[SciPy-user] FFT speed ?

[Christian Kristukat]

Samuel GARCIA wrote:
> Hello list,
> 
> I have a problem with fft speed.
> could someone try this :
> 
> from scipy import *
> import time
> 
> a = rand((186888));t1=time.time();fft(a);t2=time.time();t2-t1
> 
> # for me about 0.67791390419006348 s. on centrino 1.6GHz
> 
> a = rand((186890));t1=time.time();fft(a);t2=time.time();t2-t1
> 
> # for me about 1.622053861618042 s. on centrino 1.6GHz
> 
> a = rand((186889));t1=time.time();fft(a);t2=time.time();t2-t1
> 
> # the computation is infinit !!!! (no time to wait)
> 
> 
> Question : why is there a problem specialy for that size (186889) of fft ?
> It is not the only size with that problem.

I guess this is due to the design of fft which is made for input lengths of
2**x. If you try e.g. 262144=2**18 which is even larger than 186889 you'll see
that it's pretty fast.

Christian