Serializable dispatch queue create - ios

Question

I need to create a serializable Dispatch queue.
I'm using AudioUnit to register from the microphone, and then encoding the frame on the created queue.
I have one object encoding, so I need always the same thread accessing it.
However, when I create my queue :
queue = dispatch_queue_create("com.myapp.queue", DISPATCH_QUEUE_SERIAL);
If I dispatch to this queue with :
NSLog(#"recording callback Thread Info: %#", [NSThread currentThread]);
dispatch_async(queue, ^{
[self processAudio];
});
- (void) processAudio
{
NSLog(#"processAudio Thread Info: %#", [NSThread currentThread]);
...
}
It is mostly using the same thread until one point where I get this :
recording callback Thread Info: <NSThread: 0x7fb0ab331b90>{number = 12, name = (null)}
processAudio Thread Info: <NSThread: 0x7fb0ab210140>{number = 7, name = (null)}
recording callback Thread Info: <NSThread: 0x7fb0ab331b90>{number = 12, name = (null)}
processAudio Thread Info: <NSThread: 0x7fb0a8e33950>{number = 8, name = (null)}
recording callback Thread Info: <NSThread: 0x7fb0ab331b90>{number = 12, name = (null)}
processAudio Thread Info: <NSThread: 0x7fb0ab212bf0>{number = 13, name = (null)}
The dispatch_async will switch thread, and then stick a while with thread number 13, until it will switch again.
Is that a normal behavior although I specified that I wanted a SERIAL queue ?
Should I be worried about switching thread, when using only one instance of an object, or is it really serialized ?

Answer 1

This is expected behavior and you should not worry about it. From Apple's docs (https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html):
Serial queues (also known as private dispatch queues) execute one task at a time in the order in which they are added to the queue. The currently executing task runs on a distinct thread (which can vary from task to task) that is managed by the dispatch queue.