[Pw_forum] difference between nr1 and nrx1
Timo Thonhauser
thonhauser at MIT.EDU
Thu May 1 16:14:30 CEST 2008
Dear Stefano,
Thanks - that helped a lot! However, I have one more question:
I have to scan and work with the density in such a way that
I want to know the real space position of each grid point.
So what I do is:
ifft = 0
do zfft=0, nr3-1
do yfft=0, nr2-1
do xfft=0, nr1-1
!
ifft = ifft + 1
rho = rho_val(ifft,1) + rho_core(ifft)
...
r_pos(ifft,1) = dble(xfft)/dble(nr1)
r_pos(ifft,2) = dble(yfft)/dble(nr2)
r_pos(ifft,3) = dble(zfft)/dble(nr3)
!
end do
end do
end do
Say, we are running the code on a gray and nrx1 was different
from nr1. The loop over xfft above would stay the same and there
is not need to let it run over xfft=0 to nrx1-1, right?
As I understand it, the nrx? are only constructs to make the
fft easier on certain machines, but there are no extra real
space density grid points associated with them. If we use only
one processor then nr1*nr2*nr3 = nrxx and the charge density is
defined as rho(1:nrxx). But if nr? is different from nrx? then
nrx1*nrx2*nrx3 IS NOT nrxx, but yet the charge density still
only has rho(1:nrxx) values, right?
Thanks for all you help!
Best, Timo
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> 1. Re: difference between nr1 and nrx1 (Stefano de Gironcoli)
> 2. Re: difference between nr1 and nrx1 (Stefano de Gironcoli)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 01 May 2008 00:52:19 +0200
> From: Stefano de Gironcoli <degironc at sissa.it>
> Subject: Re: [Pw_forum] difference between nr1 and nrx1
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID: <4818F823.1020202 at sissa.it>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Dear Timo,
> due to memory access conflicts for certain architectures (cray was
> one of these) FFT routines were significantly slower if some of the
> nrx1,nrx2,nrx3 (nr1 in scalar and nr3 in parallel executions) was a
> multiple of 2 while obviously nr1,nr2,nr3 are best when they are powers
> of 2. It was therefore convenient to have the flexibility to set
> nrx1=nr1+1 (or nrx3=nr3+1) when nr1(nr3) was even.
> Now this is not anymore the case on most machines but the distinction
> between nr? and nrx? remains and in some case it is still useful.
> Best regards,
> Stefano de Gironcoli
>
> Timo Thonhauser wrote:
>
>> Dear Developers,
>>
>> What is the difference between nr1, nr2, nr3 and nrx1, nrx2, nrx3
>> in the code? For all practical purposes they always seem to be
>> the same.
>>
>> In PW/pwcom.f90 we find:
>>
>> nr1, &! fft dimension along x
>> nr2, &! fft dimension along y
>> nr3, &! fft dimension along z
>> nrx1, &! maximum fft dimension along x
>> nrx2, &! maximum fft dimension along y
>> nrx3, &! maximum fft dimension along z
>>
>> But when would nr1 differ from nrx1?
>>
>> Thanks a lot!
>> Timo
>>
>>
>>
>
>
>
> ------------------------------
>
> Message: 2
> Date: Thu, 01 May 2008 00:59:40 +0200
> From: Stefano de Gironcoli <degironc at sissa.it>
> Subject: Re: [Pw_forum] difference between nr1 and nrx1
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID: <4818F9DC.4020901 at sissa.it>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> just to complete the information, this check is done in the
> good_fft_dimension function in Modules/fft_scalar.f90
> stefano
>
> Stefano de Gironcoli wrote:
>
>> Dear Timo,
>> due to memory access conflicts for certain architectures (cray was
>> one of these) FFT routines were significantly slower if some of the
>> nrx1,nrx2,nrx3 (nr1 in scalar and nr3 in parallel executions) was a
>> multiple of 2 while obviously nr1,nr2,nr3 are best when they are powers
>> of 2. It was therefore convenient to have the flexibility to set
>> nrx1=nr1+1 (or nrx3=nr3+1) when nr1(nr3) was even.
>> Now this is not anymore the case on most machines but the distinction
>> between nr? and nrx? remains and in some case it is still useful.
>> Best regards,
>> Stefano de Gironcoli
>>
>> Timo Thonhauser wrote:
>>
>>
>>> Dear Developers,
>>>
>>> What is the difference between nr1, nr2, nr3 and nrx1, nrx2, nrx3
>>> in the code? For all practical purposes they always seem to be
>>> the same.
>>>
>>> In PW/pwcom.f90 we find:
>>>
>>> nr1, &! fft dimension along x
>>> nr2, &! fft dimension along y
>>> nr3, &! fft dimension along z
>>> nrx1, &! maximum fft dimension along x
>>> nrx2, &! maximum fft dimension along y
>>> nrx3, &! maximum fft dimension along z
>>>
>>> But when would nr1 differ from nrx1?
>>>
>>> Thanks a lot!
>>> Timo
>>>
>>>
>>>
>>>
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>
>
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--
______________________________________________________________________
Dr. TIMO THONHAUSER
Department of Materials Science and Engineering (617) 253 6026
MIT, 13-4084, 77 Massachusetts Avenue thonhauser at mit.edu
Cambridge, MA 02139-4307, USA www.mit.edu/~thonhaus
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