# KSPSetTolerances

Sets the relative, absolute, divergence, and maximum iteration tolerances used by the default KSP convergence testers.

### Synopsis

    #include "petscksp.h"
    PetscErrorCode PETSCKSP_DLLEXPORT KSPSetTolerances(KSP ksp,PetscReal rtol,PetscReal abstol,PetscReal dtol,PetscInt maxits)

Collective on KSP

### Input Parameters

| ksp | - the Krylov subspace context
| rtol | - the relative convergence tolerance (relative decrease in the residual norm)
| abstol | - the absolute convergence tolerance (absolute size of the residual norm)
| dtol | - the divergence tolerance (amount residual can increase before KSPDefaultConverged() concludes that the method is diverging)
| maxits | - maximum number of iterations to use

### Options Database Keys

| -ksp_atol <abstol> | - Sets abstol
| -ksp_rtol <rtol> | - Sets rtol
| -ksp_divtol <dtol> | - Sets dtol
| -ksp_max_it <maxits> | - Sets maxits

### Notes

Use PETSC_DEFAULT to retain the default value of any of the tolerances.

See KSPDefaultConverged() for details on the use of these parameters in the default convergence test. See also KSPSetConvergenceTest() for setting user-defined stopping criteria.

### Keywords

KSP, set, tolerance, absolute, relative, divergence,
convergence, maximum, iterations

### See Also

KSPGetTolerances(), KSPDefaultConverged(), KSPSetConvergenceTest()

Level:intermediate
Location:src/ksp/ksp/interface/itfunc.c
Index of all KSP routines
Table of Contents for all manual pages
Index of all manual pages

### Examples

src/ksp/ksp/examples/tutorials/ex1.c.html
src/ksp/ksp/examples/tutorials/ex2.c.html
src/ksp/ksp/examples/tutorials/ex7.c.html
src/ksp/ksp/examples/tutorials/ex8.c.html
src/ksp/ksp/examples/tutorials/ex15.c.html
src/ksp/ksp/examples/tutorials/ex23.c.html
src/ksp/ksp/examples/tutorials/ex1f.F.html
src/ksp/ksp/examples/tutorials/ex2f.F.html
src/ksp/ksp/examples/tutorials/ex15f.F.html
src/ksp/ksp/examples/tutorials/ex21f.F.html
src/snes/examples/tutorials/ex1.c.html