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<TITLE>KSPSetLagNorm</TITLE>
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<A NAME="KSPSetLagNorm"><H1>KSPSetLagNorm</H1></A>
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.
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscksp.h"
PetscErrorCode KSPSetLagNorm(KSP ksp,PetscBool flg)
</PRE>
<P>
Logically Collective on <A HREF="../KSP/KSP.html#KSP">KSP</A>
<P>
<H3><FONT COLOR="#CC3333">Input Parameter</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>ksp </B></TD><TD>- Krylov solver context
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>flg </B></TD><TD>- <A HREF="../Sys/PETSC_TRUE.html#PETSC_TRUE">PETSC_TRUE</A> or <A HREF="../Sys/PETSC_FALSE.html#PETSC_FALSE">PETSC_FALSE</A>
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Options Database Keys</FONT></H3>
<DT><B>-ksp_lag_norm </B> -lag the calculated residual norm
<br>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
Currently only works with <A HREF="../KSP/KSPIBCGS.html#KSPIBCGS">KSPIBCGS</A>.
<P>
Use <A HREF="../KSP/KSPSetNormType.html#KSPSetNormType">KSPSetNormType</A>(ksp,<A HREF="../KSP/KSP_NORM_NONE.html#KSP_NORM_NONE">KSP_NORM_NONE</A>) to never check the norm
<P>
If you lag the norm and run with, for example, -ksp_monitor, the residual norm reported will be the lagged one.
<P>
<H3><FONT COLOR="#CC3333">Keywords</FONT></H3>
<A HREF="../KSP/KSP.html#KSP">KSP</A>, create, context, norms
<BR>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../KSP/KSPSetUp.html#KSPSetUp">KSPSetUp</A>(), <A HREF="../KSP/KSPSolve.html#KSPSolve">KSPSolve</A>(), <A HREF="../KSP/KSPDestroy.html#KSPDestroy">KSPDestroy</A>(), <A HREF="../KSP/KSPSkipConverged.html#KSPSkipConverged">KSPSkipConverged</A>(), <A HREF="../KSP/KSPSetNormType.html#KSPSetNormType">KSPSetNormType</A>()
<BR><P><B><P><B><FONT COLOR="#CC3333">Level:</FONT></B>advanced
<BR><FONT COLOR="#CC3333">Location:</FONT></B><A HREF="../../../src/ksp/ksp/interface/itcreate.c.html#KSPSetLagNorm">src/ksp/ksp/interface/itcreate.c</A>
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