Re: Why is java consumer/producer so much faster than C++

On 24/07/2012 21:45, Melzzzzz wrote:


Right, it may well depend on how smart the std::deque::allocator_type is.

Below is a version of the BlockingQueue using spin locks. Compared to
the original version in Java it is about 2.5 times faster on my
platform, provided that you remove std::rand() from the test, as it
seems to be a real bottleneck and does not give any precious
informations about the efficiency of the Queue (which is the thing to
measure after all).

Let me know your results.

---

#include <thread>
#include <cstdlib>
#include <functional>
#include <atomic>
#include <memory>
#include <type_traits>

class spinlock_mutex {
     std::atomic_flag flag = ATOMIC_FLAG_INIT;
public:

     void lock() {
         while (flag.test_and_set(std::memory_order_acquire)) {
             std::this_thread::yield();
         };
     }

     void unlock() {
         flag.clear(std::memory_order_release);
     }
};

template <class T, class A = std::allocator<T >>
class BlockingQueue {
     const std::size_t cap_;
     A alloc_;
     T* queue_;
     std::size_t n = 0, b = 0, f = 0;
     spinlock_mutex m_;
public:

     BlockingQueue(const std::size_t cap, const A& alloc = A())
     : cap_(cap), alloc_(alloc), queue_(alloc_.allocate(cap)) { }

     ~BlockingQueue() {
         alloc_.deallocate(queue_, cap_);
     }

     static_assert(std::is_nothrow_move_constructible<T>::value,
"BlockingQueue cannot be exception-safe");

     void put(const T t) {
         for (;;) {
             m_.lock();
             if (n < cap_)
                 break;
             m_.unlock();
         }
         alloc_.construct(&queue_[f], std::move(t));
         f = ++f % cap_;
         ++n;
         m_.unlock();
     }

     T take() {
         for (;;) {
             m_.lock();
             if (n > 0)
                 break;
             m_.unlock();
         }
         const T t = std::move(queue_[b]);
         alloc_.destroy(&queue_[b]);
         b = ++b % cap_;
         --n;
         m_.unlock();
         return t;
     }
};

int main() {
     BlockingQueue<int> produced(100000);
     const int nitems = 100000000;
     std::srand(12345);

     std::function<void() > f_prod = [&](){
         int i = nitems;
         while (i-- > 0) {
             produced.put(i);
         }
     };

     std::thread producer1(f_prod);

     std::function<void() > f_cons = [&](){
         const int size = 10000;
         int arr[size];
         int i = nitems;
         while (i-- > 0) {
             arr[std::rand() % size] = produced.take();
         }
     };

     std::thread consumer1(f_cons);

     producer1.join();
     consumer1.join();
}