MiddleCodeCommon

Why does this threaded CPU loop get no speedup?

This runs two threads of pure-Python counting, but is no faster than one.

Python

import threading
def count():
    x = 0
    for _ in range(10**7): x += 1

t1 = threading.Thread(target=count)
t2 = threading.Thread(target=count)
t1.start(); t2.start(); t1.join(); t2.join()

Determine why there is no speedup and how to fix it.

The two threads are no faster than sequential — often slower, from GIL contention — because count is pure-Python CPU work and the GIL lets only one thread run bytecode at a time. To parallelize, use multiprocessing.Process or a ProcessPoolExecutor.

Believing threads give CPU parallelism in CPython
Thinking the GIL only affects I/O, not CPU loops
Expecting a longer loop to overcome the GIL

Why does the GIL not hurt I/O-bound threaded code the same way?
How does multiprocessing achieve real CPU parallelism that threads cannot?

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