i.e #pragma omp task...). However, only tasks generated by the same task region are sibling tasks (not all of them!). If you want to be sure that all generated tasks are sibling tasks, you have to use a single task region (e.g. using single construct) to generate all the explicit tasks.

Note that your code gives the correct result, because there is an implicit barrier at the end of worksharing-loop construct (#pragma omp for). To remove this barrier you have to use the nowait clause and you will see that the result will be incorrect in such a case.

Another comment is that in your case the workload is smaller than parallel overheads, so my guess is that your parallel code will be slower than the serial one.