[Pw_forum] difference between nr1 and nrx1

[Stefano de Gironcoli]

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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