Metal renderer fps cap?

SDL Development
1

I have noticed a bit of a strange behaviour with the SDL renderer on a Mac. When I use the metal renderer it seems like the fps is being capped at around 90-130 fps, even for empty scenes (no actual drawing, just clear and present). When I choose the OpenGL renderer with otherwise the exact same settings, I get uncapped frame rates that easily go into the thousands. So it does not appear to be related to the PRESENTVSYNC flag.

It seems like it does not matter if I specifically choose the metal driver or if I pass -1 to Createrenderer and let it choose by itself. As long as the metal driver is used I only get around 120 fps.

It does not seem to be bad performance, as if I make the scene much more complex I still get the same 120 fps. So it behaves more like a cap. With OpenGL I see a concrete link between scene complexity and fps, the more I draw the lower the fps. With metal it stays at around 120.

Just curious if anyone saw this behaviour and if it’s expected or maybe I’m doing something wrong.

2

Are you running fullscreen or windowed? What version of macOS?

I think when windowed there’s a cap when it comes to Metal as well. But I’ve seen it it even in code that isn’t using SDL, so I think it’s a macOS thing, not a bug in SDL.

Probably has something to do with the way the drawables swapchain interacts with the rest of the windowing system when running in windowed mode.

3

I don’t think this is a bug in SDL. It’s probably some Metal thing. I also noticed that moving the window to a different screen changed the fps to a different value. I was just curious if anyone knows about this.

My results were from running in a window, I did not try Fullscreen yet.

I am almost sure that it was different on an earlier OS version, but I don’t have any way to go back and check this. I can confirm that this is not a regression with 2.0.18 as older SDL versions have the same behavior.