CODEWITHSUNDEEP
pythonmultithreading
Terrence Brannon
Terrence Brannon

Reputation: 4968

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

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?

Upvotes: 56

Views: 19685

Answers (2)

user2864740
user2864740

Reputation: 61885

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.

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.

Upvotes: 41

Alex Martelli
Alex Martelli

Reputation: 881735

All of Python's blocking I/O primitives release the GIL while waiting for the I/O block to resolve -- it's as simple as that! They will of course need to acquire the GIL again before going on to execute further Python code, but for the long-in-terms-of-machine-cycles intervals in which they're just waiting for some I/O syscall, they don't need the GIL, so they don't hold on to it!

Upvotes: 59

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