AVAudioSession Buffer Duration on iphone6* Simulator - buffer

Question

I'm setting a preferred buffer duration of 0.0001 seconds using AVAudioSession and not getting logical results using the simulators.
[session setPreferredIOBufferDuration:self.bufferDuration error:&audioSessionError];
if (audioSessionError) {
NSLog(#"Error %ld, %#",
(long)audioSessionError.code, audioSessionError.localizedDescription);
}
The problem is that in my Audio Unit render callbacks, I always get 512 frames to process as the inNumberFrames argument.
On my device, setting the preferred buffer duration results in different buffer rates. For example, if I set self.bufferDuration and then set the AVAudioSession with 0.1, than I will get 4096 sized inNumberFrames arguments to my render callbacks. On the simulator, it will be 512.
I wanted to check if this is normal behavior (i know many things don't work identically on the simulator and device), or if this is a problem with my assumptions.

Answer 1

Note that the setPreferredIOBufferDuration setting is just a preference suggestion not a hard setting. The OS is free at run-time to choose a number of frames (duration of actual data times the sample rate) to send to audio callbacks, and to even change that number while an app is running audio in the foreground. The actual duration may vary between different iOS devices and Mac systems. The duration might also depend the sample rate or format, the audio route, whether there are any other background app audio sessions that are currently active, the audio settings used by the immediately prior app, and/or on the OSX or iOS version and version of iOS Simulator.
For a requested buffer duration of 0.0053, I seem to get 512 frames on the iOS 9.2 Simulator, and 256 frames on an iPhone 6s (only the latter matching the request, but this will not be true at all common sample rates). Some older iOS devices will not return a frame count below 256.
It is invalid to assume that inNumberFrames will correspond to an app's preferred buffer duration setting.