Re: [eigen] Getting Householder reflections right
• To: eigen@xxxxxxxxxxxxxxxxxxx
• Subject: Re: [eigen] Getting Householder reflections right
• From: Michael Meyer <mjhmeyer@xxxxxxxxx>
• Date: Sun, 10 May 2009 08:34:28 -0700 (PDT)
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```Greetings,

I looked at the code for the Hessenberg tridiagonalization of a symmetric matrix A
(Tidiagonalization::_compute).
It seems that the symmetry of A is not used to the fullest.

With v denoting the Householder vector, Q=hvv' and H=I-Q
we have to compute

A = (I-Q')A(I-Q)

The code does

A1=(I-Q')A then A=A1(I-Q).

The intermediate matrix A1 is not symmetric.
We need (h' denotes the complex conjugate of h)

A1Q=h(A1v)v' and Q'A=(AQ)'=h'(Av)v',

that is, we have two multiplications of the form (Av)v'.
If we do both multiplications in one step

A = (I-Q')A(I-Q) = A-(Q'A+AQ)+Q'AQ

then we can use that

(a) A is symmetric, so compute only the upper half
(b) the matrix Q'A+AQ = (AQ)'+AQ is symmetric
(C) we compute and save the vector w=Av, then we get AQ=h(Av)v'=hwv' and

(d) Q'AQ = |h|^2(v'Av)vv' = |h|^2(v'w)vv', where vv' is again symmetric

so we have 1.5 multiplications of the form (Av)v' (vv' is half of one because of the symmetry).
The symmetry in (b) does not do us much good since we still need all of AQ.

I am not sure it's worth going after a 25% speedup (if any) if the code gets much uglier.
The constructor (which calls _compute and so already computes the decomposition)
is already very fast (1 sec for a 1000 x 1000 matrix).

Returning the orthogonal matrix Q is much slower (39 secs) and I do not understand why since the computation is quite
similar to the one in the decomposition.

A poster on an earlier thread once commented that LAPACK is faster for the symmetric eigenvalue problem.
This may be because the algorithms are written so that they make as much use a possible of matrix-matrix multiplication
A large effort then goes into speeding up matrix-matrix multiplication and the code is probably very ugly.
It is not realistic for Eigen to try and duplicate that.

Cheers,

Mike

--- On Fri, 5/8/09, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:

> From: Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
> Subject: Re: [eigen] Getting Householder reflections right
> To: eigen@xxxxxxxxxxxxxxxxxxx
> Date: Friday, May 8, 2009, 3:49 PM
> LOL.
> First I googled for "block householder", found this paper.
> Then Rohit IMs me and points me to the same paper.
> Then you. Small world!
>
> Anyway, I was about to draw the same conclusion: The
> construction of
> the block householder reflector relies on
> ....(drumroll).... SVD !
>
> So yes, let's implement a plain SVD first.
>
> Cheers,
> Benoit
>
> 2009/5/8 Keir Mierle <mierle@xxxxxxxxx>:
> > I found this:
> >
> > http://www.jstage.jst.go.jp/article/ipsjdc/2/0/298/_pdf
> >
> > Looks complicated. I suggest implementing the
> householder
> > bidiagonalization in a straightforward way first,
> where code clarity
> > and modularity goes before speed. Then we can look at
> blocking
> > versions.
> >
> > Keir
> >
> > On Fri, May 8, 2009 at 12:33 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
> wrote:
> >> thanks for the reminder, it's actually the right
> time to look at that
> >> as it may impact the design.
> >> Benoit
> >>
> >> 2009/5/8 Rohit Garg <rpg.314@xxxxxxxxx>:
> >>> Possibly OT
> >>>
> >>> May be you should look at block wise House
> holder transformations. I'd
> >>> infact recommend that all algorithms
> is-as-far-as-possible be done in
> >>> a blockwise manner.
> >>>
> >>> On Sat, May 9, 2009 at 12:52 AM, Benoit Jacob
> <jacob.benoit.1@xxxxxxxxx>
> wrote:
> >>>> Hi,
> >>>>
> >>>> I'm wondering how to do Householder
> reflections right.
> >>>> Books (Golub&vanLoan) and libraries
> (LAPACK) seem to _always_ use the
> >>>> following packed storage for a Householder
> vector v:
> >>>> they normalize it so that v(0)=1, hence
> they don't need to store v(0)
> >>>> which is useful for storing decompositions
> in a packed way in a single
> >>>> matrix.
> >>>> However, they store separately the norm of
> v, in order to avoid having
> >>>> to recompute it everytime.
> >>>>
> >>>> I'm not saying this is always a bad idea,
> i'm saying that's not always
> >>>> a good idea.
> >>>>
> >>>> That's a good idea for the QR
> decomposition, because it allows to
> >>>> store the whole decomposition in a single
> matrix, plus a vector to
> >>>> store the norms of the v's.
> >>>>
> >>>> But right now i'm looking at
> bidiagonalization and i'm seriously
> >>>> wondering if this packed storage is a good
> idea.
> >>>> With the packed storage, i can store in a
> single matrix the bidiagonal
> >>>> matrix, and the essential parts of the
> householder vectors, but then i
> >>>> need to store in 2 separate vectors the
> norms of the householder
> >>>> vectors.
> >>>> While _without_ packed storage, i'd be
> able to store entirely in a
> >>>> single matrix (of the same size) the whole
> householder vectors, and
> >>>> then i'd store the bidiagonal matrix
> separately (obviously in a
> >>>> compact way).
> >>>> So here, I don't see any advantage in
> using packed Householder storage.
> >>>> On the other hand, NOT packing the
> householder vectors will give
> >>>> simpler code and slightly better speed.
> >>>>
> >>>> Maybe the only advantage is to make our
> bidiagonalization storage
> >>>> compatible with LAPACK. But if that's
> important, we can add conversion
> >>>> methods, and, having n^2 complexity, the
> conversion won't have a big
> >>>> overhead compared to the n^3 cost of the
> decomposition itself.
> >>>>
> >>>> Same remarks for these close relatives:
> tridiagonalization and
> >>>> hessenberg decomposition (which i'll
> rework along the way).
> >>>>
> >>>> Opinions?
> >>>>
> >>>> Cheers,
> >>>> Benoit
> >>>>
> >>>>
> >>>>
> >>>
> >>>
> >>>
> >>> --
> >>> Rohit Garg
> >>>
> >>> http://rpg-314.blogspot.com/
> >>>
> >>> Department of Physics
> >>> Indian Institute of Technology
> >>> Bombay
> >>>
> >>>
> >>>
> >>
> >>
> >>
> >
> >
> >
>
>
>

```

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