19 Sep
2013
19 Sep
'13
6:35 p.m.
On Thursday 19 September 2013 12:35:17 Aleksey
Midenkov wrote:
It's not interesting news, it's
just wrong. Qt's internal rendering
doesn't
depend on whether compositing is used or not. Even more a compositor would
not
even be able to make use of any optimizations done inside Qt for the
composited case, because there is no way for the compositor to know that
the
app "supports" it. Compositing on X11 happens behind the scenes of the
window.
The window doesn't and doesn't need to know that it gets composited.
Cheers
Martin
Maintainer of the X11 Compositor of the KDE Plasma Workspaces
My apologies; I was not referring to compositing in the window management
sense, I was referring to the new painting system code in Qt4 (Arthur:
http://www.trinitydesktop.org/docs/qt4/qt4-arthur.html), and even more
specifically to things like the backing store introduced in Qt 4.1
(http://doc.qt.digia.com/qq/qq16-background.html).
While I don't have a link to it handy, newer versions of Qt essentially
are just blasting pre-rendered pixmaps to the X server on every change
(graphics system raster). In fact, Qt is working hard on removing native
bindings and replacing them with their software-only renderer
(http://blog.qt.digia.com/blog/2009/12/18/qt-graphics-and-performance-the-ra…)
for performance reasons.
I have to ask: If the XRender system worked so well in Qt3, even over the
network, why is the XRender system working so badly in Qt4 that Nokia now
needs to fall back on a software rasterizer? What changed?
Even over a fast RDP connection (yes, RDP, not X11 native, though X11
native shows the same effect) I can tell the difference between Qt3 and
Qt4 apps just from their redraw speed alone. Essentially, *as far as I
can tell*, if your remote desktop is not blasting entire screens over the
network on each update (e.g. some old versions of VNC), you will see a
definite performance difference between Qt3 and Qt4 apps. My best *guess*
is that the graphics server does not know what changed when a Qt4 app
updates its visible contents, therefore it sends the entire contents of
the window over the remote desktop connection, even if 99% of the pixels
are the same.
I probably got some of the Qt4 stuff above wrong as usual, but I don't
really have the time or desire to keep up with the latest Qt4 information
as I don't use Qt4 very often. :-) All I know is that there *is* a
performance drop on many systems (unquantified, though this seems most
severe on non-VNC remote desktops) that so far has eluded the teams at
Nokia, the various KDE SC developers, the TDE developers, and various
application developers such as Xilinx.
Tim
Qt4
decided that they wanted to be a compositor instead of
passing compositing off to the native display server, and most of the
performance problems and drawing limitations stem from this decision.
Hmm, interesting news. Ok, that's a good reason to stay with Qt3. I
don't like how Qt4 performs either.