boost::pool_allocator significantly slower than std::allocator

Question

I am learning about memory pool, and trying to utilize boost::pool_allocator in my project. According to the documment, I made a small test about time cost:

template <typename Alloc>
void test()
{
    using namespace std::chrono;
    auto t0 = high_resolution_clock::now();
    for (int i = 0; i < 1000; ++i) {
        std::vector<int, Alloc> vec;
        for (int j = 0; j < 10000; ++j)
            vec.push_back(i + j);
    }
    auto t1 = high_resolution_clock::now();
    auto time_ms = duration<double>(t1 - t0).count() * 1e3;
    cout << "time cost: " << time_ms << " ms" << endl;
}

int main()
{
    test<std::allocator<int>>();
    test<boost::pool_allocator<int>>();
}

And the result is:

time cost: 3.97602 ms
time cost: 91.3943 ms

The Boost doc says:

Pools are generally used when there is a lot of allocation and deallocation of small objects.

So I expect boost::pool_allocator cost less time than std::allocator in the code above, but the test result shows it's much worse.

Am I using boost::pool_allocator wrong? In what situation can I obtain a speedup by using memory pool (or just Boost pool/pool_allocator)?

Answer 1

Pools are generally used when there is a lot of allocation and deallocation of small objects.

vector does not allocate "small objects". vector<T> allocates arrays. Or more specifically, a single array allocated in a slab of memory big enough to hold (at least) sizeof(T) * size bytes.

Pool allocators are fantastically bad at this allocation pattern.