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<div id="version" align=right><b>petsc-3.4.2 2013-07-02</b></div>
<A NAME="MatBlockMatSetPreallocation"><H1>MatBlockMatSetPreallocation</H1></A>
For good matrix assembly performance the user should preallocate the matrix storage by setting the parameter nz (or the array nnz). By setting these parameters accurately, performance during matrix assembly can be increased by more than a factor of 50.
<H3><FONT COLOR="#CC3333">Synopsis</FONT></H3>
<PRE>
#include "petscmat.h"
PetscErrorCode MatBlockMatSetPreallocation(Mat B,PetscInt bs,PetscInt nz,const PetscInt nnz[])
</PRE>
Collective on <A HREF="../Sys/MPI_Comm.html#MPI_Comm">MPI_Comm</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>B </B></TD><TD>- The matrix
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>bs </B></TD><TD>- size of each block in matrix
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>nz </B></TD><TD>- number of nonzeros per block row (same for all rows)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>nnz </B></TD><TD>- array containing the number of nonzeros in the various block rows
(possibly different for each row) or NULL
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
If nnz is given then nz is ignored
<P>
Specify the preallocated storage with either nz or nnz (not both).
Set nz=<A HREF="../Sys/PETSC_DEFAULT.html#PETSC_DEFAULT">PETSC_DEFAULT</A> and nnz=NULL for PETSc to control dynamic memory
allocation. For large problems you MUST preallocate memory or you
will get TERRIBLE performance, see the users' manual chapter on matrices.
<P>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../Mat/MatCreate.html#MatCreate">MatCreate</A>(), <A HREF="../Mat/MatCreateBlockMat.html#MatCreateBlockMat">MatCreateBlockMat</A>(), <A HREF="../Mat/MatSetValues.html#MatSetValues">MatSetValues</A>()
<BR>
<P>
<P><B><P><B><FONT COLOR="#CC3333">Level:</FONT></B>intermediate
<BR><FONT COLOR="#CC3333">Location:</FONT></B><A HREF="../../../src/mat/impls/blockmat/seq/blockmat.c.html#MatBlockMatSetPreallocation">src/mat/impls/blockmat/seq/blockmat.c</A>
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