Building CPython

[BartC]

On 14/05/2015 17:29, Marko Rauhamaa wrote:
> BartC <bc at freeuk.com>:
>
>> That's a shame because I wanted to tinker with the main dispatcher
>> loop to try and find out what exactly is making it slow. Nothing that
>> seems obvious at first sight.
>
> My guess is the main culprit is attribute lookup in two ways:
>
>   * Each object attribute reference involves a dictionary lookup.
>
>   * Each method call involves *two* dictionary lookups plus an object
>     creation.
>
> Tell us what you find out.

I'm just starting but I can tell you that it isn't because debug mode 
(Py_DEBUG defined) was left on by mistake!

What is interesting however is that on the very simple test I'm doing (a 
while loop incrementing a variable up to 100 million), the timings under 
Windows are:

Python 2.5       9.2 seconds
Python 3.1      13.1
Python 3.4.3    17.0
Python 3.4.3    14.3 (under Ubuntu on same machine, using the version
                      I built today)

That's quite a big range!

PyPy does it in 0.7 seconds. The same program within my own *simple* 
bytecode interpreter (not for Python) has a fastest time of 1.5 seconds 
but makes use of ASM. A version 100% in (gcc) C can manage 2 seconds.

So far I haven't find anything that explains the discrepancy (the 
languages are different, mine is simpler, but the Python code isn't 
doing anything that complicated, only LOAD_FASTs and such, and LOAD_FAST 
is apparently just an array access.

But the nearly 2:1 difference between new and old Python versions is 
also intriguing.

def whiletest():
	i=0
	while i<=100000000:
		i=i+1

whiletest()

-- 
Bartc