How does GIL affect the efficiency of multithreaded programs in Python?

GIL (Global Interpreter Lock) in Python is a mechanism that allows only one thread to execute bytecode at a time in the CPython interpreter. This means that even with multiple threads, they cannot execute Python code in parallel, limiting the efficiency of multithreaded programs, especially in CPU-intensive tasks.

However, GIL does not block I/O operations or calls in C extensions, so multithreading in Python is effective for I/O-bound tasks. For CPU-bound tasks, multiprocessing (the multiprocessing module) or alternative Python implementations without GIL are usually used.