https://docs.google.com/document/d/1pVzU8w_QF44YzUCCab990Q_WZOdhpKolCIHaiXG-sPw/edit#heading=h.gh0cw4u6nbi5

Ruby memory model

The Ruby memory model is a framework allowing to reason about programs in concurrent and parallel environment. It defines what variable writes can be observed by a particular variable read, which is essential to be able to determine if a program is correct. It is achieved by defining what subset of all possible program execution orders is allowed.

A memory model sources:

• Java memory model, and its FAQ
• Java Memory Model Pragmatics
• atomic<> Weapons 1 and 2
• C++11 interactive MM

Concurrent behavior sources of Ruby implementations:

• Source codes.
• JRuby's wiki page
• Rubinius's wiki page

A similar document for MRI was not found. Key fact about MRI is GVL (Global VM lock) which ensures that only one thread can interpret a Ruby code at any given time. When the GVL is handed from one thread to another a mutex is released by first and acquired by the second thread implying that everything done by first thread is visible to second thread. See thread_pthread.c and thread_win32.c.

This memory model was created by: comparing MRI, JRuby, JRuby+Truffle, Rubinius; taking account limitations of the implementations or their platforms; inspiration drawn from other existing memory models (Java, C++11). This is not a formal model.