CODEWITHSUNDEEP
vulkan
Bioliquid
Bioliquid

Reputation: 191

Should the amount of resource allocations be "per swap chain image"?

I just learned about uniform buffers (https://vulkan-tutorial.com/Uniform_buffers/Descriptor_layout_and_buffer) and a bit confused about the size of uniformBuffers and uniformBuffersMemory. In the tutorial it is said that:

We should have multiple buffers, because multiple frames may be in flight at the same time and we don't want to update the buffer in preparation of the next frame while a previous one is still reading from it! We could either have a uniform buffer per frame or per swap chain image.

As far as I understand "per swap chain image" approach is more optimal. Please, prove me wrong, if I am. But why do we need it to be the size of swapChainImages.size()? Isn't MAX_FRAMES_IN_FLIGHT just enough, because we have fences? As a simple example, if we have just a single frame in flight and do vkDeviceWaitIdle after each presentation then our single uniform buffer will always be available and not used by cpu/gpu so we don't need an array of them.

Upvotes: 3

Views: 765

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473272

do vkDeviceWaitIdle

OK, stop right there. There is basically only one valid reason to call that function: you need to delete every resource created by that device, because you're about the destroy the device, so you wait until all such resources are no longer being used.

Yes, if you halt the CPU's execution until the GPU stops doing stuff, then you're guaranteed that CPU writes to GPU memory will not interact with GPU reads from that memory. But you purchased this guarantee by ensuring that there will be no overlap at all between CPU execution and GPU execution. The CPU sets up some stuff, sends it to the GPU, then waits till the GPU is done, and the CPU starts up again. Everything executes perfectly synchronously. While the CPU is doing work, the GPU is doing nothing. And vice-versa.

This is not a recipe for performance. If you're going to use a graphics API designed to achieve lots of CPU/GPU overlap, you shouldn't throw that away because it's easier to work with.

Get used to multi-buffering any resources that you modify from the CPU on a regular basis. How many buffers you want to use is your choice, one that should be informed by the present mode and the like.

My question is "Do I need n buffers or m is enough?".

The situation you're describing ultimately only happens if your code wanted to have X frames in flight, but the presentation engine requires you to use a minimum of Y swap-chain images, and X < Y. So the question you're asking can be boiled down to, "if I wanted to do double-buffering, but the implementation forces 3 buffers on me, is it OK if I treat it as double-buffering?"

Yes, as long as you're not relying on the vkAcquireNextImage call to block the CPU for your synchronization. But you shouldn't be relying on that anyway, since the call itself doesn't constitute a proper barrier as far as the Vulkan execution model is concerned. You should instead block the CPU on fences tied to the actual work, not on the acquire process.

Upvotes: 4

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