[Qt-jambi-interest] QPixmap erratic performance

[Bart van Deenen]

On Monday 17 November 2008 09:07:03 you wrote:
> Bart van Deenen wrote:
> > Hi all
> >
> > I'm constructing a display from a nested tree of objects, each of which
> > contains two QPixmaps for visual effects stuff. The whole code works
> > fine (as
> > far as display content is concerned), but the performance is very
> > erratic. One function that contains two consecutive drawPixmap calls on
> > the same QPainter might take between 4 and 100 ms.
> > I've read somewhere on the web about a maximum QPixmap storage (can't
> > find where anymore). Is this what I'm running against? Is the operating
> > system (Linux in my case) or the graphics driver doing something? I don't
> > think it's
> >
> > my code, its just a single thread.
> 

> On Linux the time spent doing a drawPixmap is mostly sending a command
> to the X server and returning immediately. The actual drawing happens in
> a different process and is not visible unless the application does a
> QApplication::sync().
I'm creating them once. They're static between calls.
> Are you creating these pixmaps on the fly or are they static between calls?


> If there is sporadic time differences, my best guess would be that GC is
> kicking in frequently. Do you have a lot of allocations in your code?
Hardly any (just a few QRects and QPoints. I don't think it's the gc.

> On Windows back in the Qt 3 days, QPixmaps used GDI objects which was a
> pool of 10k objects per process. If the total number of fonts, pixmaps,
> icons, windows etc grew beyond 10.000 the process would start behaving
> "weird". This is much less a problem in Qt 4 as neither pixmaps, icons
> nor windows use native GDI handles anymore.
I'm trying to narrow it down, but can't find it yet. My mainloop code is:


    public void paintEvent(QPaintEvent g)    {
        
	// does all the combination of QPixmaps into one (root.pixmap)
        this.render(root); 

        QPainter painter = new QPainter();
        painter.begin(this);
        painter.drawPixmap(10, 30, root.pixmap);
        painter.end();
    }

    protected void timerEvent(QTimerEvent event)
    {
        if (event.timerId() == timer.timerId())  {
            update();
        }
        else  {
            super.timerEvent(event);
        }
    }

Ok. I'm starting to get somewhere. I've added instrumentation to the code (not 
shown here) which uses System.currentTimeMillis() to measure intervals, and it 
turns out that the interval between two consecutive calls to timerEvent is 
very erratic! My 'render' function (currently with a much simplified QPixmap 
tree under it) only takes a few ms. The QTimer is supposed to run at 40 ms 
interval, but the actual intervals are anywhere between 15 and 300 ms!

How's this possible?

Confused

Bart



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