The ScrollView has a high CPU load when the scrollbar appears

[TuEmb]

I'm using ScrollView and VerticalLayout to display my data, the CPU load is acceptable (~11%) when displaying just enough data to fit on the screen without needing a scrollbar. However, when I add more data and the scrollbar appears, the CPU load increases significantly (>50%). After that, I expand my window app, the scrollbar disappears and the CPU load goes down again.

I'm using Slint with Rust. You can find my source code here: https://github.com/TuEmb/can-viewer
Here is my record for this bug:

can-viewer.webm

[ogoffart]

thanks for reporting an issue. I have checked out your code, but i am not sure how to load data. Could you guide me on how to import data so i can reproduce the problem?

Otherwise, one thing to try to figure out where the problem is is to run a profiler such as valgrind or perf to find out what is taking CPU time. Btw, is it a release or debug build?
You can also see if there is a difference between the skia or the femtovg renderer.

[TuEmb]

Thanks for your reply, I built my code with release. I used femtovg and software renderer and the results are below pictures:

For skia renderer, I'm having the issue out of memory. So that I can't test with skia renderer now. I will try to use skia renderer later.

Step to run my source code with a simulation data:

1. you can create a virtual can socket on Ubuntu OS: https://www.pragmaticlinux.com/2021/10/how-to-create-a-virtual-can-interface-on-linux/#google_vignette

2. Then update my code to use "vcan0" instead of "can0" at https://github.com/TuEmb/can-viewer/blob/a58a3fdc4b60192f19abf020896190a66ecec426/src/main.rs#L29

3. you can download my material here, then run the software --> open --> choose to "can_db.dbc"
   can-viewer-simulate.zip

4. Finally you can run the script "vcan_loop.sh" to simulate CAN data

[ogoffart]

I think this is probably the same as #5041
Can you try disabling accessibility by tweaking the features of slint?

[TuEmb]

I updated the features in my project to:

slint = { version = "1.6.0", default-features = false, features = ["backend-winit", "compat-1-2", "renderer-winit-femtovg"] }

But the issue is still there

[TuEmb]

I used the valgrind to do profiling my app, but I have no experiment in reading this file. Could you help me find out what the problems are ?
callgrind.out.zip

I used callgrind to open this file:

[TuEmb]

After looking through all data I have, It seems when we use scroll view with un-fixed value of viewport-width and viewport-height, It has to allocate and deallocate a lot of time for redrawing. Because in my application, the elements inside ScrollView are flexible. I updated the ScrollView with fixed value, it seems the CPU load reduced a lot.

From the documentation, also mention about the loop inside ScrollView is not fully supported now: https://releases.slint.dev/1.6.0/docs/slint/src/language/widgets/scrollview.html

I'm looking forward to having this feature in Slint soon. Thanks for your response

[ogoffart]

Perhaps you can try to use a ListView instead of a ScrollView?

[ogoffart]

Regarding the way to use kcachegrind, i like to watch at the "Call Graph" tab and go to find which subtree is taking lots of instructions. Also you get better result by unchecking the "Cycle Detection" in the toolbar.

[TuEmb]

Thank @ogoffart,

It seems my application works more stable when I use ListView, But I got a problem with scrollbar. If I use the mouse hold the scrollbar and move up/down, the animation goes crazy.

ListView.webm