[Qt-interest] Xorg performance

[Matthew Fincham]

On 28-05-10 21:27, Clemens Eisserer wrote:
>> I think you have no idea on how to measure the real life performance of
>> an application.
>>      
> Please not again that fanboy stuff.
> This user has problems with QT on his (quite outdated) installation,
> so I don't see a problem he is asking here.
>    
>> Your test, IMHO, shows nothing. Real life Qt
>> applications do not have an "appaling" or "unusable" performance at all.
>>      
> In real-life I see QT4 applications performing quite poor at resize
> compared to QT3.
> Most likely stupid backing pixmap allocation is the cause, would be
> interesting to investigate further.
>
>    
>> I am not a specialist on this topic either, but perhaps you should
>> re-read some of the old blogs of "Graphics Ninja" Zack Russin. He
>> blogged about this kind of so-called benchmarks several times.
>>      
> I guess the problem is QT4 is now double-buffered.
> That means a lot of BLIT operations (drawing the backbuffer pixmap to
> frontbuffer),
> which is quite slow when its not hardware accelerated (like its the
> case for XAA).
> Is turning of double-buffering an option for you?
>
> Sad you are not on Linux. For Linux there is sysprof which exectly
> tells you where how many cycles are burnt.
> Maybe there are some compareable "whole-system" profilers for your system?
>
> Good luck!
>
> - Clemens
>    

The application is a video application. With Qt3 it is possible to 
display a single 4CIF stream (720x576) at 30fps with negligible Xorg 
usage. With Qt4 the application can barely manage 2 fps, with Xorg usage 
through the roof. This is the real-life performance the of the 
application - what more can I say.

I have since found that the poor performance is poor after running the 
Qt3 version of the application. The process is this. Restart system - 
Xorg starts. Run Qt4 application and performance is good. Run Qt3 
application and performance is good. Re-run Qt4 application and Xorg's 
usage goes to 100% of a single core. Restart Xorg and run Qt4 version 
and the application runs fine.

So it looks like the Qt3 version is setting something in Xorg (or maybe 
the (intel) driver) that Qt4 is not happy with. I will need to 
investigate further.

This happens with double buffering turned on and off.

Thank you Clemens for your more understanding response.

Regards
Matthew Fincham