What is the global interpreter lock (GIL) in CPython?

Question

What is a global interpreter lock and why is it an issue?

A lot of noise has been made around removing the GIL from Python, and I\'d like to understand why that is s

Answer 1

I want to share an example from the book multithreading for Visual Effects. So here is a classic dead lock situation

static void MyCallback(const Context &context){
Auto lock(GetMyMutexFromContext(context));
...
EvalMyPythonString(str); //A function that takes the GIL
...    
}

Now consider the events in the sequence resulting a dead-lock.

╔═══╦════════════════════════════════════════╦══════════════════════════════════════╗
║   ║ Main Thread                            ║ Other Thread                         ║
╠═══╬════════════════════════════════════════╬══════════════════════════════════════╣
║ 1 ║ Python Command acquires GIL            ║ Work started                         ║
║ 2 ║ Computation requested                  ║ MyCallback runs and acquires MyMutex ║
║ 3 ║                                        ║ MyCallback now waits for GIL         ║
║ 4 ║ MyCallback runs and waits for MyMutex  ║ waiting for GIL                      ║
╚═══╩════════════════════════════════════════╩══════════════════════════════════════╝