[Pw_forum] Errors in parallel 3D FFT

[Iain Bethune]

Thanks for the suggestions - I have looked a bit closer at the code, and it
appears that all the grid elements (x,y,z) where these large errors appear
are those where dffts%iplw(x) == 1 .AND. dffts%iplw(y) == 1

However, there are some grid elements which meet the above criteria that do
not have errors, so maybe I am not understanding something about how the FFT
works?

I can't think of anything else in the fft_dlay_descriptor data structure
that helps narrow down further where these errors occur - any ideas?

Cheers

- Iain

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Iain Bethune
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> -----Original Message-----
> From: Paolo Giannozzi [mailto:giannozz at democritos.it]
> Sent: 15 January 2010 17:41
> To: ibethune at epcc.ed.ac.uk; PWSCF Forum
> Subject: Re: [Pw_forum] Errors in parallel 3D FFT
> 
> 
> On Jan 14, 2010, at 11:41 , Iain Bethune wrote:
> 
> > So most of the elements have small errors (which is as expected),
> > but there
> > are a large number which have very large errors (>1e-06), and I don't
> > understand why this should happen.
> 
> nor do I, but it seems to me rather unlikely that the FFT used in QE
> is wrong.
> Try first with a simpler system in serial execution and then try more
> complex
> cases. Note that not all elements of FFTs arrays are "physical": for
> reasons
> of increased speed, it is sometimes convenient to increase the first
> dimension of arrays by 1
> 
> P.
> ---
> Paolo Giannozzi, Dept of Physics, University of Udine
> via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
> 
> 
>