Thread start overhead

[^NyAw^]

Hi,

How do you measure thread initialization time?

Using QElapsedTimer

Make a list with pointers to threads. After finishing worker sends a signal. In slot remove the worker from the list. If list empty done.

Think on that the worker thread is a thread with a loop that starts all the other threads, so there is no event loop in this worker thread.

This is my approach

MyThread::run()
{
  for (int i=0; i<qNumThreads; i++)
    m_qThreadList[i]->start();
 
  for (int i=0; i<qNumThreads; i++)
    m_qThreadList[i]->wait();
 
  //Get results
}

MyThread::run()
{
  for (int i=0; i<qNumThreads; i++)
    m_qThreadList[i]->start();

  for (int i=0; i<qNumThreads; i++)
    m_qThreadList[i]->wait();

  //Get results
}

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Thanks,

[d_stranz]

m_qThreadList[i]->wait();

Hmmm, isn't this a blocking function? If wait() doesn't return until the thread has finished, then your loop will not complete until all threads have finished, and MyThread::run() won't return either.

[^NyAw^]

Hi,

Let me be a little more presice:

MyThread::run()
    {
      while(!bExit)
      {
          get_image_from_camera
          for (int i=0; i<qNumThreads; i++)
              m_qThreadList[i]->start();
 
          for (int i=0; i<qNumThreads; i++)
              m_qThreadList[i]->wait();
 
          //Get results and emit some signal to let the GUI show results
      }
    }

MyThread::run()
    {
      while(!bExit)
      {
          get_image_from_camera
          for (int i=0; i<qNumThreads; i++)
              m_qThreadList[i]->start();
     
          for (int i=0; i<qNumThreads; i++)
              m_qThreadList[i]->wait();
     
          //Get results and emit some signal to let the GUI show results
      }
    }

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"MyThread" is running in a loop until the user stops it.
The other Threads(ThreadList) are processing Threads that compute some image processing algorithms parallel, so I need to start them and wait for them to finish to get results.

The overhead seems to be on the thread start. The OS have to create a thread, stack, copy system variables, ...

So, my question was if I could start the threads but put them to sleep with a QWaitCondition. When the "MyThread" have captured a frame from the camera, set a boolean let the threads process it. Finally what I don't know how to wait until the threads have finished.

Something like this wastes CPU on MyThread

for (int i=0; i<qNumThreads; i++)
{
    while (m_qThreadList[i]->isProcessing())
      ;
}

for (int i=0; i<qNumThreads; i++)
{
    while (m_qThreadList[i]->isProcessing())
      ;
}

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Maybe using another QWaitCondition that the threads could call "wakeOne()" ?

for (int i=0; i<qNumThreads; i++)
{
    m_qThreadList[i]->m_qWaitCondition->wait();
    //Here we know that thread "i" have finished its process
}

for (int i=0; i<qNumThreads; i++)
{
    m_qThreadList[i]->m_qWaitCondition->wait();
    //Here we know that thread "i" have finished its process
}

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Could this work?

Thanks,