Why on my Intel system does general 2D performance take a significant hit when I enable OpenGL compositing? I notice that Xorg usage goes way up, even for things like scrolling pages in a browser. This does not happen when I enable XRender compositing. Using OpenGL in Shared Memory mode is a bit better but not much. What the relationship between OpenGL and the awful performance (or perhaps the relationship between OpenGL and Xorg)? Is there any way to get OpenGL and Xorg to play better together?

This is likely related to the generally poor Intel driver OpenGL support.
Which Intel IGP do you have? I personally use a GM965 ( also known as a X3100 ) and don't find the performance too poor. However i'm probably used to it...

I'm using a 945GM on my laptop. It's a few generations old, and my cpu's not very powerful (1.06Ghz Core Solo ULV) so the performance hit taken with opengl compositing is noticeable.

Well to explain it quite bluntly: when Vista was introduced Intel cards did not support Aero - MS even invented the "Vista Capable" badge just for Intel GPUs. From a technical point of view Aero is exactly the same as the desktop effects on Linux (X) systems. Surprisingly the GPUs which are not capable for Aero are capable on X.

The poor performance is caused by a technique called Texture from Pixmap. We notice that there is quite some difference between the various driver implementations. With XRender you don't need this technique so that is faster while everything else is slower.

Scorlling in a browser is for example quite expensive. The content changes constantly, requiring lots of texture from pixmap operations and the repaints of KWin.

Thanks for the explanation. I've found a nice compromise using XRender and disabling certain effects.

One thing I find curious is that transparency effects should have anything to do with compositing per se. Maybe that's a question for another thread.

molecule-eye wrote:One thing I find curious is that transparency effects should have anything to do with compositing per se. Maybe that's a question for another thread.

That's just a restriction of X. You cannot have an alpha channel without compositing. The same restriction is known from other windowing systems, too.