Tasks on Thread Building Blocks

Question

Here is the example code:

#include <iostream>
#include <list>
#include <tbb/task.h>
#include <tbb/task_group.h>
#include <stdlib.h>
#include <boost/thread.hpp>

using namespace tbb;

 long fib(long a)
{
  if (a < 2) return 1;

  return fib(a - 1) + fib(a - 2);
}

class PrintTask 
{
public:
    void operator()()
    {
        std::cout << "hi world!: " <<  boost::this_thread::get_id() << std::endl;

        fib(50);
    }
};

int main(int argc, char** argv)
{     
    task_group group;

    for (int i = 0; i < 100; ++i)
    {
      group.run(PrintTask());
    }      

    group.wait();

    return(0);
}

Here I'm computing a big fibonacci sequence just to simulate non-blocking computation. I was specting that this code would generate more than two threads (my computer is a Core2Duo), but only the first and second tasks are called. This is the spected?

Answer 1

Massively multithreading blocking behaviour (std::cout use) is an anti-pattern in multithreading and may result in bad behaviour, because it's the wrong thing to do. In addition, TBB reserves the right to implement group.run() however the hell it likes and spawn whatever threads it likes. If you only have a dual core, and you call with heavy work, why should it spawn more than two threads? The OS and other apps will happily eat remaining background time.