[petsc-dev] PETSc LU, Lapack and Preconditioning Matrices

Barry Smith bsmith at mcs.anl.gov
Fri Dec 16 10:32:43 CST 2011

On Dec 16, 2011, at 9:52 AM, Matthew Knepley wrote:

> On Fri, Dec 16, 2011 at 9:37 AM, Dave Nystrom <dnystrom1 at comcast.net> wrote:
> I'm trying to figure out whether I can do a couple of things with petsc.
> 1.  It looks like the preconditioning matrix can actually be different from
> the full problem matrix.  So I'm wondering if I could provide a different
> preconditioning matrix for my problem and then do an LU solve of the
> preconditioning matrix using the -pc_type lu as my preconditioner.
> Yes, that is what it is for.
> 2.  When I build petsc, I use the --download-f-blas-lapack=yes option.  I'm
> wondering if petsc uses lapack under the hood or has the capability to use
> lapack under the hood when one uses the -pc_type lu option.  In particular,
> since my matrices are band matrices from doing a discretization on a 2d
> regular mesh, I'm wondering if the petsc lu solve has the ability to use the
> lapack band solver dgbsv or dgbsvx.  Or is it possible to use the lapack band
> solver through one of the external packages that petsc can interface with.
> I'm interested in this capability for smaller problem sizes that fit on a
> single node and that make sense.
> We do not have any banded matrix stuff. Its either dense or sparse right now.


    As I noted before, your band is so large that lapack type band solvers don't make sense. Using a general purpose sparse direct solver will be much more efficient than using the lapack band solver.


> 3.  I'm also wondering how I might be able to learn more about the petsc ilu
> capability.  My impression is that it does ilu(k) and I have tried it with
> k>0 but am wondering if one of the options might allow it to do ilut and
> whether as k gets big whether ilu(k) approximates lu.  I currently do not
> understand the petsc ilu well enough to know how much extra fill I get as I
> increase k and where that extra fill might be located for the case of a band
> matrix that one gets from discretization on a regular 2d mesh.
> We do not do ilu(dt). Its complicated, and we determined that it was not worth
> the effort. You can get that from Hypre is you want. Certainly, for big enough
> k, ilu(k) is lu but its a slow way to do it.
>    Matt
> Thanks,
> Dave
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener

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