Cocos2D 2.1 - High CPU usage while idle

Question

I have this iPhone app created in Cocos2d 2.1.

WHen the app first starts it presents a full screen background and a UIACtionSheet where the user has to choose one option to start the application. That's it. This actionsheet is presented by the first scene to run by the delegate (HelloWorldLayer scene).

Nothing else is loaded. Nothing is running on any thread that I am responsible for that. Even so, the CPU usage is about 20%. I have profiled that on instruments and this is what I have:

I see this [CCGLView swapBuffers] that is inside something that calls attention, ths CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION (CoreFoundation).

If I choose "show obj-c only" I see this:

Apparently the guilty is this drawScene but how can a scene with a background image and a UIActionsheet use that much CPU?

How do I trace the source of this problem?

NOTE. This is being tested on the device. On simulator CPU usage is 77%!!!!!

Answer 1

Cocos2d, like any OpenGL application, redraws the contents of the screen every frame. Typically that means 60 frames per second. Swapping the frame buffers and all the other overhead associated takes some CPU time.

So what you're observing is normal behavior, there's nothing that can be done about it. There is no concept of "idle" in a rendering engine, it draws the contents of the screen even if they didn't change (or the screen is empty) on the assumption that screen contents usually do change.

You can however pause the CCDirector to reduce framerate, conserving CPU cycles and battery. Or call stopAnimation to completely stop all updates, including scheduled selectors. However what happens to the framebuffer is undefined, usually it sticks around (ie the last rendered frame "freezes") but this may not be true for all situations and devices.

As far as Simulator is concerned: ignore it. Its performance characteristics are not the least bit comparable nor instructive.