multithreading - Why is a Python I/O bound task not blocked by the GIL?

Question

The python threading documentation states that "...threading is still an appropriate model if you want to run multiple I/O-bound tasks simultaneously", apparently because I/O-bound processes can avoid the GIL that prevents threads from concurrent execution in CPU-bound tasks.

But what I dont understand is that an I/O task still uses the CPU. So how could it not encounter the same issues? Is it because the I/O bound task will not require memory management?

question from:https://stackoverflow.com/questions/29270818/why-is-a-python-i-o-bound-task-not-blocked-by-the-gil

Answer 1

The GIL in CPython¹ is only concerned with Python code being executed. A thread-safe C extension that uses a lot of CPU might release the GIL as long as it doesn't need to interact with the Python runtime.

As soon as the C code needs to 'talk' to Python (read: call back into the Python runtime) then it needs to acquire the GIL again - that is, the GIL is to establish protection/atomic behavior for the "interpreter" (and I use the term loosely) and is not to prevent native/non-Python code from running concurrently.

Releasing the GIL around I/O (blocking or not, using CPU or not) is the same thing - until the data is moved into Python there is no reason to acquire the GIL.

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¹ The GIL is controversial because it prevents multithreaded CPython programs from taking full advantage of multiprocessor systems in certain situations. Note that potentially blocking or long-running operations, such as I/O, image processing, and NumPy number crunching, happen outside the GIL. Therefore it is only in multithreaded programs that spend a lot of time inside the GIL, interpreting CPython bytecode, that the GIL becomes a bottleneck.