Allow Multiple Frames in Flight

[cwfitzgerald]

Description

In order to get full use of the gpu, multiple frames should be in flight so the gpu doesn't have to wait for the cpu.

Repro steps

I have an example of what I think is needed to have multiple frames in flight to the hello-triangle example on a branch here: cwfitzgerald/wgpu-rs@6763396. This panics as I can't acquire more than one swapchain frame at a time.

Extra materials

https://github.com/gfx-rs/wgpu/blob/master/wgpu-core/src/swap_chain.rs#L194-L196 This code is spawning the error. To get this working, more than one outstanding frame would need to be able to be tracked, maybe up to some limit.

Note

There might be some extra complexity here, but I was discussing this on a vulkan discord channel, so I had to sift through a lot of vulkan specifics to get to how it all works. My understanding at this point is that this is all that is needed, but I could be very wrong.

[cwfitzgerald]

An interesting extension to this is what happens when a resize event is received. As far as I know, you need to get a new swapchain before the next presentation, or you'll hit vulkan issues. If there is a frame in the delay slot, and you get a resize event, there is no way in the api to cancel the presentation of that frame. I'm not entirely sure this'll be an issue, but something to keep in mind as we progress.

[kvark]

We need to move from "drop" semantics to explicit present() call - https://github.com/gfx-rs/wgpu-rs/issues/558. This will allow discarding those frames.

[cwfitzgerald]

I had no idea what I was talking about at this time.