Multiple renders to single texture without blocking MTLCommandBuffer-CodePudding

I am trying to render 3 separate things to one texture in Metal. I have a MTLTexture that is used as a destination in 3 different MTLCommandBuffers. I commit them one after another. Each MTLCommandBuffer renders to a separate part of the texture - first draws in the 0 - 1/3 part, second draws the middle 1/3 - 2/3 and the last one draws 2/3 - 1.

id<MTLTexture> dst_texture = ...;
id<MTLCommandBuffer> buffer1 = [self drawToTexture:dst_texture];
[buffer1 commit];
id<MTLCommandBuffer> buffer2 = [self drawToTexture:dst_texture];
[buffer2 commit];
id<MTLCommandBuffer> buffer3 = [self drawToTexture:dst_texture];
[buffer3 commit];

The problem is that it seems I can't share the destination texture in the different command buffers - I get glitches, sometimes I can see only partial results on the destination texture... Inside drawToTexture I use dst_texture this way:

_renderPassDescriptor.colorAttachments[0].texture = dst_texture;
_renderPassDescriptor.colorAttachments[0].loadAction = MTLLoadActionLoad;

The problem gets fixed when I call [buffer waitUntilCompleted] after each individual commit but I suppose it affects the performance and I would love to have it without blocking/waiting.

This works:

id<MTLTexture> dst_texture = ...;
id<MTLCommandBuffer> buffer1 = [self drawToTexture:dst_texture];
[buffer1 commit];
[buffer1 waitUntilCompleted];
id<MTLCommandBuffer> buffer2 = [self drawToTexture:dst_texture];
[buffer2 commit];
[buffer2 waitUntilCompleted];
id<MTLCommandBuffer> buffer3 = [self drawToTexture:dst_texture];
[buffer3 commit];
[buffer3 waitUntilCompleted];

What else I could do here to avoid waitUntilCompleted calls?

CodePudding user response：

To answer the questions "I am trying to render 3 separate things in one texture in metal", and "What else I could do here to avoid waitUntilCompleted calls?" (Hamid has already explained why the problem occurs), is that you shouldn't be using multiple command buffers for basic rendering with multiple draw calls. If you're rendering to one texture then you need one command buffer which you use to create one renderPassDescriptor that you attach the texture to. Then you need one encoder created from the renderPassDescriptor where you can encode all the draw calls and change of buffers states etc..so as I said in the comment you bind the shader set buffers etc then draw and then don't call endEncoding but set shaders and buffers again and again for how many draw calls and buffer changes you want. If you wanted to draw to multiple textures then you typically create multiple renderPassDescriptors (but still use one command buffer). Generally you use one command buffer per frame, or for a set of offscreen render passes.

CodePudding user response：

The manual synchronization is only required:

For Untracked Resources.
Across Multiple Devices.
Between a GPU and the CPU.
Between separate command queues.

otherwise, metal automatically synchronizes resources (tracked) between the command buffers even if they are running in parallel.

If a command buffer includes write or read operations on a given MTLTexture, you must ensure that these operations complete before reading or writing the MTLTexture contents. You can use the addCompletedHandler: method, waitUntilCompleted method, or custom semaphores to signal that a command buffer has completed execution.