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   <div id="version" align=right><b>petsc-3.10.3 2018-12-18</b></div>
   <div id="bugreport" align=right><a href="mailto:petsc-maint@mcs.anl.gov?subject=Typo or Error in Documentation &body=Please describe the typo or error in the documentation: petsc-3.10.3 v3.10.3 docs/manualpages/PC/PCGAMG.html "><small>Report Typos and Errors</small></a></div>
<A NAME="PCGAMG"><H1>PCGAMG</H1></A>
Geometric algebraic multigrid (AMG) preconditioner 
<H3><FONT COLOR="#CC3333">Options Database Keys</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_type &lt;type&gt; </B></TD><TD>- one of agg, geo, or classical
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_repartition  &lt;true,default=false&gt; </B></TD><TD>- repartition the degrees of freedom accross the coarse grids as they are determined
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_reuse_interpolation &lt;true,default=false&gt; </B></TD><TD>- when rebuilding the algebraic multigrid preconditioner reuse the previously computed interpolations
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_asm_use_agg &lt;true,default=false&gt; </B></TD><TD>- use the aggregates from the coasening process to defined the subdomains on each level for the <A HREF="../PC/PCASM.html#PCASM">PCASM</A> smoother
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_process_eq_limit &lt;limit, default=50&gt; </B></TD><TD>- GAMG will reduce the number of MPI processes used directly on the coarse grids so that there are around &lt;limit&gt;
equations on each process that has degrees of freedom
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_coarse_eq_limit &lt;limit, default=50&gt; </B></TD><TD>- Set maximum number of equations on coarsest grid to aim for.
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_threshold[] &lt;thresh,default=0&gt; </B></TD><TD>- Before aggregating the graph GAMG will remove small values from the graph on each level
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_threshold_scale &lt;scale,default=1&gt; </B></TD><TD>- Scaling of threshold on each coarser grid if not specified
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Options Database Keys for default Aggregation</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_agg_nsmooths &lt;nsmooth, default=1&gt; </B></TD><TD>- number of smoothing steps to use with smooth aggregation
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_sym_graph &lt;true,default=false&gt; </B></TD><TD>- symmetrize the graph before computing the aggregation
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_gamg_square_graph &lt;n,default=1&gt; </B></TD><TD>- number of levels to square the graph before aggregating it
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Multigrid options</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_mg_cycles &lt;v&gt; </B></TD><TD>- v or w, see <A HREF="../PC/PCMGSetCycleType.html#PCMGSetCycleType">PCMGSetCycleType</A>()
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_mg_distinct_smoothup </B></TD><TD>- configure the up and down (pre and post) smoothers separately, see <A HREF="../PC/PCMGSetDistinctSmoothUp.html#PCMGSetDistinctSmoothUp">PCMGSetDistinctSmoothUp</A>()
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_mg_type &lt;multiplicative&gt; </B></TD><TD>- (one of) additive multiplicative full kascade
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-pc_mg_levels &lt;levels&gt; </B></TD><TD>- Number of levels of multigrid to use.
</TD></TR></TABLE>
<P>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
In order to obtain good performance for <A HREF="../PC/PCGAMG.html#PCGAMG">PCGAMG</A> for vector valued problems you must
Call <A HREF="../Mat/MatSetBlockSize.html#MatSetBlockSize">MatSetBlockSize</A>() to indicate the number of degrees of freedom per grid point
Call <A HREF="../Mat/MatSetNearNullSpace.html#MatSetNearNullSpace">MatSetNearNullSpace</A>() (or <A HREF="../PC/PCSetCoordinates.html#PCSetCoordinates">PCSetCoordinates</A>() if solving the equations of elasticity) to indicate the near null space of the operator
See the Users Manual Chapter 4 for more details
<P>

<P>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
  <A HREF="../PC/PCCreate.html#PCCreate">PCCreate</A>(), <A HREF="../PC/PCSetType.html#PCSetType">PCSetType</A>(), <A HREF="../Mat/MatSetBlockSize.html#MatSetBlockSize">MatSetBlockSize</A>(), <A HREF="../PC/PCMGType.html#PCMGType">PCMGType</A>, <A HREF="../PC/PCSetCoordinates.html#PCSetCoordinates">PCSetCoordinates</A>(), <A HREF="../Mat/MatSetNearNullSpace.html#MatSetNearNullSpace">MatSetNearNullSpace</A>(), <A HREF="../PC/PCGAMGSetType.html#PCGAMGSetType">PCGAMGSetType</A>(), PCGAMGAGG, PCGAMGGEO, PCGAMGCLASSICAL, <A HREF="../PC/PCGAMGSetProcEqLim.html#PCGAMGSetProcEqLim">PCGAMGSetProcEqLim</A>(),
<BR><A HREF="../PC/PCGAMGSetCoarseEqLim.html#PCGAMGSetCoarseEqLim">PCGAMGSetCoarseEqLim</A>(), <A HREF="../PC/PCGAMGSetRepartition.html#PCGAMGSetRepartition">PCGAMGSetRepartition</A>(), <A HREF="../PC/PCGAMGRegister.html#PCGAMGRegister">PCGAMGRegister</A>(), <A HREF="../PC/PCGAMGSetReuseInterpolation.html#PCGAMGSetReuseInterpolation">PCGAMGSetReuseInterpolation</A>(), <A HREF="../PC/PCGAMGASMSetUseAggs.html#PCGAMGASMSetUseAggs">PCGAMGASMSetUseAggs</A>(), <A HREF="../PC/PCGAMGSetUseParallelCoarseGridSolve.html#PCGAMGSetUseParallelCoarseGridSolve">PCGAMGSetUseParallelCoarseGridSolve</A>(), <A HREF="../PC/PCGAMGSetNlevels.html#PCGAMGSetNlevels">PCGAMGSetNlevels</A>(), <A HREF="../PC/PCGAMGSetThreshold.html#PCGAMGSetThreshold">PCGAMGSetThreshold</A>(), <A HREF="../PC/PCGAMGGetType.html#PCGAMGGetType">PCGAMGGetType</A>(), <A HREF="../PC/PCGAMGSetReuseInterpolation.html#PCGAMGSetReuseInterpolation">PCGAMGSetReuseInterpolation</A>()
<P><B></B><H3><FONT COLOR="#CC3333">Level</FONT></H3>intermediate<BR>
<H3><FONT COLOR="#CC3333">Location</FONT></H3>
</B><A HREF="../../../src/ksp/pc/impls/gamg/gamg.c.html#PCGAMG">src/ksp/pc/impls/gamg/gamg.c</A>
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