Bug 502686

Thanks for the bug report. Do you notice any sluggishness in anything else, other than mouse movement? For instance, are windows slow to move around, are applications slow to open, compared to X11?

Movng to kwin because this will be compositor related

(In reply to TraceyC from comment #1)
> Thanks for the bug report. Do you notice any sluggishness in anything else,
> other than mouse movement? For instance, are windows slow to move around,
> are applications slow to open, compared to X11?

i'm only having the issue with the mouse, everything else is pretty much the same snappy and fast(for programs and moving windows) across both X11 and Wayland.

Thanks for the additional details. I found an older bug where someone reported high CPU usage moving the mouse around with Wayland but not X11 (bug 485609)

Can you use System Monitor, or the tool of your choice, to see if there's excessive CPU usage in Wayland and if it seems to correspond to moving the mouse? Thanks.

(In reply to TraceyC from comment #4)
> Thanks for the additional details. I found an older bug where someone
> reported high CPU usage moving the mouse around with Wayland but not X11
> (bug 485609)
> 
> Can you use System Monitor, or the tool of your choice, to see if there's
> excessive CPU usage in Wayland and if it seems to correspond to moving the
> mouse? Thanks.

there's no excessive CPU usage, my CPU is avaraging between 5% and 15% on idle
i think the guys in this reddit post are up to something but they've been having those issues while having an Nvidia GPU meanwhile i have an AMD GPU, https://www.reddit.com/r/kde/comments/1335gj5/mouse_feels_laggy_and_slow_on_wayland_compared_to/ .

mainly from what the user "Infradragon" said and i quote:
["I was experiencing this problem too, and it looks like its a known issue, part of the Wayland spec, and will never be fixed unless a core tenet of Wayland changes

tldr;

Wayland wants to guarantee the position of your cursor before they draw a frame, so they employ what is basically vsync for your cursor, adding a frame of latency
x11 does this asynchronously, which means your cursor can move before your compositor can respond (e.g. highlighting a button under your cursor), and can cause cursor tearing (i have not noticed this)"] (i am on 60Hz(Aka 60FPS) monitor so i can notice some jankiness at times)

Thanks for adding the additional detail. That thread on reddit and the related one on lobste.rs do present a more complex story.
I will say that we don't expect very noticable mouse input lag. 

On a system with an AMD integrated GPU, and Plasma git-master, I don't see much difference between cursor movement in Wayland vs X11. I tested with both the touchpad and a mouse.

I'll let the maintainers who are more familiar with this take it from here.

(In reply to NickTheNeko from comment #5)
> i think the guys in this reddit post are up to something but they've been
> having those issues while having an Nvidia GPU meanwhile i have an AMD GPU,
> https://www.reddit.com/r/kde/comments/1335gj5/
> mouse_feels_laggy_and_slow_on_wayland_compared_to/ .
> 
> mainly from what the user "Infradragon" said and i quote:
> ["I was experiencing this problem too, and it looks like its a known issue,
> part of the Wayland spec, and will never be fixed unless a core tenet of
> Wayland changes
> 
> tldr;
> 
> Wayland wants to guarantee the position of your cursor before they draw a
> frame, so they employ what is basically vsync for your cursor, adding a
> frame of latency
> x11 does this asynchronously, which means your cursor can move before your
> compositor can respond (e.g. highlighting a button under your cursor), and
> can cause cursor tearing (i have not noticed this)"] (i am on 60Hz(Aka
> 60FPS) monitor so i can notice some jankiness at times)
Don't jump to conclusions with random comments from Reddit - pretty much none of that is correct. For example, Asahi Lina's statements about synchronizing button highlights and the cursor position are completely and entirely hypothetical - there is no mechanism to do such a thing, and compositors process input events and draw the cursor whenever and wherever they please.


The differences between Xorg and kwin_wayland boil down to
- Xorg may use a different input driver. This can make a huge difference, as you're used to that driver's acceleration curve and perceive anything else as not snappy
- Xorg uses legacy modesetting, kwin uses atomic modesetting. The difference in latency is generally incredibly tiny (<2ms), but may be a bit larger, depending on what the GPU driver does

For the former, you can make sure that you don't have any synaptics drivers installed. For testing the latter, you can set https://invent.kde.org/plasma/kwin/-/wikis/Environment-Variables#kwin_drm_no_ams to 1 to make KWin fall back to legacy modesetting.