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<div id="version" align=right><b>petsc-3.4.2 2013-07-02</b></div>
<A NAME="MatSeqAIJSetPreallocation"><H1>MatSeqAIJSetPreallocation</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 MatSeqAIJSetPreallocation(Mat B,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-free
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>nz </B></TD><TD>- number of nonzeros per 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 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>
The AIJ format (also called the Yale sparse matrix format or
compressed row storage), is fully compatible with standard Fortran 77
storage. That is, the stored row and column indices can begin at
either one (as in Fortran) or zero. See the users' manual for details.
<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>
You can call <A HREF="../Mat/MatGetInfo.html#MatGetInfo">MatGetInfo</A>() to get information on how effective the preallocation was;
for example the fields mallocs,nz_allocated,nz_used,nz_unneeded;
You can also run with the option -info and look for messages with the string
malloc in them to see if additional memory allocation was needed.
<P>
Developers: Use nz of MAT_SKIP_ALLOCATION to not allocate any space for the matrix
entries or columns indices
<P>
By default, this format uses inodes (identical nodes) when possible, to
improve numerical efficiency of matrix-vector products and solves. We
search for consecutive rows with the same nonzero structure, thereby
reusing matrix information to achieve increased efficiency.
<P>
<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>-mat_no_inode </B></TD><TD>- Do not use inodes
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-mat_inode_limit <limit> </B></TD><TD>- Sets inode limit (max limit=5)
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-mat_aij_oneindex </B></TD><TD>- Internally use indexing starting at 1
rather than 0. Note that when calling <A HREF="../Mat/MatSetValues.html#MatSetValues">MatSetValues</A>(),
the user still MUST index entries starting at 0!
</TD></TR></TABLE>
<P>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../Mat/MatCreate.html#MatCreate">MatCreate</A>(), <A HREF="../Mat/MatCreateAIJ.html#MatCreateAIJ">MatCreateAIJ</A>(), <A HREF="../Mat/MatSetValues.html#MatSetValues">MatSetValues</A>(), <A HREF="../Mat/MatSeqAIJSetColumnIndices.html#MatSeqAIJSetColumnIndices">MatSeqAIJSetColumnIndices</A>(), <A HREF="../Mat/MatCreateSeqAIJWithArrays.html#MatCreateSeqAIJWithArrays">MatCreateSeqAIJWithArrays</A>(), <A HREF="../Mat/MatGetInfo.html#MatGetInfo">MatGetInfo</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/aij/seq/aij.c.html#MatSeqAIJSetPreallocation">src/mat/impls/aij/seq/aij.c</A>
<BR><A HREF="./index.html">Index of all Mat routines</A>
<BR><A HREF="../../index.html">Table of Contents for all manual pages</A>
<BR><A HREF="../singleindex.html">Index of all manual pages</A>
<P><H3><FONT COLOR="#CC3333">Examples</FONT></H3>
<A HREF="../../../src/mat/examples/tutorials/ex15.c.html">src/mat/examples/tutorials/ex15.c.html</A><BR>
<A HREF="../../../src/mat/examples/tutorials/ex16.c.html">src/mat/examples/tutorials/ex16.c.html</A><BR>
<A HREF="../../../src/mat/examples/tutorials/ex17.c.html">src/mat/examples/tutorials/ex17.c.html</A><BR>
<A HREF="../../../src/ksp/ksp/examples/tutorials/ex2.c.html">src/ksp/ksp/examples/tutorials/ex2.c.html</A><BR>
<A HREF="../../../src/ksp/ksp/examples/tutorials/ex3.c.html">src/ksp/ksp/examples/tutorials/ex3.c.html</A><BR>
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<A HREF="../../../src/ksp/ksp/examples/tutorials/ex18.c.html">src/ksp/ksp/examples/tutorials/ex18.c.html</A><BR>
<A HREF="../../../src/ksp/ksp/examples/tutorials/ex52.c.html">src/ksp/ksp/examples/tutorials/ex52.c.html</A><BR>
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