Question

I am running a multithreaded application(Python2.7.3) in a Intel(R) Core(TM)2 Duo CPU E7500 @ 2.93GHz. I thought it would be using only one core but using the “top” command I see that the python processes are constantly changing the core no. Enabling “SHOW THREADS” in the top command shows diffrent thread processes working on different cores.

Can anyone please explain this? It is bothering me as I know from theory that multithreading is executed on a single core.

Answer 1

First off, multithreading means the inverse, namely that multiple cores are being utilized (via threading) at the same time. CPython is indeed crippled when it comes to this, though whenever you call into C code (this includes parts of the standard library, but also extension modules like Numpy) the lock which prevents concurrent execution of Python code may be unlocked. You can still have multiple threads, they just won’t be interpreting Python at the same time (instead, they’ll take turns quite frequently). You also speak of “Python processes” — are you confusing terminology, or is this “multithreaded” Python application in fact multiprocessing? Of course multiple Python processes can run concurrently.

However, from your wording I suspect another source of confusion. Even a single thread can run on multiple cores… just not at the same time. It is up to the operating system which thread is running on which CPU, and the OS scheduler does not necessarily re-assign a thread to the same CPU where it used to run before it was suspended (it’s beneficial, as David Schwartz notes in the comments, but not vital). That is, it’s perfectly normal for a single thread/process to jump from CPU to CPU.