[Numpy-discussion] numpy and math sqrt timings

[Matthieu Brucher]

Hi,

This is a know fact, you should use Python default functions if you have
only one value.
If Numpy uses math.sqrt for floatting point number, it would have to use
cmath for complex values as well. Now, I don't know if an additionnal test
will slow down Numpy, if this is the case, then we should stay with the
current situation ; if I have a signle value to compute, I always use math
instead of Numpy.

Matthieu

2007/12/29, Bruce Sherwood <Bruce_Sherwood at ncsu.edu>:
>
> On the VPython list Scott Daniels suggested using try/except to deal
> with the problem of sqrt(5.5) being numpy.float64 and thereby making
> sqrt(5.5)*(VPython vector) not a (VPython vector), which ends up as a
> big performance hit on existing programs. I tried his suggestion and did
> some timing using the program shown below.
>
> Using "from numpy import *", the numpy sqrt(5.5) gives 5.7 microsec per
> sqrt, whereas using "from math import *" a sqrt is only 0.8 microsec.
> Why is numpy so much slower than math on this simple case? For
> completeness I also timed the old Numeric sqrt, which was 14 microsec,
> so numpy is a big improvement, but still very slow compared to math.
>
> Using Daniels's suggestion of first trying the math sqrt, falling
> through to the numpy sqrt only if the argument isn't a simple scalar,
> gives 1.3 microsec per sqrt on the simple case of a scalar argument.
> Shouldn't/couldn't numpy do something like this internally?
>
> Bruce Sherwood
>
> ----------------------------
> from math import *
> mathsqrt = sqrt
> from numpy import *
> numpysqrt = sqrt
> from time import clock
>
> # 0.8 microsec for "raw" math sqrt
> # 5.7 microsec for "raw" numpy sqrt
> # 1.3 microsec if we try math sqrt first
>
> def sqrt(x):
>     try: return mathsqrt(x)
>     except TypeError: return numpysqrt(x)
>
> # Check that numpy sqrt is invoked on an array:
> nums = array([1,2,3])
> print sqrt(nums)
>
> x = 5.5
> N = 500000
> t1 = clock()
> for n in range(N):
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
>     y = sqrt(x)
> t2 = clock()
> for n in range(N):
>     pass
> t3 = clock()
> # t3-t2 is the loop overhead (turns out negligible)
> print "%i loops over 10 sqrt's takes %.1f seconds" % (N,t2-t1)
> print "Total loop overhead = %.2f seconds (negligible)" % (t3-t2)
> print "One sqrt takes %.1f microseconds" % (1e6*((t2-t1)-(t3-t2))/(10*N))
>
>
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-- 
French PhD student
Website : http://matthieu-brucher.developpez.com/
Blogs : http://matt.eifelle.com and http://blog.developpez.com/?blog=92
LinkedIn : http://www.linkedin.com/in/matthieubrucher
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