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FFT slow in 2.9?


From: Matthias Brennwald
Subject: FFT slow in 2.9?
Date: Sun, 4 Feb 2007 19:38:04 +0100

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?

Matthias


-------
Matthias Brennwald
L├Ągernstrasse 6
CH 8037 Z├╝rich
+41 (0)44 364 17 03
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