Greetings.

I'm very new to QtConcurrent, and I have very little experience in threading in general. I went over some examples today in the Qt docs, and I can't figure out why my multi-threaded code runs slower than the single-threaded version.

This code is basically a modified version of what I found in the docs.

Qt Code: 
  1. #include <QThread>
  2. #include <QApplication>
  3. #include <qtconcurrentmap.h>
  4. #include <iostream>
  5. #include <QtConcurrentRun>
  6. using namespace std;
  7.  
  8. int Func(const int &n){
  9.  
  10.   //Some random calculations...
  11.   int a = rand() % 3, b = rand() % 4, c = rand() % 5;
  12.   int x = a * a, y = b * b, z = c * c;
  13.   int u = a + x, v = b + y, w = c + z;
  14.   int d = u - v * w;
  15.   if( d > 0 ){
  16.     d = a / x / u;
  17.   }else{
  18.     d = b / y / v;
  19.   }
  20.     for(int i = 1; i < 10; ++i){
  21.       for(int j = 1; j < 30; ++j){
  22. 	d = rand() % 5 + 3;
  23. 	d /= 2;
  24. 	d *= rand() % 4;
  25.       }
  26.     }
  27.   return d * n;
  28. }
  29.  
  30. int main(int argc, char *argv[]){
  31.  
  32.   QApplication app(argc, argv);
  33.   const int SIZE = 500000;
  34.   QVector<int> data;
  35.  
  36.   for(int i = 0; i < SIZE; ++i){
  37.     data.push_back(i);
  38.   }
  39.  
  40.   //non-concurrent version
  41.   //for(int i = 0; i < SIZE; ++i){ data[i] = Func( data[i] ); }
  42.  
  43.   //concurrent version
  44.   QtConcurrent::mapped(data, Func);
  45.  
  46.   return 0;
  47. }
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I'm running it on a dual core, and the single-threaded version runs in about 0.7s, but the using QtConcurrent it takes over 30.0s. The two cores are utilized, but not at 100%.

Also, using QtConcurrent::map() I get seg-fault, and I'm not sure why.

I'm probably doing something wrong, but can anyone tell me why QtConcurrent::mapped() call slows it down so much??