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computation time for convolution [message #88966]

Wed, 09 July 2014 01:12

fra
Messages: 3
Registered: April 2010

Junior Member

I am a little puzzled about the computation time required by different convolution routines. I need to compute several times the convolution of large arrays and I always used the convolve routine of the astrolib. Since I need to speed up the processing I compared the computation time for array of different size (but using sizes power of 2, which should be the best case for FFT) convolved with different routines. The best result (by far) is obtained with the function convol of the IDL standard library, the worst is convol_fft and convolve is somewhat in the middle. This does not make sense to me, I was sure that the FFT approach is the fastest. What am I missing or doing wrong?

These are the results:

4x 4
convolve: 0.038000107
convol: 0.00000000
convol_fft: 0.00099992752
8x 8
convolve: 0.00000000
convol: 0.00000000
convol_fft: 0.00000000
16x 16
convolve: 0.00000000
convol: 0.00000000
convol_fft: 0.00000000
32x 32
convolve: 0.00000000
convol: 0.00000000
convol_fft: 0.0010001659
64x 64
convolve: 0.00000000
convol: 0.00000000
convol_fft: 0.0019998550
128x 128
convolve: 0.00099992752
convol: 0.0010001659
convol_fft: 0.0079998970
256x 256
convolve: 0.0080001354
convol: 0.0019998550
convol_fft: 0.035000086
512x 512
convolve: 0.036000013
convol: 0.0069999695
convol_fft: 0.28600001
1024x 1024
convolve: 0.25300002
convol: 0.026999950
convol_fft: 1.4849999
2048x 2048
convolve: 1.6410000
convol: 0.11600018
convol_fft: 6.6910000
4096x 4096
convolve: 7.4190001
convol: 0.43299985
convol_fft: 26.736000

and this is the code I used for this test:

for i=2,12 do begin
a=fltarr(2l^i,2l^i)
b=a
time0=systime(1)
c=convolve(a,b)
time1=systime(1)
c=convol(a,b)
time2=systime(1)
c=convol_fft(a,b)
time3=systime(1)
print,2l^i,'x',2l^i
print,'convolve:', time1-time0
print,'convol:', time2-time1
print,'convol_fft:', time3-time2
endfor

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[Message index]

		computation time for convolution By: fra on Wed, 09 July 2014 01:12
		Re: computation time for convolution By: wlandsman on Wed, 09 July 2014 09:48
		Re: computation time for convolution By: chris_torrence@NOSPAM on Wed, 09 July 2014 17:59
		Re: computation time for convolution By: Lajos Foldy on Thu, 10 July 2014 01:47
		Re: computation time for convolution By: wlandsman on Thu, 10 July 2014 05:16
		Re: computation time for convolution By: fra on Thu, 10 July 2014 14:19
		Re: computation time for convolution By: wlandsman on Thu, 10 July 2014 19:17

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