I think the confusion here is that CPython and PyPy provide single-process concurrency (but not parallelism) without STM. The GIL allows multiple threads to exe

I think the confusion here is that CPython and PyPy provide single-process concurrency (but not parallelism) without STM. The GIL allows multiple threads to execute concurrently within a single process, but only one thread to execute at a time. PyPy is implementing an STM solution to provide single-process parallelism in Python, allowing multiple threads to execute at a time, and detecting and recovering (re-executing) when they conflict. So while STM can be used in concurrent execution without parallelism, existing Python interpreters provide concurrent execution without STM, and PyPy only needs STM to provide parallelism.