Linear solvers

By default GetDP uses linear solvers from PETSc. (You can also compile GetDP with alternative linear solvers - see the GetDP documentation.) The choice of which particular linear solver to use for a given problem is handled directly through PETSc options, which can be specified:

on the command line (e.g. getdp test.pro -solve -ksp_type cg -pc_type ilu)
in the Resolution operations, with the SetGlobalSolverOptions[] function (e.g. SetGlobalSolverOptions["-ksp_type gmres -petsc_prealloc 400"];)
in a file :
- either as default options in a .petscrc file in your home directory
- or in an arbitrary file, specified to getdp with the -solver option (e.g. getdp test.pro -solve -solver "myoptions.txt")

Here are some useful PETSc options:

# To use GMRES+ILU(3) with a relative tolerance of 1e-12 and a restart of 300:
-ksp_rtol 1.e-12
-ksp_type gmres
-ksp_gmres_restart 300
-pc_type ilu
-pc_factor_levels 3

# To use PETSc's built-in direct LU solver
-ksp_type preonly
-pc_type lu
-pc_factor_mat_solver_package petsc

# To use the direct LU solver from UMFPACK
-ksp_type preonly
-pc_type lu
-pc_factor_mat_solver_package umfpack

# To use the direct LU solver from MUMPS
-ksp_type preonly
-pc_type lu
-pc_factor_mat_solver_package mumps

# To preallocate 300 nonzero elements per line in the matrix
-petsc_prealloc 300

# To see the residual
-ksp_monitor

# To use GMRES+Jacobi with a relative tolerance of 1e-12 and a restart of 100:
-ksp_rtol 1.e-12
-ksp_type gmres
-ksp_gmres_restart 100
-pc_type jacobi
-pc_factor_levels 3