Dear Umut, I understand that in order to have an *efficient* iterative solver, an appropriate preconditioner should be used. But, the thing is that when I use a general preconditioner e.g. diagonal, the iterative solver seems to get stuck in a loop and never ends! What I expect is that the solution ends in a reasonable time without convergence (due to inappropriate precond.) or ends in longer time but with convergence. By long I mean longer than direct solver. Then, I could work on preconditioner to make it as efficient as possible. Again, I used GMRES and it never ended.
I have tested GMM++ using its direct solver and it was OK. Our code is being ported to Intel MKL (partly). We have also used PETSc before. Thanks, Danesh On 2010-09-13 11:14, Umut Tabak wrote: > On 09/13/2010 11:01 AM, Danesh Daroui wrote: >> Dear Renard, >> >> I have another question. When I run iterative solvers in GMM++, it takes >> very very long time and sometimes (specially when ILU precond. is used) >> the method >> doesn't even converge. With ILUT and ILUTP I got the error "pivot is too >> small" and i don't know why. But, with direct solvers, it is possible to >> solve it in less time. >> Is it because my matrix is not sparse enough? But I am wondering, if I >> solve a dense system with iterative solver, shouldn't it take in worse >> case, the time equal to >> direct solver? I have run all my tests using GMRES solver. Do you think >> that I may get better result with other iterative solvers in GMM++? >> >> Thanks, >> >> Danesh >> > Hi Danesh, > > Iterative solvers, most of the time, do not work without a good > preconditioner set for them and finding good preconditioners is also > pretty hard, and problem specific most of the time. > > Apart from this, convergence properties are closely related to the > spectral properties of the operator matrix(the scattering of the > eigenvalues), using iterative solvers might need serious expertise and > knowing your problems in deep, so they are not sth like black box > routines, or a shot-in-the-dark like direct(LU factorization) solvers. > > And from my experience, if your matrices are really ill-conditioned and > you can not improve the condition numbers seriously by the application > of a preconditioner, the best bet is to rely on direct solvers, PETSc > has interfaces for MUMPS solver package, it seems to be the best option > most of the time. > > Hope this helps a bit, > Umut > > _______________________________________________ > Getfem-users mailing list > [email protected] > https://mail.gna.org/listinfo/getfem-users -- Danesh Daroui Ph.D Student Lulea University of Technology http://www.ltu.se [email protected] Tel: +46-(0)920-492451 Cell phone: +46-(0)704-399847 _______________________________________________ Getfem-users mailing list [email protected] https://mail.gna.org/listinfo/getfem-users
