Re: [eigen] Allocation policy of eigen decompositions

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

2010/3/5 Adolfo Rodríguez Tsouroukdissian <dofo79@xxxxxxxxx>:
> Hello,
>
> I have a question on what the allocation policy of the eigen 3.x matrix
> decompositions will be, for this will strongly bias how I use eigen on
> different parts of my code.
>
> On the one hand, I know that the template parameters of the matrix to be
> decomposed (size, options, max size, ...) will be propagated to all the
> decomposition's members and temporaries [1], so if the input matrix is
> stack-allocated, so will be the internals of a decomposition. So far so
> good.
>
> On the other hand, I had the expectation that if heap-allocated matrices are
> used, all heap allocations of the decomposition algorithm would take place
> during the initialization phase, and none would be done in the computation
> phase (provided that the initialization and computation matrices have the
> same size in memory). This currently does not seem to be the case. For
> example,
>
> MatrixXd I(MatrixXd::Identity(16, 16));
> MatrixXd A(MatrixXd::Random(16, 16));   // Same size as I
>
> Eigen::FullPivLU<MatrixXd> lu(I);       // Allocates 5 temps
> lu.compute(A);                          // Allocates 2 extra temps
>
> Eigen::JacobiSVD<MatrixXd> svd(I);      // Allocates 4 temps
> svd.compute(A);                         // Allocates 1 extra temp
>
> I haven't checked where the extra allocations in compute() come from, but my
> question is, can and will they be prevented for 3.x? or is this an
> intentional design decision? (and if so, for what reasons).

It's not an intentional design decision, it's just that I didn't even
think about that. On regular PC hardware this is kind of a non-issue.
Now I can believe that in robotics it's different. Fortunately this is
just an implementation detail, so we can fix that at any later date.
Patches welcome :) Otherwise I might do this the next time that I
rework these decompositions but, since it's an internal thing, it's
not my priority for 3.0 (prioritizing API-impacting issues).

Benoit

>
> TIA,
>
> Adolfo
>
> [1]
> http://listengine.tuxfamily.org/lists.tuxfamily.org/eigen/2010/03/msg00095.html
>
>
> --
> Adolfo Rodríguez Tsouroukdissian, Ph. D.
>
> Robotics engineer
> PAL ROBOTICS S.L
> http://www.pal-robotics.com
> Tel. +34.93.414.53.47
> Fax.+34.93.209.11.09
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