PETSc version 3.16.5
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# KSPSetLagNorm

Lags the residual norm calculation so that it is computed as part of the MPI_Allreduce() for computing the inner products for the next iteration. This can reduce communication costs at the expense of doing one additional iteration.

### Synopsis

    #include "petscksp.h"
    PetscErrorCode  KSPSetLagNorm(KSP ksp,PetscBool flg)

Logically Collective on ksp

### Input Parameters

| ksp | - Krylov solver context
| flg | - PETSC_TRUE or PETSC_FALSE

### Options Database Keys

| -ksp_lag_norm | - lag the calculated residual norm

### Notes

Currently only works with KSPIBCGS.

Use KSPSetNormType(ksp,KSP_NORM_NONE) to never check the norm

If you lag the norm and run with, for example, -ksp_monitor, the residual norm reported will be the lagged one.

### See Also

KSPSetUp(), KSPSolve(), KSPDestroy(), KSPConvergedSkip(), KSPSetNormType(), KSPSetCheckNormIteration()

### Level

advanced

### Location

src/ksp/ksp/interface/itcreate.c
Index of all KSP routines
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Index of all manual pages