From: "Adam P. Jenkins" <thorin@...>
Date: 2005-07-30T04:41:02+09:00
Subject: Re: Ruby and threads

Yohanes Santoso wrote:
> You can test whether current python implementation still use a global
> interpreter lock or not by trying to span multiple threads, each doing
> some CPU-intensive computation (e.g. repeatedly increasing the value
> of some thread-local variable) on a MP machine, and then comparing the
> result with the same program on the same MP machine with less number
> of CPUs activated.
> 
> I can't do the above test since I don't have access to a MP machine
> anymore. So, I'm waiting for your response.

Hmm, I just tried on an 8p ia64 machine running Linux, with python 2.3. 
  I tried the following program, which starts 4 threads, which each 
execute a function which just busy loops.  I then watched what went on 
in "top" in another terminal.  If I execute the function in just one 
thread, then one of the CPUs goes to 99% usage.  If I execute 4 threads, 
then 4 of the CPUs go to about %20 usage.  So it does seem that a global 
interpreter lock is still being used in Python's implementation.  Too 
bad, though I suppose I wasn't about to go and write computationally 
intensive code in Python anyway.

from threading import Thread

nthreads = 4

def busyloop(n=2**26):
     i = 0
     while i < n:
         i = i + 1

threads = []

for i in range(nthreads):
     th = Thread(target=busyloop)
     th.start()
     threads.append(th)

for th in threads:
     th.join()