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

Dombo <dombo@disposable.invalid>
Mon, 23 Jul 2012 22:57:38 +0200
Op 22-Jul-12 23:59, Melzzzzz schreef:

I have played little bit with new C++11 features and compared,
java performance to c++.
Actually this was meant to be GC vs RAII memory management,
but boiled down to speed of BlockingQueue class in java,
and mine in c++.
It seems that java implementation is so much more efficient
but I don't know why. I even tried c++ without dynamic
memory management (except queue itself) and that is *even slower*.
Must be some quirks with a queue ;)

These are timings:
(java openjdk 1.7)
bmaxa@maxa:~/examples$ time java consprod

real 0m13.411s
user 0m19.853s
sys 0m0.960s

(c++ gcc 4.6.3)
bmaxa@maxa:~/examples$ time ./consprod

real 0m28.726s
user 0m34.518s
sys 0m6.800s

Example programs follow (I think implementations of
blocking queues are similar):
// java
import java.util.concurrent.*;
import java.util.Random;

class Vars{
public final static int nitems = 100000000;
public final static Random rnd = new Random(12345);
public final static int size = 10000;

class Producer implements Runnable {
    private final BlockingQueue<Integer> queue;
    Producer(BlockingQueue<Integer> q) { queue = q; }
    public void run() {
      try {
        int i = Vars.nitems;
        while(i-- > 0) { queue.put(produce(i)); }
      } catch (InterruptedException ex)

    Integer produce(int i) { return new Integer(i); }

  class Consumer implements Runnable {
    private final BlockingQueue<Integer> queue;
    Consumer(BlockingQueue<Integer> q)
     queue = q;
    public void run() {
      try {
        Integer[] arr = new Integer[10000];
        int i = Vars.nitems;
        while(i-- > 0) { consume(queue.take(),arr); }
      } catch (InterruptedException ex)
    void consume(Integer x, Integer[] arr)
     arr[Vars.rnd.nextInt(Vars.size)] = x;

  public class consprod {
    public static void main(String [] args) {
      BlockingQueue<Integer> q = new ArrayBlockingQueue<Integer>(100000);
      Producer p = new Producer(q);
      Consumer c = new Consumer(q);
      new Thread(p).start();
      new Thread(c).start();
      } catch(Exception e)
// c++
#include <condition_variable>
#include <mutex>
#include <thread>
#include <deque>
#include <cstdlib>

template <class T>
class BlockingQueue{
     BlockingQueue(unsigned cap):capacity_(cap)
     void put(T t)
         std::unique_lock<std::mutex> lock(m_);
         while(queue_.size() >= capacity_)c_full_.wait(lock);
     T take()
         std::unique_lock<std::mutex> lock(m_);
         T tmp = std::move(queue_.front());
         return std::move(tmp);
     bool empty()
         std::unique_lock<std::mutex> lock(m_);
         return queue_.empty();
     std::mutex m_;
     std::condition_variable c_empty_,c_full_;
     std::deque<T> queue_;
     unsigned capacity_;

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

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

     std::thread producer1(f_prod);

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

     std::thread consumer1(f_cons);

// --------------------------------------

In the C++ version at the end of the main function it waits for the
threads to complete, in the Java version the threads are not joined at
the end of the main function. I don't know if Java keeps the process
alive until all threads have terminated, but if not it would explain the

I'm a bit surpirsed that the C++ version compiles without complaints on
your compiler because there is no return statement in the main() function.

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