Question

I’m rendering OpenGL Context on a background thread with a different EAGLContext than the main thread.

I use something like this:

- (void)renderInBackground {
      EAGLContext *context = [[EAGLContext] alloc] init];
      [EAGLContext setCurrentContext:context];


       Rendering..
    }

However, even though this is performed in a background thread, when using a heavy shader, the main thread gets blocked and the UI gets stuck.

Why is the background thread blocking the main thread?
the methods are not synchronized.

Answer 1

You do have finite CPU and GPU resources to perform all of the rendering and UI interactions, so if you max out the GPU you will slow down everything else in your application.

That said, you should be able to render on a background thread without completely halting all UI elements. I do some fairly intense rendering in my open source Molecules application, all of it using a background GCD queue, yet you can still scroll in popovers and otherwise interact with the interface.

I describe the process I use in this answer, but the basic setup is a single-wide GCD queue that relies on a dispatch semaphore to prevent the enqueueing of additional rendering frames while one is still being processed. Threads and blocks have some overhead to them, so if they are being fired off faster than they can be processed, this can lead to resource exhaustion. The semaphore prevents this.

Wrapping all interactions with my OpenGL ES context in this queue provides lock-free usage of this shared resource, and I see a significant performance boost over simply running this all on the main thread on the multicore devices. As I said, I’m still able to interact with the UI during even heavy rendering here.