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Reassessing Wayland (dudemanguy.github.io)

submitted 23 hours ago by [email protected] to c/[email protected]

18 comments fedilink hide all child comments

So a bit under 3 years ago, I made my infamous Wayland rant post that is likely the most read post on this blog by miles. I should really actually write about music again one of these days, but that's a topic for another time. The language was perhaps a bit inflammatory, but I felt the criticisms I made at the time were fair. It was primarily born out some frustrations I had with the entire ecosystem, and it was not like I was the only sole voice. There are other people out there you can find that encountered their own unique Wayland problems and wrote about it.

With that post, I probably cast myself as some anti-Wayland guy which is my own doing, but I promise you that is not the case. You can check my mpv commits, and it's businesses as usual. Lots of Wayland fixes, features, and all that good stuff. Quite some time has passed since then, and it is really overdue look at the situation again with all the new developments in mind. To be frank, my original post is very outdated and it is not fair to leave it up in its current state without acknowledging the work that has been done. So in comparison to 3 years ago, I have a much more positive outlook now.

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[–] [email protected] 9 points 17 hours ago (1 children)

For example, the recently publicized about mouse latency differences is true and something I've noticed but the difference doesn't particularly bother me. Something like that is just one of those inevitable consequences of the design of Wayland being so fundamentally different.

It would be interesting if someone explained the relationship between Wayland design and mouse latency

[–] [email protected] 6 points 5 hours ago* (last edited 5 hours ago) (1 children)

It is kinda simple. X.org and older Wayland compositors use the legacy KMS API, while modern compositors use the atomic API. The atomic API lets a compositor update everything for all planes in one go, this is a good thing because either all changes apply or none at all. The legacy API only lets things be updated individually.

Now why does atomic have more latency when it comes to mousing around? It's because in a single atomic commit all properties have to be set, including cursor X & Y positions. Once an atomic commit has been sent to the kernel, it is locked and can't be changed until the next monitor vblank. This means the cursor position is ever so slightly outdated. The legacy API does not have this limit, compositors can immediately move the cursor no problem, so less perceived latency. You can particularly see this while dragging windows around, on a modern Wayland compositor the cursor will be perfectly attached to the window, but in X.org it'll be slightly ahead because of the reduced latency. Unless you don't have a compositor running, of course.

There were proposed changes to address this years ago, but those seem to have fizzled out.

Oh and this is also why the cursor movement might visibly start stuttering during heavy GPU load. This is a problem that was solved back in the 80s but here we are...

[–] [email protected] 2 points 2 hours ago

Thanks for the detailed answers. So we can say that Wayland sacrifices lower latency in exchange for higher accuracy.

According to this post Gnome allows you to change this behavior through an environment variable (MUTTER_DEBUG_ENABLE_ATOMIC_KMS=0 on Ubuntu 22.04). It should be a configurable option, considering the amount of people complaining about this mouse behavior.

Oh and this is also why the cursor movement might visibly start stuttering during heavy GPU load. This is a problem that was solved back in the 80s but here we are…

Sad, but does this problem only affect Wayland or also Xorg?