High input latency with wgpu backend

[mrjbom]

Describe the bug
In the demo application, there is a delay in the application's response to input: windows move with a delay, elements react to the mouse over them with a delay, and button presses also have a delay.
This issue is present in 0.28.1 and the master branch.

egui.delay.mp4

In my application, I also encountered this delay (Windows, Vulkan backend, egui-wgpu 0.28.1, and egui-winit 0.28.1). I found that the delay depends on the backend: it is lower on DX12 than on Vulkan.

I tested everything on release versions.
The web version with egui.rs works fine.

At first, it seemed to me that I was doing something wrong in my application when I encountered this issue, but since it also occurs in the demo application, it seems the problem isn't related to something else; maybe it's with egui-wgpu.

[emilk]

Related:

• egui-wgpu: reduce input latency by defaulting to double-buffering #3714
• Add WgpuConfiguration::desired_maximum_frame_latency #3874

Please see if changing the value of WgpuConfiguration::desired_maximum_frame_latency to Some(1) helps

[mrjbom]

Please see if changing the value of WgpuConfiguration::desired_maximum_frame_latency to Some(1) helps

Can I configure this using egui_wgpu::renderer::Renderer? I using Renderer now

[emilk]

Then you want https://docs.rs/wgpu/latest/wgpu/type.SurfaceConfiguration.html#structfield.desired_maximum_frame_latency

[mrjbom]

Then you want https://docs.rs/wgpu/latest/wgpu/type.SurfaceConfiguration.html#structfield.desired_maximum_frame_latency

I tried changing the value here from 2 to 1 and it didn't help.

[emilk]

I still suspect a latency issue with triple (or more) buffering in wgpu - maybe @Wumpf can chime in with more info

[mrjbom]

I still suspect a latency issue with triple (or more) buffering in wgpu - maybe @Wumpf can chime in with more info

Definitely a wgpu problem, there is no delay only if present mode is Immediate or Mailbox.

[mrjbom]

I tested how a pure Vulkan application behaves with the FIFO present mode, which is the default for PresentMode in wgpu, and I also encountered delays.
This seems to be an issue with FIFO itself, not wgpu only; it's probably better to choose Immediate or Mailbox to avoid delays.

[Wumpf]

I'm on vacation and also not super knowledgable about the myriad of fine details here, but maybe @cwfitzgerald wants to chime in :)
Thanks @mrjbom for digging that deep already! It's still curious that DX12 behaves differently on FIFO, worth figuring out why that is.

This sounds vaguely familiar to some issues Bevy folks reported after some severe bugs in Vulkan's synchronization code got fixed.

[mrjbom]

I tried to show and compare what delay is present and under which API and modes, I think it will be useful.

delays.compare.2.mp4

[mrjbom]

The egui_demo_app screen at the top seems to use glow.

No, the logs say that wgpu is being used