Multithread program in C++ shows the same performance as a serial one

Question

I just want to write a simple program in C++, which creates two threads and each of them fills vector by squares of integers (0, 1, 4, 9, ...). Here is my code:

Answer 1

Most of the suggestions are right: threading a task will improve the execution time only if the thread cpu load (in your case the multiplication i * i) is more important than the shared memory access load (in your case v.push_back). You can try with this code. You will see the gains of threading. And you can use the unix command

>time ./a.out 

to time your code more easily.

#include 
#include 
#include 
#include 
#include 
#include 

#define MULTI 1
#define SIZE 10000000

void fill(std::vector &v, size_t n)
{
    int sum = 0;
    for (size_t i = 0; i < n; ++i) {
        for (size_t j = 0; j < 100; ++j) {
            sum += sqrt(i*j);
        }
    }
    v.push_back(sum);
}

int main()
{
    std::vector v1, v2;
    v1.reserve(SIZE);
    v2.reserve(SIZE);
    #if !MULTI
    fill(v1, SIZE);
    fill(v2, SIZE);
    #else
    std::thread first(fill, std::ref(v1), SIZE);
    std::thread second(fill, std::ref(v2), SIZE);

    first.join();
    second.join();
    #endif
    return 0;
}