c++ - Memory management issue with deleting QQuickView in Qt5.3(mingw32)

Question

We are developing an application with Qt/Qml (Qml is embedded in the QWidget). When deleting a QWidget that contains a QQuickView (embedded), the allocated memory won't be freed entirely.

By adding a QWidget to application, about 30MB of memory will be allocated but when the widget deletes, only about 20MB of memory will be freed.

In the destructor of QWidget, I have deleted the QQuickView instance and there are no other big objects.

Also, I am pretty sure that the QQuickView doesn't release memory properly.

How can I free entire memory allocated by QQuickView?

NOTE: The code is really big (160000 Lines), therefore I can't put sample code.

Thanks in advance...

Answer 1

I've written a quick test to determine if there is an actual leak in the creation and deletion of a QQUickWidget:

class Widget : public QWidget {
    Q_OBJECT
public:
    Widget(QWidget *parent = 0) : QWidget(parent) {
        widget = 0;
        count = 0;
        resize(200, 200);
        layout = new QVBoxLayout(this);
        setLayout(layout);
        QTimer * t = new QTimer(this);
        t->setInterval(200);
        t->setSingleShot(false);
        t->start();
        connect (t, SIGNAL(timeout()), this, SLOT(toggleQuickView()));
    }

public slots:
    void toggleQuickView() {
        if (!widget) {
            widget = new QQuickWidget;
            widget->setSource(QUrl::fromLocalFile("d:\main.qml"));
            connect(widget, SIGNAL(destroyed()), this, SLOT(echo()));
            layout->addWidget(widget);
        } else {
            layout->removeWidget(widget);
            widget->deleteLater();
            widget = 0;
        }
    }

    void echo() {
        PROCESS_MEMORY_COUNTERS memcount;
        if (!GetProcessMemoryInfo(GetCurrentProcess(), &memcount, sizeof(memcount))) return;
        qDebug() << ++count << "created and destroyed," << memcount.WorkingSetSize / (1024 * 1024) << "MB memory used";
    }

private:
    QVBoxLayout * layout;
    QQuickWidget * widget;
    int count;
};

It has a timer that creates/destroys a QQuickWidget with a loaded QML file inside, and although the results initially ramp up, the memory usage stabilizes in time, indicating it is not likely there was a memory leak in the Qt code, and that if you indeed leak memory, the fault does not lie in Qt but in your own code.

Also, it is worth mentioning that the task manager actually showed the process using less memory than GetProcessMemoryInfo(), and I think the latter is the more accurate measure of the two. The task manager reading also didn't indicate any memory leaking although its value fluctuated more.

Here is the output:

1 created and destroyed, 41 MB memory used
2 created and destroyed, 44 MB memory used
3 created and destroyed, 44 MB memory used
4 created and destroyed, 48 MB memory used
5 created and destroyed, 48 MB memory used
6 created and destroyed, 48 MB memory used
7 created and destroyed, 48 MB memory used
8 created and destroyed, 48 MB memory used
9 created and destroyed, 48 MB memory used
10 created and destroyed, 48 MB memory used
11 created and destroyed, 52 MB memory used
12 created and destroyed, 52 MB memory used
13 created and destroyed, 52 MB memory used
14 created and destroyed, 52 MB memory used
15 created and destroyed, 52 MB memory used
16 created and destroyed, 52 MB memory used
17 created and destroyed, 52 MB memory used
18 created and destroyed, 52 MB memory used
19 created and destroyed, 52 MB memory used
20 created and destroyed, 52 MB memory used
21 created and destroyed, 53 MB memory used
...
50 created and destroyed, 53 MB memory used
...
100 created and destroyed, 53 MB memory used
...
200 created and destroyed, 53 MB memory used
...
500 created and destroyed, 53 MB memory used