OpenCV parallel_for not using multiple processors

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/

Answer 2

The problem is most likely that your loop body is too small.

It appears all you are doing is assigning a pointer in one vector to another.

You really need to think of a parallel for as an inefficient for loop, that is the work inside each iteration needs to be large enough so that you wouldn't dream of getting speedups by unrolling the loop because in addition to the usual decrement, compare and jump that can go on you also have a few interlocked instructions and perhaps a virtual function call or two and some allocations.

So instead of copying a pointer try doing a good amount of real math or work on a large array of data.