If you're interested in continuing to use KDE, I'd recommend adding LIBGL_ALWAYS_SOFTWARE=1 to your environment, and unsetting it only for applications that you want to actually use 3d accel. KDE + nouveau = fail for now. I'm told that some newer versions of ... Qt? KDE? something? detect nouveau and auto-disable the GL backends now. Not sure on the precise specifics.

Apparently Qt/KDE's threading can be disabled with

QSG_RENDER_LOOP=basic

Try putting that into /etc/environment and logging out/in (to make sure that it's there).

Thanks Ilia. Do you mean that I need to add both LIBGL_ALWAYS_SOFTWARE=1 and QSG_RENDER_LOOP=basic to /etc/environment or either of them? By the way, any of them slows down graphics processing so that watching Youtube or even scrolling pages in browser becomes nightmare due to lags. How can I fight this?

Try it with just

QSG_RENDER_LOOP=basic

in your /etc/environment. Note you need to log out/in in order for changes to take effect.

I have added QSG_RENDER_LOOP=basic to /etc/environment and then logged out/in, but Chromium is still lagging. According to 'top', kwin_x11 consumes about 90% of CPU.

Does running "glxinfo" show that you're using nouveau or llvmpipe?

# glxinfo | grep -E "nouveau | llvmpipe"
    Device: llvmpipe (LLVM 3.7, 128 bits) (0xffffffff)
OpenGL renderer string: Gallium 0.4 on llvmpipe (LLVM 3.7, 128 bits)

(In reply to Alexander from comment #7)
> # glxinfo | grep -E "nouveau | llvmpipe"
>     Device: llvmpipe (LLVM 3.7, 128 bits) (0xffffffff)
> OpenGL renderer string: Gallium 0.4 on llvmpipe (LLVM 3.7, 128 bits)

OK, so you're still using llvmpipe. You must have a LIBGL_ALWAYS_SOFTWARE=1 left somewhere. (check "env" to see whether that's the case.)

Thanks. I have added the following to ~/.bashrc:
export "LIBGL_ALWAYS_SOFTWARE=1"

Then I checked 'env' as advised:

# env | grep QSG
QSG_RENDER_LOOP=basic
# env | grep LIBGL
LIBGL_ALWAYS_SOFTWARE=1

However, there is still llvmpipe in glxinfo output.

The system works with no lags, but it froze twice within 5 minutes. So now I have the same as I had initially.

(In reply to Alexander from comment #9)
> Thanks. I have added the following to ~/.bashrc:
> export "LIBGL_ALWAYS_SOFTWARE=1"
> 
> Then I checked 'env' as advised:
> 
> # env | grep QSG
> QSG_RENDER_LOOP=basic

That's good.

> # env | grep LIBGL
> LIBGL_ALWAYS_SOFTWARE=1

That's bad. You don't want this.

Oh, I see. I thought you meant I should add this by saying "You must have a LIBGL_ALWAYS_SOFTWARE=1 left somewhere." My apologies.

I have removed it and now I have 
# glxinfo | grep -E "nouveau | llvmpipe"
    Vendor: nouveau (0x10de)

Will test it for some time. Thank you.

No, it froze again.

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