CODEWITHSUNDEEP
c++vulkan
Nowine
Nowine

Reputation: 196

Why I don't need memory barriers when starting drawing on an acquired swapchain image?

I'm learning Vulkan and my experience with memory barriers was quite good until I have to deal with memory visibility.

I feel like I have to use a memory barrier each time I start using a ressource for reading when I was previously writing on it, and inversly. A bit like if there was a state on the memory which says if it's used for writing or for reading. I know that the rationals for this are related to cache management, but at a higher level that's how I see it.

Bad things start when I don't see memory barriers, where according to my (very likely wrong) understanding they should be.

For example, if I want to draw something and present it on the screen, there is no memory barrier to make a transition from a swapchain image used for presentation (and thus for reading) to an image used for drawing (and thus for writing). And when I finish drawing, there is no barrier in the reverse order aswell.

I have seen the same thing happen when copying a staging host visible buffer to a device local buffer. You write something in the mapped memory, flush it, and then start recording the copy in a command buffer without putting any barrier to transition from a host writable memory to transfer read memory. So I'd like to know what I misunderstand or what implicits things make everything work out of the box.

Upvotes: 2

Views: 315

Answers (1)

krOoze
krOoze

Reputation: 13276

No barrier between presentation is illegal. The swapchain image must be in VK_IMAGE_LAYOUT_PRESENT_SRC_KHR for presentation. And it must be in different layout when your app does write something to the image. Only way to achieve this is with barrier-like primitive.

Writes to mapped memory is one rare exception. Writes to mapped memory are automatically visible to any subsequent vkQueueSubmit. See Host Write Ordering Guarantees chapter of the specification.

Why the tutorial does not have barriers there is because it covers synchronization in the next chapter you assumably did not reach. They do so with Subpass Dependencies. The layout transitions that are part of that are shown in earlier chapter about render passes.

Upvotes: 2

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