Question

I just saw in the new OpenCV 2.4.3 that they added a universal parallel_for. So following this example, I tried to implement it myself. I got it all functioning with my code, but when I timed its processing vs a similar loop done in a typical serial fashion with a regular “for” command, the results were insignificantly faster, or often a tiny bit slower!

I thought maybe this had something to do with my pushing into vectors or something (I’m a pretty big noob to parallel processing), so I set up a test loop of just running through a big number and it still doesn’t work.

Code:

class Parallel_Test : public cv::ParallelLoopBody
{
private:
double* const mypointer;



public:
Parallel_Test(double* pointer)
: mypointer(pointer){

}
     void operator() (const Range& range) const
{
         //This constructor needs to be here otherwise it is considered an abstract class.
//             qDebug()<<"This should never be called";
}

    void operator ()(const cv::BlockedRange& range) const
    {

        for (int x = range.begin(); x < range.end(); ++x){

            mypointer[x]=x;

        }


    }



};


 //TODO Loop pixels in parallel
     double t = (double)getTickCount();

    //TEST PARALELL LOOPING AT ALL
    double data1[1000000];



        cv::parallel_for(BlockedRange(0, 1000000),  Parallel_Test(data1));

        t = ((double)getTickCount() - t)/getTickFrequency();
        qDebug() << "Parallel TEST time " << t << endl;


        t = (double)getTickCount();

        for(int i =0; i<1000000; i++){

            data1[i]=i;
        }
        t = ((double)getTickCount() - t)/getTickFrequency();
        qDebug() << "SERIAL Scan time " << t << endl;

output:

Parallel TEST time  0.00415479 

SERIAL Scan time  0.00204597 

Answer 1

Wow! I found the answer! “parallel_for” and “parallel_for_” (with a trailing underscore!) are totally different. You need the trailing underscore to make it work! Otherwise it will just run your loop in serial and you will have to use a BLOCKEDRANGE instead of a range! AHH!

Thanks to @Daniil Osokin and especially @Vladislav Vinogradov for pointing this out!

So again you code will need to look something like this:
cv::parallel_for_(Range(0, 1000000), Parallel_Test(data1));

More updated details at: http://answers.opencv.org/question/3730/how-to-use-parallel_for/