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


M

Melzzzzz

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)
[email protected]:~/examples$ time java consprod

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

(c++ gcc 4.6.3)
[email protected]:~/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) {
try{
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)
{
e.printStackTrace();
}
}
}
//-----------------------------------------
// c++
#include <condition_variable>
#include <mutex>
#include <thread>
#include <deque>
#include <cstdlib>

template <class T>
class BlockingQueue{
public:
BlockingQueue(unsigned cap):capacity_(cap)
{
}
void put(T t)
{
std::unique_lock<std::mutex> lock(m_);
while(queue_.size() >= capacity_)c_full_.wait(lock);
queue_.push_back(std::move(t));
c_empty_.notify_one();
}
T take()
{
std::unique_lock<std::mutex> lock(m_);
while(queue_.empty())c_empty_.wait(lock);
T tmp = std::move(queue_.front());
queue_.pop_front();
c_full_.notify_one();
return std::move(tmp);
}
bool empty()
{
std::unique_lock<std::mutex> lock(m_);
return queue_.empty();
}
private:
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::srand(12345);


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);

producer1.join();
consumer1.join();
}
// --------------------------------------
 
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J

Joshua Maurice

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)
[email protected]:~/examples$ time java consprod

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

(c++ gcc 4.6.3)
[email protected]:~/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) {
   try{
     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)
     {
      e.printStackTrace();
     }
   }
 }
//-----------------------------------------
// c++
#include <condition_variable>
#include <mutex>
#include <thread>
#include <deque>
#include <cstdlib>

template <class T>
class BlockingQueue{
public:
    BlockingQueue(unsigned cap):capacity_(cap)
    {
    }
    void put(T t)
    {
        std::unique_lock<std::mutex> lock(m_);
        while(queue_.size() >= capacity_)c_full_.wait(lock);
        queue_.push_back(std::move(t));
        c_empty_.notify_one();
    }
    T take()
    {
        std::unique_lock<std::mutex> lock(m_);
        while(queue_.empty())c_empty_.wait(lock);
        T tmp = std::move(queue_.front());
        queue_.pop_front();
        c_full_.notify_one();
        return std::move(tmp);
    }
    bool empty()
    {
        std::unique_lock<std::mutex> lock(m_);
        return queue_.empty();
    }
private:
    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::srand(12345);

    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);

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


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.)

Also,
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.
 
M

Melzzzzz

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.

I have used -O2.
Also, you might be testing the difference between std::rand() and
Java's Random.
Possibly.


Also, why did you use dynamic allocation in the C++ code for an int?

Actually with non dynamical allocation seems to be slower ;)
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.)

That was necessary in order to put unique_ptr in queue.
Also,
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.

I have removed that statement , removed called to rand, left
only take, made int's non dynamic
and I got following time:

[email protected]:~/examples$ g++ -Wall -O2 -pthread -std=c++0x consprod1.cpp -oconsprod1
consprod1.cpp: In lambda function:
consprod1.cpp:58:19: warning: unused variable ‘size’ [-Wunused-variable]
[email protected]:~/examples$ time ./consprod1

real 0m23.335s
user 0m22.177s
sys 0m16.417s

with java, I have removed rand too...
[email protected]:~/examples$ javac consprod.java
[email protected]:~/examples$ time java consprod

real 0m12.491s
user 0m21.001s
sys 0m0.524s

Still twice as fast as c++.
What it looks like, is that c++ spends much more time in syscalls
(futex) than java and that explains big difference.
 
L

Luca Risolia

Still twice as fast as c++.
What it looks like, is that c++ spends much more time in syscalls
(futex) than java and that explains big difference.

A lock-free "BlockingQueue" would probably be more efficient.
You can also try to implement your Queue with std::array instead of
std::deque and see how it performs.

In the C++ version change:

while (queue_.size() >= capacity_)c_full_.wait(lock);
with
c_full_.wait(lock, [this]{return queue_.size() < capacity_;});

and
while (queue_.empty())c_empty_.wait(lock);
with
c_empty_.wait(lock, [this]{return !queue_.empty();});

Also use a std::lock_guard in empty() instead of std::unique_lock. The
former is faster.

(On a side note, the C++ version you have given does not have the best
possible interface for a thread-safe queue)
 
J

Juha Nieminen

In comp.lang.c++ Melzzzzz said:
I even tried c++ without dynamic
memory management (except queue itself) and that is *even slower*.

If two C++ programs are otherwise identical, except that one uses
'new int' and the other just uses the ints by value, and the former
is faster than the latter, then there's something horribly wrong in
the way you are measuring, or something else. I don't think it's
physically possible for 'new int' to be faster than using ints by
value under any possible circumstance, even if we assumed a highly
optimized version of 'new' that does magic under the hood to be 10
times faster than the regular 'new'.
std::unique_lock<std::mutex> lock(m_);
produced.put(std::unique_ptr<int>(new int(i)));
arr[std::rand()%size] = produced.take();

I don't know what's the cause of the slowness in your program, but
I quoted above the three possible culprits I would first investigate
(which happen to be in order of likeliness).

Locks are slow. They are probably slower than 'new'.

'new' is slow, especially when compared to java's.

std::rand() is slow, although probably does not account for that much
slowness, but could be a minor contributing factor.
 
M

Melzzzzz

A lock-free "BlockingQueue" would probably be more efficient.
You can also try to implement your Queue with std::array instead of
std::deque and see how it performs.

I will try. Seems that java ArrayBlockingQueue is more efficient
because if that.
In the C++ version change:

while (queue_.size() >= capacity_)c_full_.wait(lock);
with
c_full_.wait(lock, [this]{return queue_.size() < capacity_;});

and
while (queue_.empty())c_empty_.wait(lock);
with
c_empty_.wait(lock, [this]{return !queue_.empty();});

Also use a std::lock_guard in empty() instead of std::unique_lock.
The former is faster.

Yes, I got speed up of few seconds, thanks.
(On a side note, the C++ version you have given does not have the
best possible interface for a thread-safe queue)

I have just copied Java BlockingQueue interface, which is of course
suited for Java. Don't know actually how ideal interface would look
like.
 
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M

Melzzzzz

If two C++ programs are otherwise identical, except that one uses
'new int' and the other just uses the ints by value, and the former
is faster than the latter, then there's something horribly wrong in
the way you are measuring, or something else.

I think that pressure on condition variable is greater with non
dynamic allocation, therefore it is slower.
If I reduce queue capacity on eg 1000 (more pressure on condition
variable) than it is much slower than with capacity of 100000.


I don't think it's
physically possible for 'new int' to be faster than using ints by
value under any possible circumstance, even if we assumed a highly
optimized version of 'new' that does magic under the hood to be 10
times faster than the regular 'new'.

That's because new is very fast and don;t take much time of program
in this example.
std::unique_lock<std::mutex> lock(m_);
produced.put(std::unique_ptr<int>(new int(i)));
arr[std::rand()%size] = produced.take();

I don't know what's the cause of the slowness in your program, but
I quoted above the three possible culprits I would first investigate
(which happen to be in order of likeliness).

Locks are slow. They are probably slower than 'new'.

I think that locks are not that slow in comparison to
condition variables. I have feeling that somehow
Java has less pressure on condition variables
in it's queue implementation.
'new' is slow, especially when compared to java's.

In this example new takes very little time.
std::rand() is slow, although probably does not account for that much
slowness, but could be a minor contributing factor.

This also takes little time.
Here is difference between new and rand and without.
Both versions are optimized now as per Luca's suggestion.


(with new and rand array assignment)
[email protected]:~/examples$ time ./consprod

real 0m25.127s
user 0m32.202s
sys 0m5.540s
(without)
[email protected]:~/examples$ time ./consprod1

real 0m23.883s
user 0m23.009s
sys 0m17.153s

Java:
(without rand and array assignment)
[email protected]:~/examples$ time java consprod

real 0m12.825s
user 0m21.833s
sys 0m0.592s

(with rand and array assignment)
[email protected]:~/examples$ time java consprod

real 0m15.571s
user 0m25.634s
sys 0m1.168s

Java pays higher price for random assignments, I think.
 
J

Juha Nieminen

In comp.lang.c++ Melzzzzz said:
I think that pressure on condition variable is greater with non
dynamic allocation, therefore it is slower.

That sentence doesn't make any kind of sense.
I don't think it's

That's because new is very fast and don;t take much time of program
in this example.

'new' is a very slow operation in C++, and even if it weren't, it just
can't be faster than using an integer by value. It's physically impossible.
(The pointer that points to the allocated int is also an integral value at
the low level, so by allocating something dynamically you are only adding
to the amount of values being handled, and the overall complexity of the
program.)

Can you give any scenario where handling pointers would be faster than
handling ints?
In this example new takes very little time.

You are executing tens of thousands of 'new's. That is slow.
 
M

Melzzzzz

That sentence doesn't make any kind of sense.
I has sense in this context. I will provide example.
'new' is a very slow operation in C++, and even if it weren't, it just
can't be faster than using an integer by value. It's physically
impossible. (The pointer that points to the allocated int is also an
integral value at the low level, so by allocating something
dynamically you are only adding to the amount of values being
handled, and the overall complexity of the program.)

Yes, but overall complexity can be masked with something else like in
this case.
Can you give any scenario where handling pointers would be faster than
handling ints?

This scenario is exactly example. My queue is limited to capacity
nodes. When capacity is reached, producer waits on condition
variable until it is emptied.
In first example queue was 100000 nodes so slow down with plain ints
isn't just that noticabe.
But if I put just 1000 nodes here is what happens:
(dynamic ints , unique_ptr, random function and array assignement)
[email protected]:~/examples$ time ./consprod

real 0m23.523s
user 0m32.390s
sys 0m10.581s

but look at this:
(no dynamic ints, no random function, no assignment)
:
[email protected]:~/examples$ time ./consprod1

real 0m35.654s
user 0m30.882s
sys 0m34.202s

it's more than 10 seconds slower! just ints!

You are executing tens of thousands of 'new's. That is slow.

It is slow but waiting on condition is much slower so,
faster producer increases number of times condition waits,
so actually that makes program slower...
 
Z

Zoltan Juhasz

I have played little bit with new C++11 features and compared,
java performance to c++.

My take on this:

I guess the Java BlockingQueue is a lock-free implementation
of the blocking circular-buffer concept. Your implementation
uses a global, single lock around operations, the _m mutex,
which essentially serializes your code (plus the synchronization
overhead). Your code might be executed concurrently, but not in
parallel for sure.

What is wrong with your code? That you compare a naive,
inefficient blocking queue implementation, with an industrial
strength solution.

- look up a lock-free, single producer / single consumer
circular-buffer implementation
- use rvalue semantics to move items around, no need for
ptrs; at very least specialize the container for built-in types
- eliminate noise (i.e. rand)

The most important part: profile your code to find out
where you spend your time, whether you have some kind of
contention etc. Performance optimization is a tricky business,
especially in a multi-threaded environment, naive
implementations usually produce catastrophic results (see above).

-- Zoltan
 
M

Melzzzzz

My take on this:

I guess the Java BlockingQueue is a lock-free implementation
of the blocking circular-buffer concept. Your implementation
uses a global, single lock around operations, the _m mutex,
which essentially serializes your code (plus the synchronization
overhead). Your code might be executed concurrently, but not in
parallel for sure.
This is implementation of put/take methods of Java queue:

public void put(E e) throws InterruptedException {
244: if (e == null) throw new NullPointerException();
245: final E[] items = this.items;
246: final ReentrantLock lock = this.lock;
247: lock.lockInterruptibly();
248: try {
249: try {
250: while (count == items.length)
251: notFull.await();
252: } catch (InterruptedException ie) {
253: notFull.signal(); // propagate to non-interrupted
thread 254: throw ie;
255: }
256: insert(e);
257: } finally {
258: lock.unlock();
259: }
260: }

310: public E take() throws InterruptedException {
311: final ReentrantLock lock = this.lock;
312: lock.lockInterruptibly();
313: try {
314: try {
315: while (count == 0)
316: notEmpty.await();
317: } catch (InterruptedException ie) {
318: notEmpty.signal(); // propagate to non-interrupted thread
319: throw ie;
320: }
321: E x = extract();
322: return x;
323: } finally {
324: lock.unlock();
325: }
326: }
327:
What is wrong with your code? That you compare a naive,
inefficient blocking queue implementation, with an industrial
strength solution.

Heh, I don't think so. This implementation is pretty classic.
It does well for blocking queue, but this test is too
much for it. It gets contented with mutex/condition_variable
calls, so does Java, but for some reason, much less.

- look up a lock-free, single producer / single consumer
circular-buffer implementation

Java ArrayBlockingQueue use circular buffer but is
not lock free, rather implementation is identical,
as I see (one mutex, two condition variables)
- use rvalue semantics to move items around, no need for
ptrs; at very least specialize the container for built-in types
- eliminate noise (i.e. rand)

The most important part: profile your code to find out
where you spend your time, whether you have some kind of
contention etc. Performance optimization is a tricky business,
especially in a multi-threaded environment, naive
implementations usually produce catastrophic results (see above).

Agreed. I think that I am close. Somehow (probably because it
is faster? c++ puts much more pressure on queue than Java does).
Perhaps someone can confirm or deny this.
 
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H

Howard Hinnant

I made a few changes to your code:

1. I only signal if the capacity becomes non-zero or non-full.

2. If the queue is the queue has got some items in it and the mutex is locked, the put thread yields to the take thread. If the queue has some free space in it and the mutex is locked, the take thread yields to the put thread.

3. Traffic in int instead of unique_ptr<int>.


#include <condition_variable>
#include <mutex>
#include <thread>
#include <deque>
#include <cstdlib>

template <class T>
class BlockingQueue{
public:
BlockingQueue(unsigned cap):capacity_(cap)
{
}
void put(T t)
{
std::unique_lock<std::mutex> lock(m_, std::try_to_lock);
int retry = 0;
while (!lock.owns_lock())
{
if (queue_.size() > capacity_/4 && ++retry < 1000)
{
std::this_thread::yield();
}
else
{
lock.lock();
}
}
while(queue_.size() >= capacity_)c_full_.wait(lock);
queue_.push_back(std::move(t));
if (queue_.size() == 1)
c_empty_.notify_one();
}
T take()
{
std::unique_lock<std::mutex> lock(m_, std::try_to_lock);
int retry = 0;
while (!lock.owns_lock())
{
if (queue_.size() < 3*capacity_/4 && ++retry < 1000)
{
std::this_thread::yield();
}
else
{
lock.lock();
}
}
while(queue_.empty())c_empty_.wait(lock);
T tmp = std::move(queue_.front());
queue_.pop_front();
if (queue_.size() == capacity_-1)
c_full_.notify_one();
return tmp;
}
bool empty()
{
std::unique_lock<std::mutex> lock(m_);
return queue_.empty();
}
private:
std::mutex m_;
std::condition_variable c_empty_,c_full_;
std::deque<T> queue_;
unsigned capacity_;
};

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();
}

On my system this sped the C++ solution up considerably (to about 13.7 seconds). I didn't time the Java solution.
 
D

Dombo

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)
[email protected]:~/examples$ time java consprod

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

(c++ gcc 4.6.3)
[email protected]:~/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) {
try{
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)
{
e.printStackTrace();
}
}
}
//-----------------------------------------
// c++
#include <condition_variable>
#include <mutex>
#include <thread>
#include <deque>
#include <cstdlib>

template <class T>
class BlockingQueue{
public:
BlockingQueue(unsigned cap):capacity_(cap)
{
}
void put(T t)
{
std::unique_lock<std::mutex> lock(m_);
while(queue_.size() >= capacity_)c_full_.wait(lock);
queue_.push_back(std::move(t));
c_empty_.notify_one();
}
T take()
{
std::unique_lock<std::mutex> lock(m_);
while(queue_.empty())c_empty_.wait(lock);
T tmp = std::move(queue_.front());
queue_.pop_front();
c_full_.notify_one();
return std::move(tmp);
}
bool empty()
{
std::unique_lock<std::mutex> lock(m_);
return queue_.empty();
}
private:
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::srand(12345);


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);

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

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
difference.

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.
 
M

Melzzzzz

I made a few changes to your code:


On my system this sped the C++ solution up considerably (to about
13.7 seconds). I didn't time the Java solution.

Yes, that's it, on my system:
[email protected]:~/examples$ time ./consprod2

real 0m9.478s
user 0m10.797s
sys 0m7.196s

which is more than twice speed of original program!
(and faster than java)
 
L

Luca Risolia

I have just copied Java BlockingQueue interface, which is of course
suited for Java. Don't know actually how ideal interface would look
like.

In C++11 the ideal interface for a generic thread-safe and
exception-safe queue should at least provide std::shared_ptr<T> take()
and/or void take(T&). T take() is limited to those types having no-throw
copy constructors or move-constructors. This also means that you should
check the type at compile-time by using type traits. However, in your
test case you used move-semantic everywhere, so that is not problem.
Also note that the term "pop" instead of "take" is more in the spirit of
C++.
 
H

Howard Hinnant

&gt; I made a few changes to your code:
&gt;
&gt;
&gt; On my system this sped the C++ solution up considerably (to about
&gt; 13.7 seconds). I didn't time the Java solution.

Yes, that's it, on my system:
[email protected]:~/examples$ time ./consprod2

real 0m9.478s
user 0m10.797s
sys 0m7.196s

which is more than twice speed of original program!
(and faster than java)

At second glance I wasn't that fond of my solution and tweaked it as below:


void put(T t)
{
std::unique_lock<std::mutex> lock(m_, std::try_to_lock);
while (!lock.owns_lock())
{
if (queue_.size() > capacity_/4)
{
for (int i = 0; i < 3250; ++i)
std::this_thread::yield();
lock.try_lock();
}
else
{
lock.lock();
break;
}
}
while(queue_.size() >= capacity_)c_full_.wait(lock);
queue_.push_back(std::move(t));
if (queue_.size() == 1)
c_empty_.notify_one();
}
T take()
{
std::unique_lock<std::mutex> lock(m_, std::try_to_lock);
int retry = 0;
while (!lock.owns_lock())
{
if (queue_.size() < 3*capacity_/4)
{
for (int i = 0; i < 3250; ++i)
std::this_thread::yield();
lock.try_lock();
}
else
{
lock.lock();
break;
}
}
while(queue_.empty())c_empty_.wait(lock);
T tmp = std::move(queue_.front());
queue_.pop_front();
if (queue_.size() == capacity_-1)
c_full_.notify_one();
return tmp;
}

I am admittedly coding to the benchmark (which is not a really great idea). But I got the time on my system down from 13.7 seconds to about 9.7 seconds (another 40%).
 
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J

Joshua Maurice

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..

It's an exception in C and C++ for main only. Running off the end of
main is allowed, and is equivalent to return 0, itself equivalent to
exit(0), IIRC.
 
M

Melzzzzz

At second glance I wasn't that fond of my solution and tweaked it as
below:


void put(T t)
{
std::unique_lock<std::mutex> lock(m_, std::try_to_lock);
while (!lock.owns_lock())
{
if (queue_.size() > capacity_/4)
{
for (int i = 0; i < 3250; ++i)
std::this_thread::yield();
lock.try_lock();
}
else
{
lock.lock();
break;
}
}
while(queue_.size() >= capacity_)c_full_.wait(lock);
queue_.push_back(std::move(t));
if (queue_.size() == 1)
c_empty_.notify_one();
}
T take()
{
std::unique_lock<std::mutex> lock(m_, std::try_to_lock);
int retry = 0;
while (!lock.owns_lock())
{
if (queue_.size() < 3*capacity_/4)
{
for (int i = 0; i < 3250; ++i)
std::this_thread::yield();
lock.try_lock();
}
else
{
lock.lock();
break;
}
}
while(queue_.empty())c_empty_.wait(lock);
T tmp = std::move(queue_.front());
queue_.pop_front();
if (queue_.size() == capacity_-1)
c_full_.notify_one();
return tmp;
}

I am admittedly coding to the benchmark (which is not a really great
idea). But I got the time on my system down from 13.7 seconds to
about 9.7 seconds (another 40%).

On my system speed up is not that big.
(new version)
[email protected]:~/examples$ time ./consprod3

real 0m9.296s
user 0m10.061s
sys 0m8.069s

(old version)
[email protected]:~/examples$ time ./consprod2

real 0m9.496s
user 0m10.917s
sys 0m7.292s
[email protected]:~/examples$

(java with array assignment)
[email protected]:~/examples$ time java consprod

real 0m16.098s
user 0m25.674s
sys 0m1.932s
 
N

none

I made a few changes to your code:

1. I only signal if the capacity becomes non-zero or non-full.
2. If the queue is the queue has got some items in it and the mutex is locked, the put thread yields to the take
thread. If the queue has some free space in it and the mutex is locked, the take thread yields to the put
thread.

Are you sure about the usefulness of #2?

Doing a few tests myself, adding this in m queue seems to reduce performance by some 20%:
while (!lock.owns_lock())
{
if (queue_.size() > capacity_/4)
{
for (int i = 0; i < 3250; ++i)
std::this_thread::yield();
lock.try_lock();
}
else
{
lock.lock();
break;
}
}

3. Traffic in int instead of unique_ptr<int>.


Yannick
 
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M

Melzzzzz

Are you sure about the usefulness of #2?

Problem is that we need to reduce waiting on condition to minimum
as more condition waits program is slower.
Ideally there would be no condition waits (maximum speed).
Doing a few tests myself, adding this in m queue seems to reduce
performance by some 20%: while (!lock.owns_lock())
{
if (queue_.size() > capacity_/4)
{
for (int i = 0; i < 3250; ++i)
std::this_thread::yield();
lock.try_lock();
}
else
{
lock.lock();
break;
}
}

This is second version, on my system first version seems better
for smaller capacities...
 

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