[petsc-users] Erro in solution

[Danesh Daroui]

On Mon, 2011-05-09 at 11:35 -0500, Barry Smith wrote:
> The code is very slow for one of two reasons
> 
> 1) the matrix preallocation is not correct and it is spending a great deal of time in the MatSetValues() calls. To check this run the code with -info and grep the output for malloc. My guess is that you are not preallocating for the diagonal and hence the preallocation is not enough. See http://www.mcs.anl.gov/petsc/petsc-as/documentation/faq.html#efficient-assembly
> 


I don't think that this is the source of the problem. Filling can be
more efficient (maybe?) but as I observed, the most time consuming part
is when I call KSPSolve.


> 2) the convergence of the solver is very slow. Run with -ksp_monitor and see how many iterations it is taking to converge. Is it thousands? If so, you will need to select a better preconditioner or use a direct solver -pc_type lu.
> 


A direct solver? Never! :) I am escaping from time complexity of direct
solvers, otherwise I already have a LAPACK based code that works well.


>    Barry
> 
> 
> 
> On May 9, 2011, at 11:02 AM, Danesh Daroui wrote:
> 
> > 
> > Hi again,
> > 
> > The issue is solved and it was due to a bug in my code. But now the
> > problem is that PETSc is extremely slow when I try to solve even a small
> > equation. I fill the matrix and right-hand-side and call the solver as
> > below:
> > 
> > ierr=KSPCreate(PETSC_COMM_WORLD, &ksp);
> > ierr=KSPSetOperators(ksp, Mp, Mp, DIFFERENT_NONZERO_PATTERN);
> > ierr=KSPSetTolerances(ksp, 1.e-2/Msize, 1.e-50, PETSC_DEFAULT,
> > 			PETSC_DEFAULT);
> > ierr=KSPSetFromOptions(ksp);
> > ierr=KSPSolve(ksp, bp, xp);
> > 
> > The Msize is the size of right-hand-side vector. I use C++ and am
> > running PETSc in sequential mode so no MPI is used, but anyway, it
> > shouldn't be that slow! Am I right or I am missing something?
> > 
> > Thanks,
> > 
> > Danesh
> > 
> > 
> > 
> > 
> > On Sun, 2011-05-08 at 14:41 +0200, dan at ltu.se wrote:
> >> Hi,
> >> 
> >> I checked my code and there is no zero on the diagonal. I also modified the code
> >> so if there is any zero on the diagonal it will be stored anyway. After this
> >> change the problem still exists! I am not sure if I have called PETSc functions
> >> correctly or in correct sequence. But anyway, it returns the error:
> >> 
> >> [0]PETSC ERROR: Object is in wrong state!
> >> 
> >> 
> >> that and the rest of the error message is exactly as I posted before. Any idea
> >> to solve this problem?
> >> 
> >> Thanks,
> >> 
> >> D.
> >> 
> >> 
> >> 
> >> Quoting Barry Smith <bsmith at mcs.anl.gov>:
> >> 
> >>> 
> >>>   Danesh,
> >>> 
> >>>    Some of the PETSc solvers (like the default ILU) require that you put
> >>> entries on all the diagonal locations of the matrix, even if they are zero.
> >>> So you should explicitly put a 0 on the diagonal locations that have 0 and
> >>> the issue will go away.
> >>> 
> >>>    Barry
> >>>