Hi Sunil,

this is likely not a problem of uBLAS, but one of the principal problems
of using sparse matrices. Depending on the type of matrix either random
access or multiplication performance is efficient.
For the compressed_matrix, random access is rather costly *unless* you
can control the way in which elements are added to the matrix. If you
can assemble the (row_major) compressed_matrix row-by-row with ascending
column indices, this should take no time at all.
If you can't do this, use a different matrix type for assembly, e.g.
mapped_matrix (which offers efficient random access, but bad
computational performance) and construct the compressed_matrix from
there.

See Gunter Winkler's page for details:
http://www.guwi17.de/ublas/matrix_sparse_usage.html

Kind reagrds,
Jörn

On Wed, 2010-04-21 at 03:09 +0200, Sunil Thomas wrote:
> Hi all,
>
>    I've been using boost 1.34 ublas library, especially the
> compressed_matrix class for sparse matrices in
>
> compressed row storage form. But I noticed that simply accessing an
> element of the matrix (to assign
>
> it a value, for example) slows my application down to unusable levels,
> for problems of the order of just
>
> 80,000 unknowns. I've identified the program is there and yes, I am
> allocating the memory as I should
>
> be for the matrix, - for example here is a snippet (of important
> lines):
>
>
> ************************************************************************************
> matrix_A = compressed_matrix(nelem_a(), nelem_a(), nonzeros());    //
> allocation
>
> matrix_A(uic1, uic1) += -trans; // assignment
>
> matrix_A(uic2, uic2) += -trans;  // assignment
>
> ************************************************************************************
>
> where all variables (and/or functions), e.g. uic1, uic2, trans,
> neleme_a(), nonzeros(), etc.. are all well-defined
>
> (this is all been checked thoroughly). Commenting  out the two
> assignment statements for example reduced
>
> my overall run time from 110 seconds to 0 (practically zero), for
> 80000 runs. Has anyone encountered this
>
> problem and know of a solution? I've heard a lot of stories about how
> boost::ublas is just not up there in
>
> performance and I certainly hope I am missing something trivial. Do
> later versions of boost address this
>
> better?
>
>
>
> Greatly appreciate any help.
>
> Thanks,
>
> Sunil.
>
>

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