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<B> Next:</B> <A NAME="tex2html2556" HREF="node36.html">Call the ScaLAPACK Routine</A>
<B>Up:</B> <A NAME="tex2html2554" HREF="node33.html">Four Basic Steps Required </A>
<B> Previous:</B> <A NAME="tex2html2548" HREF="node34.html">Initialize the Process Grid</A>
<BR> <P>
<H2><A NAME="SECTION04242000000000000000">Distribute the Matrix on the Process Grid</A></H2>
<P>
All global matrices must be distributed
on the process grid prior to the invocation of a ScaLAPACK routine.
It is the user's responsibility to perform this data distribution.
For further information on the appropriate data distribution, please refer to
Chapter <A HREF="node68.html#chap2dbsd">4</A>.
<P>
Each global matrix that is to be distributed across the process grid must
be assigned an <EM>array descriptor</EM><A NAME="824"> </A><A NAME="825"> </A><A NAME="826"> </A>. Details of the entries in the array
descriptor can be found in section <A HREF="node77.html#secdesc1">4.3.3</A>. This array
descriptor is
most easily initialized with a call to a ScaLAPACK TOOLS routine called
DESCINIT<A NAME="828"> </A> and <EM>must</EM> be set prior to the
invocation of a ScaLAPACK routine.
<P>
As an example, the array descriptors<A NAME="830"> </A> for
the matrices in
figures <A HREF="node28.html#figgrida9">2.2</A> and <A HREF="node28.html#figgridb9">2.3</A> are assigned with
the following code excerpt from the example program in
section <A HREF="node28.html#example1">2.3</A>.
<PRE> CALL DESCINIT( DESCA, M, N, MB, NB, RSRC, CSRC, ICTXT, MXLLDA,
$ INFO )
CALL DESCINIT( DESCB, N, NRHS, NB, NBRHS, RSRC, CSRC, ICTXT,
$ MXLLDB, INFO )</PRE>
These two calls to DESCINIT are equivalent to the assignment
statements:
<PRE> DESCA( 1 ) = 1
DESCA( 2 ) = ICTXT
DESCA( 3 ) = M
DESCA( 4 ) = N
DESCA( 5 ) = MB
DESCA( 6 ) = NB
DESCA( 7 ) = RSRC
DESCA( 8 ) = CSRC
DESCA( 9 ) = MXLLDA
*
DESCB( 1 ) = 1
DESCB( 2 ) = ICTXT
DESCB( 3 ) = N
DESCB( 4 ) = NRHS
DESCB( 5 ) = NB
DESCB( 6 ) = NBRHS
DESCB( 7 ) = RSRC
DESCB( 8 ) = CSRC
DESCB( 9 ) = MXLLDB</PRE>
<P>
Details of the entries in the array descriptor can be found in
section <A HREF="node77.html#secdesc1">4.3.3</A>.
<P>
A simplistic mapping of the global matrix in figure <A HREF="node28.html#figgrida9">2.2</A> to a
process grid is accomplished in the example program in section <A HREF="node28.html#example1">2.3</A>
via a call to the subroutine MATINIT. Please note that the routine
MATINIT is not a ScaLAPACK routine and is used in this example
program for demonstrative purposes only.
<P>
Appendix <A HREF="node180.html#example2">C.1</A> provides a more detailed example program, which
reads a matrix from a file, distributes it onto a process grid,
and then writes the solution to a file.
<P>
<BR> <HR>
<P><ADDRESS>
<I>Susan Blackford <BR>
Tue May 13 09:21:01 EDT 1997</I>
</ADDRESS>
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