Need help with multithreading

qounce

Hi. I have a function which I want to call with few instances in several threads. I suppose to use something like that:

void MainWindow::btn_clicked () {
    for (int i = 0; i < 10; ++i) {
        // different parts computing in different threads
        Function(a,b); // thread 1
        Function(b,c); // thread 2

        // after tasks in all threads are done
        UpdateMap(); // update QGrapchicScene
        sleep(delay); // pause
    }
}

But during cycle is proceeding GUI should not be frozen.
So, what is the best practice to do that? QThreads or QConcurrent or something else?

SGaist

Hi,

QtConcurrent sounds like a good choice. However, the UpdateMap function should be connected to a QFutureWatcher to only run once all operations are done.

Why the call to sleep ?

qounce

I need pause just to see updated map.
Well, I'm trying to find out how to use QtConcurrent right now, but there is no significant success. I try:

future1 = new QFuture<void>;
watcher1 = new QFutureWatcher<void>;
future2 = new QFuture<void>;
watcher2 = new QFutureWatcher<void>;
connect(watcher1, SIGNAL(finished()),
               this, SLOT(S()));
connect(watcher2, SIGNAL(finished()),
               this, SLOT(S()));

size_t s = fieldSizeI / 2;
for (size_t i = 0; i < 10; ++i) {
    *future1 = QtConcurrent::run(field, &Field::PerformIteration, 0, s);
    *future2 = QtConcurrent::run( field, &Field::PerformIteration, s, 2*s);

    watcher1->setFuture(*future1);
    watcher2->setFuture(*future2);
}

void MainWindow::S() {
    ++workCounter;
    if (workCounter == 2) {
        field->AdoptNewState();
        UpdateMap();
        workCounter = 0;
    }
}

It seems I'am doing it absolutely wrong. And I can't find an appropriate example.

SGaist

QFutureWatcher *watcher1 = new QFutureWatcher<void>;
QFutureWatcher *watcher2 = new QFutureWatcher<void>;
connect(watcher1, SIGNAL(finished()),
               this, SLOT(S()));
connect(watcher2, SIGNAL(finished()),
               this, SLOT(S()));
 
size_t s = fieldSizeI / 2;

    QFuture<void> future1 = QtConcurrent::run(field, &Field::PerformIteration, 0, s);
QFuture<void> future2 = QtConcurrent::run( field, &Field::PerformIteration, s, 2*s);
 
    watcher1->setFuture(future1);
    watcher2->setFuture(future2);
}

qounce

Thanks. I used your code to construct the thing below. It looks not so hot but work just how I need.

// call this function to start
void MainWindow::on_btn_iter_clicked()
{
    maxIter = 5;
    s = fieldSizeI / threadsAmount;
    NewIteration();
}

void MainWindow::NewIteration() {
    std::vector<QFutureWatcher<void> *> watchers (threadsAmount);
    for (size_t i = 0; i < threadsAmount; ++i) {
        watchers[i] = new QFutureWatcher<void>;
        connect (watchers[i], SIGNAL (finished()), this, SLOT (S()));
    }
    typedef std::vector<QFuture<void> > futures (threadsAmount);
    for (size_t i = 0; i < threadsAmount; ++i) {
        futures[i] = QtConcurrent::run(field, &CA_Field::PerformIteration,
                                       i*s, (i+1)*s );
        watchers[i]->setFuture(futures[i]);
    }
}

void MainWindow::S() {
    ++workCounter;
    if (workCounter == threadsAmount) {
        field->AdoptNewState();
        workCounter = 0;
        (--maxIter != 0) ? NewIteration() : UpdateHexMap();
    }
}