Re: dual screen: some operations are slow (nouveau driver)

[Vincent Lefevre <vincent@vinc17.net>]

Some additional information...

When I change the configuration to

  xrandr --output DP-3 --auto --output eDP-1 --auto --right-of DP-3

(usual dual screen settings), /var/log/Xorg.0.log shows:

  modeset(0): Allocate new frame buffer 7040x2160 stride

because 3840 + 3200 = 7040 and max(2160,1800) = 2160.

And when I do

  xrandr --output eDP-1 --off

to disable the laptop screen, /var/log/Xorg.0.log shows:

  modeset(0): Allocate new frame buffer 3840x2160 stride

corresponding to the external 4K monitor. I was wondering whether the
performance issue with the dual screen settings could be related to
the size of the frame buffer. So I tried

  xrandr --output DP-3 --auto --output eDP-1 --auto --same-as DP-3

in which case, the frame buffer size is still 3840x2160, and both
screens show the same content (except that the laptop screen, which
has a lower definition, does not show some bottom and right parts).
But even in this case, I notice the same performance issue.

I would add that if I switch to a Linux console (Ctrl-Alt-F1), I get
the same contents on both screens (though only 3200x1800 is used),
and everything is very fast: I can't "move windows" there, but the
scrolling tests do not show any performance issue, unlike with xorg.

Any idea?

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)