Skip to main content
`Ctrl`+`K`

# KSPSetFromOptions

Sets `KSP` options from the options database. This routine must be called before `KSPSetUp()` if the user is to be allowed to set the Krylov type.

## Synopsis

    #include "petscksp.h"
    PetscErrorCode KSPSetFromOptions(KSP ksp)

Collective

## Input Parameter

- ksp - the Krylov space context

## Options Database Keys

- -ksp_rtol rtol - relative tolerance used in default determination of convergence, i.e. if residual norm decreases by this factor than convergence is declared

- -ksp_atol abstol - absolute tolerance used in default convergence test, i.e. if residual norm is less than this then convergence is declared

- -ksp_divtol tol - if residual norm increases by this factor than divergence is declared

- -ksp_max_it maxits - maximum number of linear iterations

- -ksp_min_it minits - minimum number of linear iterations to use, defaults to zero

- -ksp_reuse_preconditioner (true|false) - reuse the previously computed preconditioner

- -ksp_converged_use_initial_residual_norm - see `KSPConvergedDefaultSetUIRNorm()`

- -ksp_converged_use_min_initial_residual_norm - see `KSPConvergedDefaultSetUMIRNorm()`

- -ksp_converged_maxits - see `KSPConvergedDefaultSetConvergedMaxits()`

- -ksp_norm_type (none|preconditioned|unpreconditioned|natural) - see `KSPSetNormType()`

- -ksp_check_norm_iteration it - do not compute residual norm until iteration number it (does compute at 0th iteration) works only for `KSPBCGS`, `KSPIBCGS`, and `KSPCG`

- -ksp_lag_norm - compute the norm of the residual for the ith iteration on the i+1 iteration; this means that one can use the norm of the residual for convergence test WITHOUT an extra `MPI_Allreduce()` limiting global synchronizations. This will require 1 more iteration of the solver than usual.

- -ksp_guess_type - Type of initial guess generator for repeated linear solves

- -ksp_fischer_guess model,size - uses the Fischer initial guess generator for repeated linear solves

- -ksp_constant_null_space - assume the operator (matrix) has the constant vector in its null space

- -ksp_test_null_space - tests the null space set with `MatSetNullSpace()` to see if it truly is a null space

- -ksp_knoll - compute initial guess by applying the preconditioner to the right-hand side

- -ksp_monitor_cancel - cancel all previous convergene monitor routines set

- -ksp_monitor - print residual norm at each iteration

- -ksp_monitor draw::draw_lg - plot residual norm at each iteration, see `KSPMonitorResidual()`

- -ksp_monitor_true_residual - print the true l2 residual norm at each iteration, see `KSPMonitorTrueResidual()`

- -all_ksp_monitor optional_filename - print residual norm at each iteration for ALL KSP solves, regardless of their prefix. This is useful for `PCFIELDSPLIT`, `PCMG`, etc that have inner solvers and you wish to track the convergence of all the solvers

- -ksp_monitor_solution [ascii binary or draw][:filename][:format option] - plot solution at each iteration

- -ksp_monitor_singular_value - monitor extreme singular values at each iteration

- -ksp_converged_reason - view the convergence state at the end of the solve

- -ksp_use_explicittranspose - transpose the system explicitly in `KSPSolveTranspose()`

- -ksp_error_if_not_converged - stop the program as soon as an error is detected in a `KSPSolve()`, `KSP_DIVERGED_ITS` is not treated as an error on inner solves

- -ksp_converged_rate - view the convergence rate at the end of the solve

## Note

To see all options, run your program with the `-help` option or consult KSP: Linear System Solvers

## See Also

KSP: Linear System Solvers, `KSP`, `KSPSetOptionsPrefix()`, `KSPResetFromOptions()`, `KSPSetUseFischerGuess()`

## Level

beginner

## Location

src/ksp/ksp/interface/itcl.c

## Examples

src/ksp/ksp/tutorials/ex1.c
src/ksp/ksp/tutorials/ex100.c
src/dm/impls/stag/tutorials/ex2.c
src/dm/impls/stag/tutorials/ex3.c
src/dm/impls/swarm/tutorials/ex1f90.F90
src/ksp/ksp/tutorials/ex12.c
src/dm/impls/stag/tutorials/ex8.c
src/ksp/ksp/tutorials/ex11.c
src/dm/impls/stag/tutorials/ex1.c
src/dm/impls/stag/tutorials/ex4.c

## Implementations

KSPSetFromOptions_BCGS() in src/ksp/ksp/impls/bcgs/bcgs.c
KSPSetFromOptions_BCGSL() in src/ksp/ksp/impls/bcgsl/bcgsl.c
KSPSetFromOptions_CG() in src/ksp/ksp/impls/cg/cg.c
KSPSetFromOptions_PIPELCG() in src/ksp/ksp/impls/cg/pipelcg/pipelcg.c
KSPSetFromOptions_PIPEPRCG() in src/ksp/ksp/impls/cg/pipeprcg/pipeprcg.c
KSPSetFromOptions_Chebyshev() in src/ksp/ksp/impls/cheby/cheby.c
KSPSetFromOptions_FCG() in src/ksp/ksp/impls/fcg/fcg.c
KSPSetFromOptions_PIPEFCG() in src/ksp/ksp/impls/fcg/pipefcg/pipefcg.c
KSPSetFromOptions_FETIDP() in src/ksp/ksp/impls/fetidp/fetidp.c
KSPSetFromOptions_GCR() in src/ksp/ksp/impls/gcr/gcr.c
KSPSetFromOptions_PIPEGCR() in src/ksp/ksp/impls/gcr/pipegcr/pipegcr.c
KSPSetFromOptions_AGMRES() in src/ksp/ksp/impls/gmres/agmres/agmres.c
KSPSetFromOptions_DGMRES() in src/ksp/ksp/impls/gmres/dgmres/dgmres.c
KSPSetFromOptions_FGMRES() in src/ksp/ksp/impls/gmres/fgmres/fgmres.c
KSPSetFromOptions_GMRES() in src/ksp/ksp/impls/gmres/gmres.c
KSPSetFromOptions_LGMRES() in src/ksp/ksp/impls/gmres/lgmres/lgmres.c
KSPSetFromOptions_PGMRES() in src/ksp/ksp/impls/gmres/pgmres/pgmres.c
KSPSetFromOptions_PIPEFGMRES() in src/ksp/ksp/impls/gmres/pipefgmres/pipefgmres.c
KSPSetFromOptions_HPDDM() in src/ksp/ksp/impls/hpddm/hpddm.cxx
KSPSetFromOptions_LCD() in src/ksp/ksp/impls/lcd/lcd.c
KSPSetFromOptions_LSQR() in src/ksp/ksp/impls/lsqr/lsqr.c
KSPSetFromOptions_MINRES() in src/ksp/ksp/impls/minres/minres.c
KSPSetFromOptions_QCG() in src/ksp/ksp/impls/qcg/qcg.c
KSPSetFromOptions_Richardson() in src/ksp/ksp/impls/rich/rich.c
KSPSetFromOptions_TSIRM() in src/ksp/ksp/impls/tsirm/tsirm.c

Index of all KSP routines
Table of Contents for all manual pages
Index of all manual pages