Why is Qtopia much faster?

Carsten Haitzler (The Rasterman) raster at openmoko.org
Mon Jul 21 20:52:49 CEST 2008

On Mon, 21 Jul 2008 13:11:25 +0200 "Yorick Moko" <yorickmoko at gmail.com> babbled:

> This might be a stupid question, but it isn't the first and will not
> be the last stupid one I ask :).
> I have glimpsed at the 2007.2, ASU and qtopia image (the latest
> builds) and a very noticable difference is the speed.
> Will this improve in the ASU and the 2007.2 or should I not get my
> hopes on? Qtopia just responds and seems to load everything faster.

he real major problem is qt/qtopia and the x11 output. it is amazingly
inefficient. it does a whole set of no-no's when it comes to performance (like
uploading and downloading pixmaps and converting to/from image formats...
repeatedly). if you want things  to work well in x11:

1. only upload when you need a pixmap or data on screen... dont upload until
you need it.
2. never repeatedly upload if u can do it just once, so defer uploads of pixel
data as late as possible.
3. NEVER download (get images) from x. this will kill performance as it 1,
stalls your app into waiting on x, 2, needs to wait for data to be read from
video ram (slow - even on desktop systems), 3, causes yet more context
switching which is expensive on ARM.

one reason EFL gets the performance it does is it has an almost 1 way pipeline
with everything deferred as much a possible. be it software rendering or
xrender etc. - everything happens as late as possible. it never downloads data
from x... and man all of this really helps.

Carsten Haitzler (The Rasterman) <raster at openmoko.org>

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