What are the Best Alternatives to `std::vector` in OpenMP Parallel For Loops?

C OpenMP Parallel For Loop: Alternatives to std::vector

OpenMP's parallel for loops provide a convenient way to parallelize code. However, using shared data structures within these loops can introduce performance bottlenecks. One commonly used data structure, std::vector, may not sempre be the best choice for shared use in parallel loops.

Alternatives to std::vector

For optimal performance and thread safety in parallel for loops, consider these alternatives to std::vector:

std::vector with User-Defined Reductions

OpenMP 4.0 introduces user-defined reductions, allowing you to define custom reduction operations for custom data structures. This approach can improve performance by avoiding the overhead of locking shared data.

Example:

#pragma omp declare reduction(merge : std::vector<int> : omp_out.insert(omp_out.end(), omp_in.begin(), omp_in.end()))

std::vector<int> vec;
#pragma omp parallel for reduction(merge: vec)
for (int i = 0; i < 100; i++) vec.push_back(i);

Ordered Vector

If the order of elements in the shared vector is crucial, consider the following approach:

std::vector<int> vec;
#pragma omp parallel
{
    std::vector<int> vec_private;
    #pragma omp for schedule(static) nowait
    for (int i = 0; i < N; i++) vec_private.push_back(i);

    #pragma omp for schedule(static) ordered
    for (int i = 0; i < omp_get_num_threads(); i++) {
        #pragma omp ordered
        vec.insert(vec.end(), vec_private.begin(), vec_private.end());
    }
}

Custom Parallel Vector Class

For complex shared data structures, you may need to implement custom parallel vector classes to handle resizing during the loop while ensuring thread safety and efficient performance.

Example:

class ParallelVector {
private:
    std::vector<int> data;
    std::atomic<size_t> size;

public:
    void push_back(int value) {
        size++;
        data.push_back(value);
    }

    size_t getSize() {
        return size.load();
    }
};

ParallelVector vec;

#pragma omp parallel
{
    #pragma omp for
    for (int i = 0; i < 100; i++) vec.push_back(i);
}

The choice of alternative to std::vector depends on the specific requirements of your parallel loop. Consider factors such as thread safety, performance, and ease of implementation to select the most appropriate solution for your application.

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