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

Joshua Maurice <>
Sun, 22 Jul 2012 15:42:55 -0700 (PDT)
On Jul 22, 2:59 pm, Melzzzzz <> wrote:

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>(1=


     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);


What g++ optimization options did you use? If you didn't compile with
proper optimization flags (e.g. at least -O2), then the numbers you
have are meaningless.

Also, you might be testing the difference between std::rand() and
Java's Random.

Also, why did you use dynamic allocation in the C++ code for an int?
If your original goal was to compare the Java way to the C++ way, you
are not doing it right. This is especially troubling after you
apparently went through the effort to make the queue support move-only
types. (Not sure. I'm still new to move-semantics.)


        std::unique_ptr<int> arr[size];


            arr[std::rand()%size] = produced.take();

I wonder if it's possible to optimize that into a simple assignment,
or whether there will be a call to delete() - or at least a branch to
test if the internal member pointer is null.

Just off the top of my head.

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