Refreshing with repaint() stops working - c++

Question

Today I encountered a problem with repaint() function from QT libraries. Long story short, I got a slot where I train my neural network using BP algorithm. I had tested the whole algorithm in console and then wanted to move it into GUI Application. Everything works fine except refreshing. Training of neural networks is a process containing a lot of computations, which are made in bp_alg function (training) and licz_mse function (counting a current error). Variable ilosc_epok can be set up to 1e10. Therefore the whole process may last even several hours. Thats why I wanted to display a current progress after each 100000 epochs (the last if contition). wyniki is an object of QTextEdit class used for displaying the progress. Unfortunately, repaint() doesnt work as intended. At the beginning it refreshes wyniki in GUI, but after some random time it stops working. When the external loop is finished, it refreshes once again showing all changes.
I tried to change frequency of refreshing, but sooner or later it always stops (unless the whole training process stops early enough because of satisfying the break condition). It looks like at some moment of time the application decides to stop refreshing because of too many computations. Imo it shouldnt happen. I was looking for a solution among older questions and managed to solve the problem when I used qApp->processEvents(QEventLoop::ExcludeUserInputEvents); instead of wyniki->repaint();. However, Im still curious why repaint() stops working just like that.
Below I paste a part of the code with the problematic part. Im using QT Creator 2.4.1 and QT Libraries 4.8.1 if it helps.
unsigned long int ile_epok;
double mse_w_epoce;
for (ile_epok=0; ile_epok<ilosc_epok; ile_epok++) { //external loop of training
mse_w_epoce = 0;
for (int i=0; i<zbior_uczacy_rozmiary[0]; i++) { //internal loop of training
alg_bp(zbior_uczacy[i], &zbior_uczacy[i][zbior_uczacy_rozmiary[1]]);
mse_w_epoce += licz_mse(&zbior_uczacy[i][zbior_uczacy_rozmiary[1]]);
}
//checking break condition
if (mse_w_epoce < warunek_stopu) {
wyniki->append("Zakończono uczenie po " + QString::number(ile_epok) + " epokach, osiągając MSE: " + QString::number(mse_w_epoce));
break;
}
//problematic part
if ((ile_epok+1)%(100000) == 0) {
wyniki->append("Uczenie w toku, po " + QString::number(ile_epok+1) + " epokach MSE wynosi: " + QString::number(mse_w_epoce));
wyniki->repaint();
}
}

Answer 1

You're blocking your GUI thread, so repaints will not work, it's just plainly bad design. You're never supposed to block the GUI thread.
If you insist on doing the work in the GUI thread, you must forcibly chop the work into small chunks and return to the main event loop after each chunk. Nested event loops are evil, so don't even think you'd want one. All this has a bad code smell, so stay away.
Alternatively, simply move your computation QObject to a worker thread and do the work there.
The code below demonstrates both techniques. It's easy to notice that the chopping-up-of-work requires to maintain loop state inside of the worker object, not merely locally in the loop. It's messier, the code smells bad, again - avoid it.
The code works under both Qt 4.8 and 5.1.
//main.cpp
#include <QApplication>
#include <QThread>
#include <QWidget>
#include <QBasicTimer>
#include <QElapsedTimer>
#include <QGridLayout>
#include <QPlainTextEdit>
#include <QPushButton>
class Helper : private QThread {
public:
using QThread::usleep;
};
class Trainer : public QObject {
Q_OBJECT
Q_PROPERTY(float stopMSE READ stopMSE WRITE setStopMSE)
float m_stopMSE;
int m_epochCounter;
QBasicTimer m_timer;
void timerEvent(QTimerEvent * ev);
public:
Trainer(QObject *parent = 0) : QObject(parent), m_stopMSE(1.0) {}
Q_SLOT void startTraining() {
m_epochCounter = 0;
m_timer.start(0, this);
}
Q_SLOT void moveToGUIThread() { moveToThread(qApp->thread()); }
Q_SIGNAL void hasNews(const QString &);
float stopMSE() const { return m_stopMSE; }
void setStopMSE(float m) { m_stopMSE = m; }
};
void Trainer::timerEvent(QTimerEvent * ev)
{
const int updateTime = 50; //ms
const int maxEpochs = 5000000;
if (ev->timerId() != m_timer.timerId()) return;
QElapsedTimer t;
t.start();
while (1) {
// do the work here
float currentMSE;
#if 0
for (int i=0; i<zbior_uczacy_rozmiary[0]; i++) { //internal loop of training
alg_bp(zbior_uczacy[i], &zbior_uczacy[i][zbior_uczacy_rozmiary[1]]);
currentMSE += licz_mse(&zbior_uczacy[i][zbior_uczacy_rozmiary[1]]);
}
#else
Helper::usleep(100); // pretend we're busy doing some work
currentMSE = 2E4/m_epochCounter;
#endif
// bail out if we're done
if (currentMSE <= m_stopMSE || m_epochCounter >= maxEpochs) {
QString s = QString::fromUtf8("Zakończono uczenie po %1 epokach, osiągając MSE: %2")
.arg(m_epochCounter).arg(currentMSE);
emit hasNews(s);
m_timer.stop();
break;
}
// send out periodic updates
// Note: QElapsedTimer::elapsed() may be expensive, so we don't call it all the time
if ((m_epochCounter % 128) == 1 && t.elapsed() > updateTime) {
QString s = QString::fromUtf8("Uczenie w toku, po %1 epokach MSE wynosi: %2")
.arg(m_epochCounter).arg(currentMSE);
emit hasNews(s);
// return to the event loop if we're in the GUI thread
if (QThread::currentThread() == qApp->thread()) break; else t.restart();
}
m_epochCounter++;
}
}
class Window : public QWidget {
Q_OBJECT
QPlainTextEdit *m_log;
QThread *m_worker;
Trainer *m_trainer;
Q_SIGNAL void startTraining();
Q_SLOT void showNews(const QString & s) { m_log->appendPlainText(s); }
Q_SLOT void on_startGUI_clicked() {
QMetaObject::invokeMethod(m_trainer, "moveToGUIThread");
emit startTraining();
}
Q_SLOT void on_startWorker_clicked() {
m_trainer->moveToThread(m_worker);
emit startTraining();
}
public:
Window(QWidget *parent = 0, Qt::WindowFlags f = 0) :
QWidget(parent, f), m_log(new QPlainTextEdit), m_worker(new QThread(this)), m_trainer(new Trainer)
{
QGridLayout * l = new QGridLayout(this);
QPushButton * btn;
btn = new QPushButton("Start in GUI Thread");
btn->setObjectName("startGUI");
l->addWidget(btn, 0, 0, 1, 1);
btn = new QPushButton("Start in Worker Thread");
btn->setObjectName("startWorker");
l->addWidget(btn, 0, 1, 1, 1);
l->addWidget(m_log, 1, 0, 1, 2);
connect(m_trainer, SIGNAL(hasNews(QString)), SLOT(showNews(QString)));
m_trainer->connect(this, SIGNAL(startTraining()), SLOT(startTraining()));
m_worker->start();
QMetaObject::connectSlotsByName(this);
}
~Window() {
m_worker->quit();
m_worker->wait();
delete m_trainer;
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Window w;
w.show();
return a.exec();
}
#include "main.moc"