Comment by bee_rider
5 hours ago
If our mouse cursors are going to have half a frame of latency, I guess we will need 60Hz or 120Hz desktops, or whatever.
I dunno. It does seem a bit odd, because who was thinking about the framerates of, like, desktops running productivity software, for the last couple decades? I guess I assumed this would never be a problem.
Mouse cursor latency and window compositing latency are two separate things. I probably did not do a good enough job conveying this. In a typical Linux setup, the mouse cursor gets its own DRM plane, so it will be rendered on top of the desktop during scanout right as the video output goes to the screen.
There are two things that typically impact mouse cursor latency, especially with regards to Wayland:
- Software-rendering, which is sometimes used if hardware cursors are unavailable or buggy for driver/GPU reasons. In this case the cursor will be rendered onto the composited desktop frame and thus suffer compositor latency, which is tied to refresh rate.
- Atomic DRM commits. Using atomic DRM commits, even the hardware-rendered cursors can suffer additional latency. In this case, the added latency is not necessarily tied to frame times or refresh rates. Instead, its tied to when during the refresh cycle the atomic commit is sent; specifically, how close to the deadline. I think in most cases we're talking a couple milliseconds of latency. It has been measured before, but I cannot find the source.
Wayland compositors tend to use atomic DRM commits, hence a slightly more laggy mouse cursor. I honestly couldn't tell you if there is a specific reason why they must use atomic DRM, because I don't have knowledge that runs that deep, only that they seem to.
Mouse being jumpy shouldn’t be related to refresh rate. The mouse driver and windowing system should keep track of the mouse position regardless of the video frame rate. Yes, the mouse may jump more per frame with a lower frame rate, but that should only be happening when you move the mouse a long distance quickly. Typically, when you do that, you’re not looking at the mouse itself but at the target. Then, once you’re near it, you slow down the movement and use fine motor skills to move it onto the target. That’s typically much slower and frame rate won’t matter much because the motion is so much smaller.