Vulkan: Sync of UniformBuffer and StoreBuffer

Question

UniformBuffers and StoreBuffers are updated, on the CPU side, using memcpy. How does synchronization work for those descriptor types? Does using memcpy imply that the application waits for memcpy to upload data to the GPU prior to continuing to next statement? If so, does this mean that barriers are not needed for sync'ing these types of buffers?

Answer 1

Synchronization works the same way for any memory resource: with certain rare exceptions, if you've changed memory, you need a memory dependency to ensure visibility of those changes. The synchronization system doesn't care whether it's used as a UBO or whatever. It cares about the nature of the source operation (the host) and the destination operation (reading from certain shader stages).

For host-to-device memory operations, you need to perform a form of synchronization known as a "domain operation". Fortunately, vkQueueSubmit automatically performs a domain operation on any host writes made visible before the vkQueueSubmit call. So if you write stuff to GPU-visible memory, then call vkQueueSubmit (either in the same thread or via CPU-side inter-thread communication), any commands in that submit call (or later ones) will see the values you wrote.

Assuming you have made them visible. Writes to host-coherent memory are always visible to the GPU, but writes to non-coherent memory must be made visible via a call to vkFlushMappedMemoryRanges.

If you want to write to memory asynchronously to the GPU process that reads it, you'll need to use an event. You write to the memory, make it visible if needs be, then set the event. The GPU commands that read from it would wait on the event, using VK_ACCESS_HOST_WRITE_BIT as the source access, and VK_PIPELINE_STAGE_HOST_BIT as the source stage. The destination access and stage are determined by how you plan to read from it.

Vulkan knows nothing about memcpy. It doesn't care how you modify the memory; it only cares that you do so in accord with its rules.