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## Re: FFT slow in 2.9?

**From**: |
David Bateman |

**Subject**: |
Re: FFT slow in 2.9? |

**Date**: |
Sun, 04 Feb 2007 21:47:15 +0100 |

**User-agent**: |
Thunderbird 1.5.0.7 (X11/20060921) |

Matthias Brennwald wrote:
>* Dear all,*
>* *
>* on my Mac (PowerBook G4), fft seems to be about four times slower in *
>* Octave 2.9.9 (binary from HPC) than in Octave 2.1.71 (binary from *
>* Fink). Also, 2.9.9 does not seem to use fft-wisdom (the same fft *
>* would be faster after the first run):*
>* *
>* on 2.9.9:*
>* octave:1> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 1.985802 seconds.*
>* octave:2> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 1.803986 seconds.*
>* octave:3> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 1.811325 seconds.*
>* octave:4> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 2.165604 seconds.*
>* *
>* on 2.1.71:*
>* octave:1> x=rand(1,2^20); tic; fft(x); toc*
>* ans = 0.76503*
>* octave:2> x=rand(1,2^20); tic; fft(x); toc*
>* ans = 0.50110*
>* octave:3> x=rand(1,2^20); tic; fft(x); toc*
>* ans = 0.50244*
>* octave:4> x=rand(1,2^20); tic; fft(x); toc*
>* ans = 0.49133*
>* *
>* and on Matlab 7.1:*
>* >> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 0.751125 seconds.*
>* >> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 0.492581 seconds.*
>* >> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 0.473516 seconds.*
>* >> x=rand(1,2^20); tic; fft(x); toc*
>* Elapsed time is 0.486890 seconds.*
>* *
>* Any ideas why this is?*
>* *
Is this version linked to FFTW? If it isn't octave falls back to using
the old FFTPACK code that is in libcruft that as you've seen is about 4
times slower.. The absence of fftw_wisdom is a dead give away that FFTW
is missing.. Note that fftw_wisdom will disappear in 2.9.10 to be
replaced with a matlab compatible fftw function in any case.
D.

**FFT slow in 2.9?**, *Matthias Brennwald*, `2007/02/04`
**Re: FFT slow in 2.9?**,
*David Bateman* **<=**