Multithreaded not efficient: Debugging False Sharing?

Question

I have the following code, that starts multiple Threads (a threadpool) at the very beginning (startWorkers()). Subsequently, at some point i have a container full of myWorkObject instances, which I want to process using multiple worker threads simulatenously. The myWorkObject are completely isolated from another in terms of memory usage. For now lets assume myWorkObject has a method doWorkIntenseStuffHere() which takes some cpu time to calculate.

When benchmarking the following code, i have noticed that this code does not scale well with the number of threads, and the overhead for initializing/synchronizing the worker threads exceeds the benefit of multithreading unless there are 3-4 threads active. I've looked into this issue and read about the false-sharing problem and i assume my code suffers from this problem. However, I'd like to debug/profile my code to see whether there is some kind of starvation/false sharing going on. How can I do this? Please feel free to critize anything about my code as I'm still learning a lot about memory/cpu and multithreading in particular.

#include <boost/thread.hpp>

class MultiThreadedFitnessProcessingStrategy
{
public:
    MultiThreadedFitnessProcessingStrategy(unsigned int numWorkerThreads):
        _startBarrier(numWorkerThreads + 1),
        _endBarrier(numWorkerThreads + 1),
        _started(false),
        _shutdown(false),
        _numWorkerThreads(numWorkerThreads)
    {
        assert(_numWorkerThreads > 0);
    }


    virtual ~MultiThreadedFitnessProcessingStrategy()
    {
        stopWorkers();
    }


void startWorkers()
{
    _shutdown = false;
    _started = true;

    for(unsigned int i = 0; i < _numWorkerThreads;i++)
    {
        boost::thread*  workerThread = new boost::thread(
                boost::bind(&MultiThreadedFitnessProcessingStrategy::workerTask, this,i)
        );
        _threadQueue.push_back(new std::queue<myWorkObject::ptr>());
        _workerThreads.push_back(workerThread);
    }
}


void stopWorkers()
{
    _startBarrier.wait();
    _shutdown = true;
    _endBarrier.wait();

    for(unsigned int i = 0; i < _numWorkerThreads;i++)
    {
        _workerThreads[i]->join();
    }

}

void workerTask(unsigned int id)
{

    //Wait until all worker threads have started.
    while(true)
    {
        //Wait for any input to become available.
        _startBarrier.wait();

        bool queueEmpty = false;
        std::queue<SomeClass::ptr >* myThreadq(_threadQueue[id]);

        while(!queueEmpty)
        {

            SomeClass::ptr myWorkObject;

            //Make sure queue is not empty,
            //Caution: this is necessary if start barrier was triggered without queue input (e.g., shutdown) , which can happen.
            //Do not try to be smart and refactor this without knowing what you are doing!
            queueEmpty = myThreadq->empty();


            if(!queueEmpty)
            {
                chromosome = myThreadq->front();
                assert(myWorkObject);
                myThreadq->pop();
            }

            if(myWorkObject)
            {
                myWorkObject->doWorkIntenseStuffHere();
            }
        }

        //Wait until all worker threads have synchronized.
        _endBarrier.wait();

        if(_shutdown)
        {
            return;
        }
    }
}


void doWork(const myWorkObject::chromosome_container &refcontainer)
{

    if(!_started)
    {
        startWorkers();
    }

    unsigned int j = 0;
    for(myWorkObject::chromosome_container::const_iterator it = refcontainer.begin();
            it != refcontainer.end();++it)
    {
        if(!(*it)->hasFitness())
        {
            assert(*it);
            _threadQueue[j%_numWorkerThreads]->push(*it);
            j++;
        }
    }

    //Start Signal!
    _startBarrier.wait();

    //Wait for workers to be complete
    _endBarrier.wait();

}


    unsigned int getNumWorkerThreads() const
    {
        return _numWorkerThreads;
    }

    bool isStarted() const
    {
        return _started;
    }


private:

    boost::barrier _startBarrier;
    boost::barrier _endBarrier;

    bool _started;
    bool _shutdown;

    unsigned int _numWorkerThreads;

    std::vector<boost::thread*> _workerThreads;

    std::vector< std::queue<myWorkObject::ptr >* > _threadQueue;


};

Answer 1

If you're on Linux, there is a tool called valgrind, with one of the modules doing cache effects simulation (cachegrind). Please take a look at

http://valgrind.org/docs/manual/cg-manual.html

Answer 2

Sampling-based profiling can give you a pretty good idea whether you're experiencing false sharing. Here's a previous thread that describes a few ways to approach the issue. I don't think that thread mentioned Linux's perf utility. It's a quick, easy and free way to count cache misses that might tell you what you need to know (am I experiencing a significant number of cache misses that correlates with how many times I'm accessing a particular variable?).

If you do find that your threading scheme might be causing a lot of conflict misses, you could try declaring your myWorkObject instances or the data contained within them that you're actually concerned about with __attribute__((aligned(64))) (alignment to 64 byte cache lines).