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

[Chris Angelico]

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));
}


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