[petsc-users] direct solver for preconditioner

[Barry Smith]

On May 12, 2010, at 1:19 PM, hxie at umn.edu wrote:

> Hi,
> 
> Is the mumps the best option as a direct solver?

   There is no "best option" for any solver; it always depends on the problem you are solving and there is no "recipe" to select the best. We have found that MUMPS generally works better than others.

> Should the size of the linear system be less than 1d5 for mumps?

   There is no hard limit on the size. It again depends on the problem. But, of course, the larger the problem, the more fill and hence longer time it will take or it may run out of memory.

> Should I run the following to set mumps as the solver for the preconditioner?
> 
> mpirun -np 6 ~/bin/cats3d.1 -pc_type asm -ksp_type gmres -sub_ksp_type preonly -sub_pc_type lu -sub_pc_factor_mat_solver_package mumps

  You can certainly use these options. Often, when using overlapping Schwarz we have found the using a direct solver on the subproblems is OVER KILL, that is it takes a lot longer time and doesn't improve the convergence rate of the linear solver over using ILU for each subproblem. This is, of course, problem specific and there may be cases where using a direct solver is best.


   Barry

> 
> Bests,
> Hui