When running two Dispatch() calls consecutively, like:

m_computeCommandList->Dispatch(111, 1, 1);
m_computeCommandList->Dispatch(555, 1, 1);

Is it guaranteed that the second Dispatch() will run after the first Dispatch() on the GPU? Or, could they run concurrently on the GPU?

Just to clarify, there is no more C code in between those two Dispatch() calls.

CodePudding user response：

Like in other graphics API, when you execute command calls on CPU side it leads to putting these commands to a command queue. It guarantees that commands will be processed in the order of queue, First-In-First-Out.

However, on GPU everything becomes massive parallel and concurrent. We can't know on what processing unit the actual execution will be scheduled, or what threads from what Dispatch will be finished earlier. Typically it's not a problem if there are no resources (buffers, textures) shared between invocations, and we need to synchronize only the end of frame.

If there is resource sharing, there is a possibility of some memory conflicts ("write-read", "write-write" or "read-write"). Here we need to use resource barriers that allow us to organize access to these resources. Using different options for barriers you can reach the consecutive execution of different Dispatch calls.

For example, a transition from D3D12_RESOURCE_STATE_UNORDERED_ACCESS to D3D12_RESOURCE_STATE_NON_PIXEL_SHADER_RESOURCE|D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE will wait for ALL preceding Graphics and Compute shader execution to complete, and block ALL subsequent Graphics and Compute shader execution.

Enhanced barriers in DirectX 12 allow you to get fine-tuned control on resource and execution synchronization.