gh-116167: Allow disabling the GIL

Re:gh-116167: Allow disabling the GIL by orf on Monday March 11, 2024 @03:56PM (#39672588)

But this would now make the reference count increment require a conditional? It’s a very hot path, and this would cause a slowdown for single-threaded Python code.

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Re:gh-116167: Allow disabling the GIL by vlovich123 on Monday March 11, 2024 @04:53PM (#39673120)

It's already taking a conditional. Take a look at the PEP:

    if (op->ob_tid == _Py_ThreadId())
      op->ob_ref_local = new_local;
    else
      atomic_add(&op->ob_ref_shared, 1 << _Py_SHARED_SHIFT);

So you're either getting a correct branch prediction or an atomic operation which will dominate the overhead of the branch anyway. All this is saying is in the else branch where you're doing the atomic add, create a new PythonObj instance that has `ob_tid` equal to `_Py_ThreadId`. This presumes that Py_INCREF changes the return type from void to `PythonObj*` and this propagates out so that futher on-thread references use the newer affinity (branch condition is always taken to the non-atomic add instead of the atomic one). It's easier said than done and there may be technical reasons why that's difficult / not possible, but worth exploring eventually so that access by multiple threads of a single object doesn't degrade to taking atomic reference counts constantly.

https://peps.python.org/pep-0703/

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