[Pw_forum] How to parallelize phonons

["Alejandro Rébola"]

Dear William,

Thank you so much for your answer, it was very helpful,

Alejandro

On Wed, March 28, 2012 2:47 pm, William Parker wrote:
> Dear Alejandro,
>
> The images in phonon calculations are groupings of the irreducible
> representations of the phonon modes.  Hence, the maximum number of images
> is the number of representations.
>
> There is a speedup as one increases the number of images parallelized
> over, but the speedup increases irregularly with image number since
> different representations require different amounts of time to calculate.
> In my limited experience, the time taken by single representations can
> vary at least up to a factor of two.
>
> The second step does generate all of the right dynamical matrix entries,
> but I have been unable to get the recover step to complete successfully
> yet, though I am currently working on this (if anyone else on the list has
> any comments on this, please speak up).
>
> Running on parallel with one image but parallelizing over k-points and FFT
> grid works properly.  I don't have any experience with the GRID method so
> perhaps someone else can say something about that.
>
> I'm not aware of any more documentation beyond what you quoted.
>
> A simple example I run my tests on is two-atom Si with a 2x1x1 q-mesh (two
> q-points, six representations), parallelizing over two images.
> With only one CPU per k-point pool, the commands look like:
>
> mpiexec -n 1 pw.x -npool 1 -inp si.scf.cg.in >& si.scf.cg.out
> mpiexec -n 2 ph.x -nimage 2 -npool 1 -inp si.ph.in >& si.ph.out
> mpiexec -n 1 ph.x -nimage 1 -npool 1 -inp si.ph_recover.in >&
> si.ph_recover.out
>
> where "diff si.ph.in si.ph_recover.in" returns "recover = .true."
>
> --William
>
> On Mar 28, 2012, at 1:34 PM, Alejandro Rébola wrote:
>
>> Dear all,
>>
>> I'm trying to run phonon calculations using ph.w on a cluster (NERSC),
>> and
>> since I have a big number of atoms I wanted to parallelize it as much
>> and
>> efficiently as possible. I've been reading the documentation, and I've
>> seen the GRID example, I was going to use this but then I saw the
>> following at the end of the INPUT_PH.html file:
>>
>> On parallel machines the q point and the irreps calculations can be
>> split
>> automatically. The procedure is the following:
>>
>> " [...]
>>
>> 1) run pw.x with nproc processors and npools pools.
>> 2) run ph.x with nproc*nimage processors, npools pools and nimage
>> images.
>> 3) run ph.x with the same input and recover=.true. on nproc processors
>>   and npools pools and only one image.
>>
>> During the first ph.x run the phonon code split the total amount of
>> work into nimage copies. Each image runs with different q and/or
>> representations.  The second run of ph.x is the final run that
>> collects all the data calculated by the images and writes the files
>> with the dynamical matrices."
>>
>> Since I'm not very familiar with the terminology used here (I'm
>> completely
>> lost) I would like to ask some questions:
>> 1) If I'm running on a cluster like NERSC, would I get any advantage
>> from
>> using the GRID method or should I just use the method outlined above?
>> 2) In that case, what does it mean by images for a phonon calculation
>> (I'm
>> just familiar with images for NEB). Where could I find more
>> documentation
>> about this or some example?
>> Thank you in advance,
>>
>> Alejandro Rébola
>>
>>
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>
> *********************************************************
>   William D. Parker                 phone: (630) 252-4834
>   Computational Postdoctoral Fellow   fax: (630) 252-4798
>   MSD-212, Rm. C-215
>   Argonne National Laboratory
>   9700 S. Cass Ave.
>   Argonne, IL 60439
> *********************************************************
>
>
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