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<div id="version" align=right><b>petsc-3.7.5 2017-01-01</b></div>
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<A NAME="KSPCGUseSingleReduction"><H1>KSPCGUseSingleReduction</H1></A>
Merge the two inner products needed in CG into a single MPI_Allreduce() call.
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscksp.h"
PetscErrorCode KSPCGUseSingleReduction(KSP ksp,PetscBool flg)
</PRE>
Logically Collective on <A HREF="../KSP/KSP.html#KSP">KSP</A>
<P>
<H3><FONT COLOR="#CC3333">Input Parameters</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>ksp </B></TD><TD>- the iterative context
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>flg </B></TD><TD>- turn on or off the single reduction
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Options Database</FONT></H3>
<DT><B>-ksp_cg_single_reduction</B> -
<br>
<P>
The algorithm used in this case is described as Method 1 in Lapack Working Note 56, "Conjugate Gradient Algorithms with Reduced Synchronization Overhead
Distributed Memory Multiprocessors", by E. F. D'Azevedo, V. L. Eijkhout, and C. H. Romine, December 3, 1999. V. Eijkhout creates the algorithm
initially to Chronopoulos and Gear.
<P>
It requires two extra work vectors than the conventional implementation in PETSc.
<P>
See also <A HREF="../KSP/KSPPIPECG.html#KSPPIPECG">KSPPIPECG</A>, <A HREF="../KSP/KSPPIPECR.html#KSPPIPECR">KSPPIPECR</A>, and <A HREF="../KSP/KSPGROPPCG.html#KSPGROPPCG">KSPGROPPCG</A> that use non-blocking reductions.
<P>
<H3><FONT COLOR="#CC3333">Keywords</FONT></H3>
CG, conjugate gradient, Hermitian, symmetric, set, type, <A HREF="../KSP/KSPPGMRES.html#KSPPGMRES">KSPPGMRES</A>
<BR><P><B><P><B><FONT COLOR="#CC3333">Level:</FONT></B>intermediate
<BR><FONT COLOR="#CC3333">Location:</FONT></B><A HREF="../../../src/ksp/ksp/impls/cg/cgtype.c.html#KSPCGUseSingleReduction">src/ksp/ksp/impls/cg/cgtype.c</A>
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