[Python-Dev] FAT Python (lack of) performance

[Ryan Gonzalez]

On January 25, 2016 9:59:36 PM CST, Chris Angelico <rosuav at gmail.com> wrote:
>On Tue, Jan 26, 2016 at 2:32 PM, INADA Naoki <songofacandy at gmail.com>
>wrote:
>>
>> I know.
>> But people compares language speed by simple microbench like
>fibbonacci.
>> They doesn't use listcomp or libraries to compare *language* speed.
>>
>
>Well, that's a stupid way to decide on a language. Here, look: Python
>is faster than C. Proof!
>
>rosuav at sikorsky:~$ time python3 fib.py
>2880067194370816120
>
>real 0m0.033s
>user 0m0.032s
>sys 0m0.000s
>rosuav at sikorsky:~$ cat fib.py
>import functools
>
>@functools.lru_cache()
>def fib(n):
>    if n < 2: return n
>    return fib(n-2) + fib(n-1)
>
>print(fib(90))
>
>rosuav at sikorsky:~$ gcc fib.c && time ./a.out
>1134903170
>
>real 0m9.104s
>user 0m9.064s
>sys 0m0.000s
>rosuav at sikorsky:~$ cat fib.c
>#include <stdio.h>
>
>unsigned long fib(unsigned long n)
>{
>    if (n < 2) return n;
>    return fib(n-2) + fib(n-1);
>}
>
>int main()
>{
>    printf("%lu\n",fib(45));
>}
>

*cough* -O3 *cough*

>
>Algorithmic differences - even subtle ones - can easily outdo choice
>of language for run-time performance. And if you try to write a true C
>equivalent of that Python code, good luck - I'll have the Python one
>written and running while you're still trying to figure out how to
>write a cache, much less how to keep the syntax clean as you add a
>cache to an existing function.
>
>Of course, rewriting the whole thing to work iteratively instead of
>double-recursively will make a dramatic difference to both programs.
>That's an unsubtle algorithmic difference, and if you're coding like
>that, you probably can't see the difference in performance between any
>two languages at anything up to a machine word (about the 90th or so
>Fibonacci number, on a 64-bit system) - all you'll see is the startup
>performance. As soon as you go beyond a machine word, Python massively
>trumps C, because its default integer type is a bignum. Going beyond a
>machine word in C is a hassle. Going beyond a machine word in Python 2
>is almost insignificant - hey look, now your repr has an 'L' on the
>end, and performance is immeasurably worse. In Python 3, there's no
>machine word whatsoever.
>
>So, yeah... Python beats C for Fibonacci calculation, too. You just
>have to not be stupid with your benchmarking.
>
>ChrisA
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