Python is darn fast (was: How fast is Python)
Tim Hochberg
tim.hochberg at ieee.org
Sun Aug 24 22:40:24 CEST 2003
Michele Simionato wrote:
> I finally came to the conclusion that the exceeding good performance
> of Psyco was due to the fact that the function was called a million
> times with the *same* argument. Evidently Psyco is smart enough to
> notice that. Changing the argument at each call
> (erfc(0.456) -> i/1000000.0) slows down Python+Psyco at 1/4 of C speed.
> Psyco improves Python performance by an order of magnitude, but still it
> is not enough :-(
This is not suprising. Last I checked, Psyco does not fully compile
floating point expressions. If, I rememeber correctly (though every time
try to delve too deeply into Psyco my brains start oozing out my ears),
there are three ways a in which a given chunk of code evaluated. At one
level, which I'll call #1, Psyco generates the machine code(*) for the
expression. At a second level, Psyco calls out to C helper functions,
but still works with unboxed values. At the third level, Psyco punts and
creates a Python object and hands things off to the interpreter.
Most integer functions operate at level #1, so they tend to be quite
fast. Most floating point operations operate at level #2, so they have a
certain amount of overhead, but are still much faster than unpsyco
(sane?) Python. I believe the reason for this is that x86 floating point
operations are very messy, so Armin punted...
(*) Armin is working on virtual machine implementation of Psyco, so it
should be available on non x86 machines soon.
FWIW,
-tim
> I was too optimistic!
>
> Here I my numbers for Python 2.3, Psyco 1.0, Red Hat Linux 7.3,
> Pentium II 366 MHz:
>
> $ time p23 erf.py
> real 0m3.245s
> user 0m3.164s
> sys 0m0.037s
>
> This is more than four times slower than optimized C:
>
> $ gcc erf.c -lm -O3
> $ time ./a.out
> real 0m0.742s
> user 0m0.725s
> sys 0m0.002s
>
> Here is the situation for pure Python
>
> $time p23 erf.jy
> real 0m27.470s
> user 0m27.162s
> sys 0m0.023s
>
> and, just for fun, here is Jython performance:
>
> $ time jython erf.jy
> real 0m44.395s
> user 0m42.602s
> sys 0m0.389s
>
> ----------------------------------------------------------------------
>
> $ cat erf.py
> import math
> import psyco
> psyco.full()
>
> def erfc(x):
> exp = math.exp
>
> p = 0.3275911
> a1 = 0.254829592
> a2 = -0.284496736
> a3 = 1.421413741
> a4 = -1.453152027
> a5 = 1.061405429
>
> t = 1.0 / (1.0 + p*x)
> erfcx = ( (a1 + (a2 + (a3 +
> (a4 + a5*t)*t)*t)*t)*t ) * exp(-x*x)
> return erfcx
>
> def main():
> erg = 0.0
>
> for i in xrange(1000000):
> erg += erfc(i/1000000.0)
>
> if __name__ == '__main__':
> main()
>
> --------------------------------------------------------------------------
>
> # python/jython version = same without "import psyco; psyco.full()"
>
> --------------------------------------------------------------------------
>
> $cat erf.c
> #include <stdio.h>
> #include <math.h>
>
> double erfc( double x )
> {
> double p, a1, a2, a3, a4, a5;
> double t, erfcx;
>
> p = 0.3275911;
> a1 = 0.254829592;
> a2 = -0.284496736;
> a3 = 1.421413741;
> a4 = -1.453152027;
> a5 = 1.061405429;
>
> t = 1.0 / (1.0 + p*x);
> erfcx = ( (a1 + (a2 + (a3 +
> (a4 + a5*t)*t)*t)*t)*t ) * exp(-x*x);
>
> return erfcx;
> }
>
> int main()
> {
> double erg=0.0;
> int i;
>
> for(i=0; i<1000000; i++)
> {
> erg = erg + erfc(i/1000000.0);
> }
>
> return 0;
> }
>
> Michele Simionato, Ph. D.
> MicheleSimionato at libero.it
> http://www.phyast.pitt.edu/~micheles/
> ---- Currently looking for a job ----
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