[Numpy-discussion] numpy speed question

Gökhan Sever gokhansever@gmail....
Thu Nov 25 15:34:24 CST 2010


On Thu, Nov 25, 2010 at 4:13 AM, Jean-Luc Menut <jeanluc.menut@free.fr> wrote:
> Hello all,
>
> I have a little question about the speed of numpy vs IDL 7.0. I did a
> very simple little check by computing just a cosine in a loop. I was
> quite surprised to see an order of magnitude of difference between numpy
> and IDL, I would have thought that for such a basic function, the speed
> would be approximatively the same.
>
> I suppose that some of the difference may come from  the default data
> type of 64bits in numpy and 32 bits in IDL. Is there a way to change the
> numpy default data type (without recompiling) ?
>
> And I'm not an expert at all, maybe there is a better explanation, like
> a better use of the several CPU core by IDL ?
>
> I'm working with windows 7 64 bits on a core i7.
>
> any hint is welcome.
> Thanks.
>
> Here the IDL code :
> Julian1 = SYSTIME( /JULIAN , /UTC )
> for j=0,9999 do begin
>   for i=0,999 do begin
>     a=cos(2*!pi*i/100.)
>   endfor
> endfor
> Julian2 = SYSTIME( /JULIAN , /UTC )
> print, (Julian2-Julian1)*86400.0
> print,cpt
> end
>
> result:
> % Compiled module: $MAIN$.
>        2.9999837
>
>
> The python code:
> from numpy import *
> from time import time
> time1 = time()
> for j in range(10000):
>     for i in range(1000):
>         a=cos(2*pi*i/100.)
> time2 = time()
> print time2-time1
>
> result:
> In [2]: run python_test_speed.py
> 24.1809999943
>
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>

Vectorised numpy version already blow away the results.

Here is what I get using the IDL version (with IDL v7.1):

IDL> .r test_idl
% Compiled module: $MAIN$.
       4.0000185

I[10]: time run test_python
43.305727005

and using a Cythonized version:

from math import pi

cdef extern from "math.h":
    float cos(float)

cpdef float myloop(int n1, int n2, float n3):
    cdef float a
    cdef int i, j
    for j in range(n1):
        for i in range(n2):
            a=cos(2*pi*i/n3)

compiling the setup.py file python setup.py build_ext --inplace
and importing the function into IPython

from mycython import myloop

I[6]: timeit myloop(10000, 1000, 100.0)
1 loops, best of 3: 2.91 s per loop


-- 
Gökhan


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