As the NSOperationQueue
Class Reference said:
In iOS, operation queues do not use Grand Central Dispatch
to execute operations. They create separate threads for non-concurrent operations and launch concurrent operations from the current thread.
But I found in iOS 4.2.1,queue always create threads for concurrent or non-concurrent operations. And I think queue don't care concurrent or non-concurrent at all, because a breakpoint in isConcurrent never be broken. So I think the Reference was wrong...