How does Apple's Objective-C runtime do multithreaded reference counting without degraded performance?

Question

So I was reading this article about an attempt to remove the global interpreter lock (GIL) from the Python interpreter to improve multithreading performance and saw somethin

Answer 1

In addition to what bbum said, a lot of the most frequently thrown around objects in Cocoa override the normal reference counting mechanisms and store a refcount inline in the object, which they manipulate with atomic add and subtract instructions rather than locking.

(edit from the future: Objective-C now automatically does this optimization on modern Apple platforms, by mixing the refcount in with the 'isa' pointer)