CODEWITHSUNDEEP
vulkan
maxest
maxest

Reputation: 91

Vulkan - How to efficiently copy data to CPU *and* wait for it

Let's say I want to execute the following commands:

cmd_buff start
dispatch (write to texture1)
copy (texture1 on gpu to buffer1 host-visible)
dispatch (write to texture2)
cmd_buff end

I'd like to know as soon as possible when buffer1's data are available.

My idea here is to have a waiting thread on which I'd wait for the copy to have completed. What I'd do is first split the above list of cmds into:

cmd_buff_1 start
dispatch (write to texture1)
copy (texture1 on gpu to buffer1 host-visible)
cmd_buff_1 end

and:

cmd_buff_2 start
dispatch (write to texture2)
cmd_buff_2 end

Now, I'd call vkQueueSubmit with cmd_buff_1 and with some fence1, followed by a call to another vkQueueSubmit with cmd_buff_2 with NULL fence.

On the waiting thread I'd call vkWaitForFences( fence1 ).

That's how I see such an operation. However, I'm wondering if that is optimal and if there was actually any way to put a direct sync still within cmd_buff_1 so that I wouldn't need to split the cmd buffer into two?

Upvotes: 1

Views: 1338

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473272

Never break up submit operations just to test fences; submit operations are too heavyweight to do that. If the CPU needs to check to see if work on the GPU has reached a specific point, there are many options other than a fence.

The simplest mechanism for something like this is to use an event. Set the event after the transfer operation, then use vkGetEventStatus on the CPU to see when it is ready. That's a polling function, so a waiting CPU thread won't immediately wake up when the data is ready (but then, there's no guarantee that would happen with a non-polling function either).

If timeline semaphores are available to you, you can wait for them to reach a particular counter value on the CPU with vkWaitSemaphores. This requires that you break the batch up into two batches, but they can both be submitted in the same submit command.

Upvotes: 3

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