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Ublas : |
Subject: Re: [ublas] Question on ublas performance (resending as subscriber).
From: Sunil Thomas (sgthomas27_at_[hidden])
Date: 2010-04-21 18:11:48
Thanks Nasos - I was going to try that next anyhow. Will let you know if I
get a major improvement.
In a related problem, I find that reverse iterator is not working as
expected.. (for compressed_matrix
and mapped_matrix). I really doubt that it is a problem of my usage (but
that is possible). Here is a
snippet of my usage from ILU back substitution (P is the matrix -
compressed, or mapped):
for(itmr1 i1 = P.rbegin1(); i1 != P.rend1(); ++i1) {
itm2 i2 = i1.begin(); i = (int)i2.index1();
for(; i2 != i1.end(); ++i2) {
j = (int) i2.index2();
if(j <= i) continue;
x(i) -= (*i2)*x(j);
}
x(i) *= P(i,i);
}
Greatly appreciate if anybody knows why this crashes as soon as it
enters outer loop...let me know if you need
any further info. Its been a while since I used it, I had given up on it and
used a combination of a forward iterator
loop and a usual nested loop inside it. I have no such problem with forward
iterator.
Thanks in advance,
Sunil.
On Wed, Apr 21, 2010 at 2:53 PM, Nasos Iliopoulos <nasos_i_at_[hidden]>wrote:
> Thomas,
>
> replace sparse_matrix with compressed_matrix. Also try the following:
>
> use the generalized vector of compressed_vector A as in the link to build
> the stiffness matrix, I think this is faster than every other sparse
> container (mapped, compressed, coordinate). If you need a storage type like
> compressed (most likely if you use a classical sparse solver), just assign
> it after the generalized is filled by your stiffness matrix construction
> algorithm like:
> compressed_matrix<double> K=A;
>
> (I am unsure that in earlier versions of uBlas the above is optimum, the
> latest svn though should be ok. What version are you using?)
>
> The downside of this approach is that it requires twice the memory.
>
> Let us know if it works for you.
>
> Best
> Nasos
>
> ------------------------------
> Date: Wed, 21 Apr 2010 13:51:56 -0700
> From: sgthomas27_at_[hidden]
> To: ublas_at_[hidden]
> Subject: Re: [ublas] Question on ublas performance (resending as
> subscriber).
>
>
> Hi Jorn,
>
> Your answer correctly identified the problem. It appears for my problem,
> mapped_matrix is a
> much better (by factor of 110), if not the best choice as far as assembling
> goes. It also hasn't
> hurt traversal that badly, it appears
>
> In the link you attached (thank you!), the author uses something called
> "sparse_matrix" in
> boost::numeric::ublas - does this even exist? Atleast in version 1.34, it
> gives me a compile
> error, saying no sparse_matrix type exists in boost::numeric::ublas (it was
> the first thing I
> tested before going for mapped_matrix) and the documentation online doesn't
> mention
> sparse_matrix as a type of sparse storage at all.
>
> Anyway, thanks a lot for all of your help!
>
> Regards,
> Sunil.
>
> 2010/4/21 Jörn Ungermann <j.ungermann_at_[hidden]>
>
> 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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