QThread execution freezes my GUI

Question

I'm new to multithread programming. I wrote this simple multi thread program with Qt. But when I run this program it freezes my GUI and when I click inside my widow, it responds that your program is not responding . Here is my widget class. My thread starts to count an integer number and emits it when this number is dividable by 1000. In my widget simply I catch this number with signal-slot mechanism and show it in a label and a progress bar.

   Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    MyThread *th = new MyThread;
    connect( th, SIGNAL(num(int)), this, SLOT(setNum(int)));
    th->start();
}


void Widget::setNum(int n)
{
    ui->label->setNum( n);
    ui->progressBar->setValue(n%101);
}

and here is my thread run() function :

void MyThread::run()
{
    for( int i = 0; i < 10000000; i++){
        if( i % 1000 == 0)
            emit num(i);
    }
}

thanks!

Answer 1

The problem is with your thread code producing an event storm. The loop counts very fast -- so fast, that the fact that you emit a signal every 1000 iterations is pretty much immaterial. On modern CPUs, doing a 1000 integer divisions takes on the order of 10 microseconds IIRC. If the loop was the only limiting factor, you'd be emitting signals at a peak rate of about 100,000 per second. This is not the case because the performance is limited by other factors, which we shall discuss below.

Let's understand what happens when you emit signals in a different thread from where the receiver QObject lives. The signals are packaged in a QMetaCallEvent and posted to the event queue of the receiving thread. An event loop running in the receiving thread -- here, the GUI thread -- acts on those events using an instance of QAbstractEventDispatcher. Each QMetaCallEvent results in a call to the connected slot.

The access to the event queue of the receiving GUI thread is serialized by a QMutex. On Qt 4.8 and newer, the QMutex implementation got a nice speedup, so the fact that each signal emission results in locking of the queue mutex is not likely to be a problem. Alas, the events need to be allocated on the heap in the worker thread, and then deallocated in the GUI thread. Many heap allocators perform quite poorly when this happens in quick succession if the threads happen to execute on different cores.

The biggest problem comes in the GUI thread. There seems to be a bunch of hidden O(n^2) complexity algorithms! The event loop has to process 10,000 events. Those events will be most likely delivered very quickly and end up in a contiguous block in the event queue. The event loop will have to deal with all of them before it can process further events. A lot of expensive operations happen when you invoke your slot. Not only is the QMetaCallEvent deallocated from the heap, but the label schedules an update() (repaint), and this internally posts a compressible event to the event queue. Compressible event posting has to, in worst case, iterate over entire event queue. That's one potential O(n^2) complexity action. Another such action, probably more important in practice, is the progressbar's setValue internally calling QApplication::processEvents(). This can, recursively call your slot to deliver the subsequent signal from the event queue. You're doing way more work than you think you are, and this locks up the GUI thread.

Instrument your slot and see if it's called recursively. A quick-and-dirty way of doing it is

void Widget::setNum(int n)
{
  static int level = 0, maxLevel = 0;
  level ++;
  maxLevel = qMax(level, maxLevel);
  ui->label->setNum( n);
  ui->progressBar->setValue(n%101);
  if (level > 1 && level == maxLevel-1) {
    qDebug("setNum recursed up to level %d", maxLevel);
  }
  level --;
}

What is freezing your GUI thread is not QThread's execution, but the huge amount of work you make the GUI thread do. Even if your code looks innocuous.

Side Note on processEvents and Run-to-Completion Code

I think it was a very bad idea to have QProgressBar::setValue invoke processEvents(). It only encourages the broken way people code things (continuously running code instead of short run-to-completion code). Since the processEvents() call can recurse into the caller, setValue becomes a persona-non-grata, and possibly quite dangerous.

If one wants to code in continuous style yet keep the run-to-completion semantics, there are ways of dealing with that in C++. One is just by leveraging the preprocessor, for example code see my other answer.

Another way is to use expression templates to get the C++ compiler to generate the code you want. You may want to leverage a template library here -- Boost spirit has a decent starting point of an implementation that can be reused even though you're not writing a parser.

The Windows Workflow Foundation also tackles the problem of how to write sequential style code yet have it run as short run-to-completion fragments. They resort to specifying the flow of control in XML. There's apparently no direct way of reusing standard C# syntax. They only provide it as a data structure, a-la JSON. It'd be simple enough to implement both XML and code-based WF in Qt, if one wanted to. All that in spite of .NET and C# providing ample support for programmatic generation of code...

Answer 2

The way you implemented your thread, it does not have its own event loop (because it does not call exec()). I'm not sure if your code within run() is actually executed within your thread or within the GUI thread.

Usually you should not subclass QThread. You probably did so because you read the Qt Documentation which unfortunately still recommends subclassing QThread - even though the developers long ago wrote a blog entry stating that you should not subclass QThread. Unfortunately, they still haven't updated the documentation appropriately.

I recommend reading "You're doing it wrong" on Qt Blog and then use the answer by "Kari" as an example of how to set up a basic multi-threaded system.

Answer 3

But when I run this program it freezes my GUI and when I click inside my window, it responds that your program is not responding.

Yes because IMO you're doing too much work in thread that it exhausts CPU. Generally program is not responding message pops up when process show no progress in handling application event queue requests. In your case this happens.

So in this case you should find a way to divide the work. Just for the sake of example say, thread runs in chunks of 100 and repeat the thread till it completes 10000000.

Also you should have look at QCoreApplication::processEvents() when you're performing a lengthy operation.