Matthew Knepley writes: > 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.
Thanks. I think I will try that and see what sort of results I get. This sounds like a very encouraging discovery to me. > > 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. OK. I had always been used to thinking of a banded system as sparse, relatively speaking, when compared to a full system. Based also on Barry's response, I guess I am not well enough educated on the nuances of sparse versus banded. For instance, when I use "-ksp_type preonly -pc_type lu" to solve one of my systems, I had assumed that the LU factorization computed by petsc was really filling in the 2*nx+1 bandwidth even though petsc might not be explicitly using the banded nature of the matrix. So I am not sure at all what is going on under the hood in petsc for this set of solver options. Nor do I really know how to find out without reading the source code which might be fairly daunting. > > 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. Thanks. I need to experiment more with ilu(k) on a couple of my linear systems. > Matt > > > > Thanks, > > > > Dave
