`eframe` on Windows high CPU usage due to excessive calls to `glutin::context::make_not_current`

[gerwin3]

I noticed that my application CPU usage was much higher than expected on Windows. On a similar machine I was seeing 1%-2% usage on Linux, and around 20%-30% on Window while moving the mouse in window (just mouse movement).

After some optimization I got my rendering loop down to less than 1ms and still the CPU usage was too high. I then sampled the CPU with https://github.com/nico-abram/blondie/tree/main and this is the flamegraph that it produces:

As you can see, almost 90% of the time, it is calling the make_not_current function, which seems really excessive...

EDIT: I found in the docs here that this call flushes the rendering context so probably rendering is just very slow... Still not sure why though.

Screenshots

Another flamegraph with Windows Performance Analyzer (similar results):

This shows the call stack a bit deeper:

• opengl32d.dll wglMakeCurrent
• opengl32d.dll bMakeNoCurrent
• opengl32d.dll wglMakeCurrent
• opengl32d.dll wglMakeCurrent
• opengl32d.dll wglMakeCurrent
• KernelBase SetEvent

Desktop (please complete the following information):

• OS: Windows
• Version: egui 0.26

[emilk]

I think this was introduced in #3172 when we added support for multiple viewports. When there are multiple viewports we need to switch to which has the "current" gl context, and we unfortunately pay this cost even when there is only one viewport, but that should be fixable.

[gerwin3]

Is there a workaround for now (i.e. options to disable multi-viewport)? (Apart from reverting to an older version)

[emilk]

This is still an issue on Windows.

If someone wants to work on this, please take a look at

• eframe: Early-out from context switching the glow backend #4284
• Only avoid glow context switching on Windows #4296

[gerwin3]

Seems that the reverted fix (#4284), even when it wasn't reverted before still did not fix the issue. CPU load does not noticeably decrease and still hovers around 30% on my machine in the hello_world example.

[emilk]

@gerwin3 With a single viewport or multiple?

In either case: I suggest if you want this fixed you start rolling up your selves ;)

Maybe start verifying wether or not this was introduced in #3172