30 December, 2007

performance and Qt 4.4 again

My previous blog resulted in an large amount of reactions, many of which warrant a reply from me.

I blogged about bad drawing performance, and the big question I had was: what was causing it.

Firstly, a few video's were posted which showed the issue:
http://bw.uwcs.co.uk/kde3_resize.ogg
http://bw.uwcs.co.uk/kde4_resize.ogg

I hope the person posting them won't mind the bandwidth sucking....

Now I ended my previous blog with mentioning this issue seemingly was a bug in Qt, and more precisely, QScrollBar, based on the profiling one reader did in the blog before that. But in the comments section another issue came up: first mentioned by Benji, who discovered to his surprise his 3 year old laptop with integrated (Intel) graphics performed better than his NVidia GeForce 8800GTS... Quite a clue as to where the issue might be, I'd say. An article talking about the upcoming (now out) NVidia driver mentioned quite a huge improvement in XRENDER performance, so that might make a difference. I've compiled and installed the driver, and unfortunately, no visible difference. I can show the graph, but just take my word: Konqi still takes between 10% and 20% CPU when scrolling (that's with dualcores, so it's actually between 20 and 40% of a 2.4 ghz AMD core...). Resizing still barely goes over 5 frames a second (mostly a lot less).

So it probably isn't the NVidia driver - somebody with an old ATI Radeon (pretty good FOSS drivers available) also has the issue. But I do wonder about the apparently good Intel performance. More convincing against the NVidia-does-it case is an comment from Rui, who mentions sub-standard Qt 4 performance on Mac OS X:
Hi, FWIW, on Mac OS X, the last.fm client, which bundles some version of Qt4, is also a lot slower redrawing itslef when resizing than, say, a Finder window, even if said Finder window is displaying the new coverflow view of files.

I'll try getting some numbers later. Maybe checking Qt4's performance on windows would be constructive too. By checking on other systems we are eliminating a whole bunch of probable places where the slowdown could be occurring. This kind of performance problem is quite difficult to measure especially because there are so many variables going under (system frameworks) and above (the app itself) Qt.


So, were are we at. A short overview:

- It is not the Oxygen style.
- It is not Qt doublebuffering.
- It is not 3D/compositing stuff.
- It is not the (lack of) alien widgets.
- It is most likely not the XRENDER performance of the NVidia drivers (but maybe Intel does something right?)

So, we're back at Qt again. Why does Qt 4 drawing seem to perform so much worse on some hardware? It could be that Qt 4 is much more dependent on hardware acceleration - and if that's not done properly by the drivers, drawing suffers. I wouldn't know. But if it IS drivers, the new NVidia driver should probably fix it - and it doesn't. I know there is more to drivers than just XRENDER, so I should probably let others talk about that. Actually, that would be a good thing - maybe a graphics guru (our own Graphics Ninja perhaps?) could chime in, say something?


----DISCLAIMER
Now I also have a disclaimer, as my previous blog also resulted in a comment blessing me by the name of 'whistleblower', and someone else wasn't to happy with the "this is Qt's fault".

I think I was clear enough on this, but I want to repeat it: I don't think this is a huge issue. Sure, it looks bad, but that's about it. The faster application startup we have in KDE 4 far outweighs the slower drawing. Performance is much more than drawing, and imho having to wait for an application to start is much more annoying.

Secondly, my measurements are of course entirely unscientific. After all, the difference on can see with the bare eye must be relatively large to really count. And it might be that there isn't one cause but there are several causes. Maybe it is a combination of the double-buffering, bad drivers, Oxygen style and lack of alien widgets. But I have tried resizing dolphin with the QtCurve style and Qt 4.4 (aka alien widgets) with the new NVidia driver - and I still clearly could see it draw. Oh, and turning the double-buffering of leads to 100% cpu usage when resizing even a slight bit so that won't help either ;-)

Third, maybe it isn't Qt's fault at all - it might be that the way Qt4 works (more hardware acceleration, more use of animations, transparency, etc) just exposes bad performance in X.org and/or graphics drivers. Seriously, I find that very likely. Seeing how many Qt developers seem to care about performance, it seems unlikely they would release something which is so much slower than Qt 3. So it is very well possible it works fine on THEIR hardware. Apparently, the FOSS Intel drivers fare pretty well, so maybe they all have these nice MacBooks and such ;-)

Last, I didn't file a bug. First because I'm still trying to figure out what exactly causes this, and secondly I'm simply not knowledgeable enough to start profiling apps. And also because this process seems to work rather well - with the help from the many readers of this blog, I feel we're getting closer to finding the reason of the issue.

So, while I look forward to a comment from someone who can tell me he/she found the issue, this is in no way a blocker for KDE 4.0 or a horrible thing for Qt 4.x. What I would do is recommend to the KWin developers to set KWin to NOT display content in resizing windows by default. That wouldn't look incredibly cool, but it would look far better than horribly slow resizing windows. Lubos? Rivo? Are you guys reading this?