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I've just installed a fresh 64-bit 13.04 system, and set up the fglrx-legacy driver from the makson96/fglrx PPA (as suggested by this bug report), since my Radeon HD 3840 is one of those no longer supported by the current driver. The installation was successful, as can be seen from the following output:

z1@foyle:~$ fglrxinfo
display: :0.0  screen: 0
OpenGL vendor string: Advanced Micro Devices, Inc.
OpenGL renderer string: ATI Radeon HD 3800 Series
OpenGL version string: 3.3.11672 Compatibility Profile Context

z1@foyle:~$ glxinfo
name of display: :0.0
display: :0  screen: 0
direct rendering: Yes
server glx vendor string: ATI
server glx version string: 1.4
... snipped long list of extensions and modes

I used amdcccle to set up a multi-display desktop like I had in 12.04. X starts fine, and I get composition effects like transparent windows and the blurry launcher background. There are no errors in /var/log/Xorg.0.log.

The only problem is that screen updates are incredibly slow. Scrolling a Firefox window makes Firefox use 30-50%, and Xorg 50-80% of a CPU core, producing a poor framerate. Video is somewhat better: a DVD-resolution video can play in VLC with compiz using just over 30% of a core, and Xorg about 15%. It's about the same amount whether it plays at the original resolution in a window or fullscreen. (I have "Unredirect Fullscreen Windows" turned on.)

Turning on or off the anti-tear feature in amdcccle makes no difference; nor does "Sync To VBlank" in CompizConfig.

The CPU usage suggests that Xorg is doing something stupid like compositing on the CPU rather than the GPU, but I can't find any way to confirm this directly. How can I make composition put less load on the CPU?

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