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# KSPSetTolerances
Sets the relative, absolute, divergence, and maximum iteration tolerances used by the default `KSP` convergence testers. 
## Synopsis
```
#include "petscksp.h" 
#include "petscmat.h" 
PetscErrorCode KSPSetTolerances(KSP ksp, PetscReal rtol, PetscReal abstol, PetscReal dtol, PetscInt maxits)
```
Logically Collective


## Input Parameters

- ***ksp -*** the Krylov subspace context
- ***rtol -*** the relative convergence tolerance, relative decrease in the (possibly preconditioned) residual norm
- ***abstol -*** the absolute convergence tolerance   absolute size of the (possibly preconditioned) residual norm
- ***dtol -*** the divergence tolerance,   amount (possibly preconditioned) residual norm can increase before `KSPConvergedDefault()` 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 `KSPConvergedDefault()` for details how these parameters are used in the default convergence test.  See also `KSPSetConvergenceTest()`
for setting user-defined stopping criteria.


## See Also
 [](ch_ksp), `KSPGetTolerances()`, `KSPConvergedDefault()`, `KSPSetConvergenceTest()`, `KSP`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/interface/itfunc.c.html#KSPSetTolerances">src/ksp/ksp/interface/itfunc.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex1.c.html">src/ksp/ksp/tutorials/ex1.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex15.c.html">src/ksp/ksp/tutorials/ex15.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex15f.F90.html">src/ksp/ksp/tutorials/ex15f.F90</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex18.c.html">src/ksp/ksp/tutorials/ex18.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex1f.F90.html">src/ksp/ksp/tutorials/ex1f.F90</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex2.c.html">src/ksp/ksp/tutorials/ex2.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex23.c.html">src/ksp/ksp/tutorials/ex23.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex2f.F90.html">src/ksp/ksp/tutorials/ex2f.F90</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex4.c.html">src/ksp/ksp/tutorials/ex4.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex52f.F90.html">src/ksp/ksp/tutorials/ex52f.F90</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex6.c.html">src/ksp/ksp/tutorials/ex6.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ksp/ksp/interface/itfunc.c)


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