# KSPSetDiagonalScale#

Tells KSP to symmetrically diagonally scale the system before solving. This actually CHANGES the matrix (and right hand side).

## Synopsis#

#include "petscksp.h"
#include "petscmat.h"
PetscErrorCode KSPSetDiagonalScale(KSP ksp, PetscBool scale)


Logically Collective

## Input Parameters#

• ksp - the KSP context

• scale - PETSC_TRUE or PETSC_FALSE

## Options Database Key#

• -ksp_diagonal_scale -

• -ksp_diagonal_scale_fix - scale the matrix back AFTER the solve

## Notes#

Scales the matrix by D^(-1/2) A D^(-1/2) [D^(1/2) x ] = D^(-1/2) b where D_{ii} is 1/abs(A_{ii}) unless A_{ii} is zero and then it is 1.

BE CAREFUL with this routine: it actually scales the matrix and right hand side that define the system. After the system is solved the matrix and right hand side remain scaled unless you use KSPSetDiagonalScaleFix()

This should NOT be used within the SNES solves if you are using a line search.

If you use this with the PCType PCEISENSTAT preconditioner than you can use the PCEisenstatSetNoDiagonalScaling() option, or -pc_eisenstat_no_diagonal_scaling to save some unneeded, redundant flops.

## See Also#

KSP: Linear System Solvers, KSPGetDiagonalScale(), KSPSetDiagonalScaleFix(), KSP

advanced

## Location#

src/ksp/ksp/interface/itfunc.c

Edit on GitLab