[GRASS5] i.fft segemntation fault

[Stephan Holl]

Hello Glynn, 

thanks for pointing this out.

On Thu, 10 Jun 2004 09:45:45 +0100 Glynn Clements
<glynn.clements at virgin.net> wrote:

> 
> Stephan Holl wrote:
> 
> > > I tried to use i.fft in GRASS 5.3 (CVS) but only get errors like:
> > > 
> > > a) Starting FFT...
> > > fftw: BUG in executor: invalid plan
> > > 
> > > b) Starting FFT...
> > > Speicherzugriffsfehler (segmentation fault)
> > > 
> > > Does this module work at all? Is there a special way to use it?
> > > I installed fftw RPMs. Is this version broken?
> > 
> > I can second that behavior described above. (... on debian sarge)
> > 
> > But I figured out, that reducing the size of a window will make
> > i.fft work.
> > My original regions, where it segfaults:
> 
> > rows:       11923
> > cols:       25151
> 
> > rows:       5445
> > cols:       6519
> 
> 11923 * 25151 * 16 bytes (2 doubles) = 4798005968
> 
> I.e. ~4.8Gb, which is likely to significantly exceed the amount of
> available memory (and, on 32-bit platforms, the amount of memory which
> can be allocated to a single process, regardless of how much total RAM
> or swap is available).
> 
> Actually, you would need half as much again (i.e. ~7.2Gb); i.fft reads
> the entire map into an array of doubles (8 bytes per cell), but FFTW
> needs an array of fftw_complex values (2 doubles, 16 bytes per cell).
> 
> More generally, maps that large won't work with anything which needs
> to read the entire map into memory (which includes i.fft), unless you
> have a 64-bit system and a lot of RAM (swap wouldn't be much use on
> something like an FFT calculation).

so the only possible solution would be tiling the map into smaller
pices which fit into my RAM (768MB) and calculating the fft on the small
tiles.

Thanks 
	Stephan Holl

-- 
Stephan Holl

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http://www.gdf-hannover.de

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