Using Metal API in background thread on macOS

my code calculates parts (tiles) of the final image on concurrent threads within CGContext. I need to pass the partial result of each calculation (vertex data) within a thread to the GPU for rendering by using MPSImageGaussianBlur. After that, I create the final bitmap from the resulting MTLTexture and draw it back to CGContext and the original thread. All render passes, on every thread, utilize a common commandQueue (created on the main thread). Each render pass initiates and terminates within the same thread. Other threads do not require access to resources from other threads/render passes as all data are isolated within their respective threads. However, I'm uncertain how to call Metal API ( encode commands, etc.) from threads other than the main thread in this circumstance. Despite this, I cannot modify the way I calculate vertex data (in concurrent threads). Do you have any advice on how to carry out this task securely? Thank you.

You can use the MTLCommandQueue in multiple threads. It doesn't matter where the command queue is created. The methods of command queue are thread safe, which means you can create a command buffer and fill it out on a background thread and then commit it. You can either enqueue your MTLCommandBuffers before giving them away to a background thread, if the command buffers need to go in a specific order. Or you can just commit them as they are ready.

Keep in mind, that the even though APIs on MTLDevice, MTLCommandQueue are safe, the APIs on MTLCommandBuffer and everything below, so any command encoder, are not thread safe. So basically, you can give each thread a command buffer, but you can't give a single command buffer to more than one thread. The only exception here is parallel render command encoders, which let you create "sub-encoders", that you can then also share between multiple threads.