KSPComputeExtremeSingularValues#

Computes the extreme singular values for the preconditioned operator. Called after or during KSPSolve().

Synopsis#

#include "petscksp.h" 
#include "petscmat.h" 
PetscErrorCode KSPComputeExtremeSingularValues(KSP ksp, PetscReal *emax, PetscReal *emin)

Not Collective

Input Parameter#

ksp - iterative context obtained from KSPCreate()

Output Parameters#

emax - maximum estimated singular value
emin - minimum estimated singular value

Options Database Key#

-ksp_view_singularvalues - compute extreme singular values and print when KSPSolve() completes.

Notes#

One must call KSPSetComputeSingularValues() before calling KSPSetUp() (or use the option -ksp_view_eigenvalues) in order for this routine to work correctly.

Many users may just want to use the monitoring routine KSPMonitorSingularValue() (which can be set with option -ksp_monitor_singular_value) to print the extreme singular values at each iteration of the linear solve.

Estimates of the smallest singular value may be very inaccurate, especially if the Krylov method has not converged. The largest singular value is usually accurate to within a few percent if the method has converged, but is still not intended for eigenanalysis. Consider the excellent package SLEPc if accurate values are required.

Disable restarts if using KSPGMRES, otherwise this estimate will only be using those iterations after the last restart. See KSPGMRESSetRestart() for more details.