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## Re: eig on several matrices in a vectorized manner

 From: Juan Pablo Carbajal Subject: Re: eig on several matrices in a vectorized manner Date: Wed, 24 Oct 2018 00:56:08 +0200

```Hi,

Here is (at least I think so) a simplified version of your example

N = 500;
sz = 16;
printf ('Run test for %d matrices of size %d\n', N, sz);

for k = 1:N
M{k} = rand (sz); M{k} = M{k}.' * M{k};
endfor

printf ('** Interpreter loop\n');
tic
for k = 1:N
eig (M{k});
endfor
toc

printf ('** C++ loop\n');
tic
cellfun (@eig, M, 'UniformOutput', false);
toc

printf ('** Blk matrix\n');
printf ('  - Assembly\n');
tic
MM = blkdiag (cellfun (@sparse, M, 'UniformOutput', false){:});
toc
printf ('  - eig\n');
tic
eig (MM);
toc

printf ('** Parallel loop\n');
tic
parcellfun (nproc (), @eig, M, 'UniformOutput', false);
toc

Run test for 500 matrices of size 16
** Interpreter loop
Elapsed time is 0.0342941 seconds.
** C++ loop
Elapsed time is 0.0255151 seconds.
** Blk matrix
- Assembly
Elapsed time is 0.105091 seconds.
- eig
Elapsed time is 57.415 seconds.
** Parallel loop
parcellfun: 500/500 jobs done
Elapsed time is 0.783254 seconds.

The timing results show that the block matrix is slower and also has
memory issues (for moderate matrix sizes (not FEM!!) will need batch
blk matrices).
The other solutions are comparable, the parallel one getting better
with the length of the loop.

What is the costly part? the length of the loop (number of matrices)
or the call to eig (size of the matrices)

If it is the length of the loop then parallelization and low level
regarding loops).
If it is the call to eig, then: can you use eigs? (i.e. how many and
which eigenvalues do you care about?). Can you compress you matrix set
and then call eig in only a few generating matrices?

On Mon, Oct 22, 2018 at 8:57 AM nicolas neuman <address@hidden> wrote:
>
> Hi, thank you for the prompt response!
> The problem I want to apply this for is simulation of magnetic resonance
> spectra. Basically the matrices are functions of a magnetic field B, and I
> have to find the values B0 for which energy (eigenvalue) differences in the
> energy matrix match a certain value (microwave frequency). Finding of the
> values B0 is done adaptively, by dividing the interval [Bmin,Bmax] in halfs,
> and finding by approximation the solutions. My algorithm is probably very far
> from efficient at this point, but I estimate I need to diagonalize tens of
> matrices per cycle of the adaptive scheme.
> It is highly possible that the actual diagonalization is not the time
> consuming part, but then the short loops I run to find the eigenvalue
> differences are what is slowing everything down. I should rethink the whole
> thing.
> Anyway, below I send the code for the small benchmark I made. Perhaps there
> is a much faster way of doing this.
> Thank you very much!
> Best,
> Nicolas
>
> %begin
>
> clear all, close all
> more off
>
> disp(['Size of each matrix: ',' Number of matrices: ',' Loop to block ratio:
> ']);
> for pMatSize = [2,4,8,16,32,64,128]
>     for nMat = 2:20
>         SizeMatrices = pMatSize;
>         Nmatrices = nMat;
>         eigA = zeros(SizeMatrices,Nmatrices);
>         timeLoop = zeros(Nmatrices,1);
>
>         augA = cell(Nmatrices,1);
>         for k = 1:Nmatrices
>             tmpA = rand(SizeMatrices,SizeMatrices);
>             augA{k} = tmpA'*tmpA;
>         end
>
>         tic
>         for k = 1:Nmatrices
>             eigA(:,k) = eig(augA{k});
>         end
>         timeLoop = toc;
>
>         augAmat = blkdiag(augA{:});
>         tic
>         eigaugA = eig(augAmat);
>         timeBlock = toc;
>
>         disp([num2str(SizeMatrices) '        ' num2str(Nmatrices) '        '
> num2str(timeLoop/timeBlock)]);
>     end
> end
>
>
> %end
>
>
>
> On Sun, Oct 21, 2018 at 11:06 PM Juan Pablo Carbajal <address@hidden> wrote:
>>
>> Hi,
>>
>> It would be useful to have a simple code to test your results.
>>
>> Also, how many matrices do you need to process? for a few hundrerds,
>> looping is not the source of cost, it is just the cost of eig itself.
>> I do not see much point on vectorizing eig (maybe I can be convinced
>> otehrwise) so I think the options for speed up here are:
>>
>> 1. paralelization
>> 2. sending the loop a level bellow e.g. cellfun (do not expect huge
>> improvements)
>>
>> Alternatively, (and risky) if your matrix set is homogenous in size
>> (ie.e. all matrice have the same size), try to reduce the set into a
>> basis of matrices and try exploit Horn's conjecture.
>>
>> : https://terrytao.wordpress.com/tag/schur-horn-inequalities/
>> On Fri, Oct 19, 2018 at 5:02 PM niconeuman <address@hidden> wrote:
>> >
>> > Greetings,
>> > this is my first post in this mailing-list. I used Matlab for many years
>> > but
>> > since 2018 I switched to Octave, and plan to keep using it in my research.
>> > I have to calculate eigenvalues of multiple matrices which have all the
>> > same
>> > size (2x2,4x4,8x8, etc) and are Hermitian, repeatedly.
>> > Running this on a loop seems to me to be inefficient. I have tried to do a
>> > small benchmark test in which I compare looping, changing the size of the
>> > matrices and the number of matrices; against generating a supermatrix by
>> > using blkdiag, and taking the eigenvalues all at once.
>> > The blkdiag approach is only faster when the size of the matrices are 2x2,
>> > but for larger systems it becomes quite slower than looping. I imagined
>> > this
>> > has to do with the use of memory by the large block supermatrices. So I
>> > tried to convert the individual matrices to sparse before making the block
>> > supermatrix. This has not improved the timings for the block approach.
>> > Perhaps I'm doing something wrong, but in any case, is there some way of
>> > broadcasting the eig function.
>> > I imagine I can write the Cholesky algorithm so that it acts on the
>> > elements
>> > of multiple matrices at the same time, and taylor that to my problem. But
>> > if
>> > there is some solution already implemented I think it would be better.
>> > Thank you very much!
>> > Best regards,
>> > Nicolás Neuman
>> >
>> >
>> >
>> > --
>> > Sent from: http://octave.1599824.n4.nabble.com/Octave-General-f1599825.html
>> >
>> >
>
>
>
> --
> Dr. Nicolas Neuman
>
> Institut für Chemie und Biochemie, Anorganische Chemie
> Freie Universität Berlin,
> Fabeckstraße 34–36, 14195, Berlin (Germany)
>
> Instituto de Desarrollo Tecnológico para la Industria Química - INTEC
> UNL-CONICET
> CCT-CONICET Santa Fe
> S3000ZAA Santa Fe - Santa Fe - Argentina
>
> Jefe de Trabajos Prácticos
> Departamento de Física
> Fac. de Bioquímica y Ciencias Biológicas