与@synchronized(self)关联的开销是多少?

I have been working on an object class for my iPhone app that lazy loads images when they are first requested by another part of the app. I decided to make the loading of the image thread safe so the same image doesn't get loaded twice by accident, however I am curious about the overhead of making an @synchronized(self) call every time the accessor is run, like so:I have been working on an object class for my i