Dual 4K slowness due to xorg threading concept?

[Thomas Lübking]

On Freitag, 2. Oktober 2015 12:59:30 CEST, Christopher Barry wrote:

>> Problem:
>> "Graphics stuff" seems to work smoothly on lower resolutions (e.g. 2x 
>> 2560x1600). On high resolution (2x 3840x2160) still everything 
>> (moving/resizing windows, playing videos, scrolling through lists,
>> text views or consoles, opening menus) goes smoothly but only as long
>> as single windows are really small (and as long as there are not too
>> many of them). As soon as a window becomes half the size of a screen
>> or even full screen, operations mentioned before become horribly slow
>> (K-Menu takes up to 8 secs to even open up) and CPU usage of the Xorg
>> process is vastly increasing (to about 100%). I don't really have an
>> idea of what's going wrong here. There are no apparent errors etc.
>> The only thing that seems really suspect to me is the 100% CPU which I
>> would attribute to a (multi-)threading issue... I'd really appreciate
>> if you could give me a hand and point me to the right direction to get
>> things working.

You're likely running into texture/shader limitations and fallover to some software emulation - the GPU is indeed not fully suited for such resolutions.

=> suspend the compositor (SHIFT+Alt+F12, no shadows, no translucency) or use the XRender backend (kcmshell[4] kwincompositing")
*Maybe* even disabling the blur filter and setting the scale method to anything but "accurate" (ie. try smooth) might aid.

Cheers,
Thomas