Getting stuck at 40 fps even with using CADisplayLink?

Question

At first I used NSTimer and ran into the problem of the frame rate being stuck at 40 frames per second. I've read about using CADisplayLink to fix the problem. It seemed to work for a while, but recently, the app starts at 60, then after a little while (about 5 - 20 seconds in) the app starts to run at 40 - 41 fps and gets stuck there. And I'm culling so that there's less draws when objects are out of view, but yet it stays stuck there. A unusual way that does seem to remedy the problem for a moment (about 10 seconds) is to disconnect the wire from the bottom of the iphone and then connect it again. Obviously, this is not ideal, but just wondering why does this happen and why does it somewhat fix it, as if it resets certain values or something. ANY help will be greatly appreciated, thanks.

Answer 1

Have you checked to see whether you're actually dropping frames? Are you using the CPU/GPU to the extent that you can only draw 40 frames in a second, or is the problem somewhere in the software?

Remember that if a CADisplayLink is unable to fire for a screen refresh (due to the main thread being busy doing things like drawing the previous frame) then it may skip it, as it won't have enough time to complete. So if some of your frames take more than 0.01666... seconds to draw, you'll miss the interval to draw the frames right after, and the slowdown may appear worse than it is.

Also remember that a mobile device is not always capable of drawing everything quickly. The A4 processor in an iPhone 4 is very good, but it's still a tall order to fill 960x640 screen pixels 60 times a second. The vast majority of your users will not notice if your frame rate is 40fps instead of 60fps--the human eye doesn't generally notice judder or screen lag until you're around 20fps and lower.