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<H1><A NAME="SECTION04440000000000000000">In-Core Narrow Band and Tridiagonal Matrices</A></H1>
<P>
The ScaLAPACK routines solving
narrow-band and tridiagonal
linear systems assume their
operands to be distributed
according to the block-column
and block-row data distribution
schemes.  Specifically, the
narrow band or tridiagonal 
coefficient matrix is 
distributed in a block-column
fashion, and the dense matrix of
right hand side vectors is 
distributed in a block-row 
fashion. This section presents
these distributions and 
demonstrates how the 
ScaLAPACK software
encodes this essential
information as well as
the related software
conventions.
<P>
The <EM>block</EM> data layout has
been selected for narrow band
matrices.  Divide-and-conquer
algorithms have been implemented
in ScaLAPACK because these 
algorithms offer a much greater
scope for exploiting parallelism
than the corresponding adapted
dense algorithms. The narrow 
band or tridiagonal coefficient
matrix is partitioned into 
blocks. The inherent parallelism
of these divide-and-conquer
methods is limited by the 
number of these blocks because
each block is processed 
independently; hence, it
is necessary to choose the
number of blocks at least
equal to the desired parallelism.
However, because the size of the
reduced system is proportional to
the number of blocks, and solving
this reduced system is the major
parallelism bottleneck, it follows
that a <EM>block</EM> layout in which
each process has exactly one 
block allows maximum exploitation
of parallelism while minimizing 
the size of the reduced system.
<P>
<BR> <HR>
<UL><A NAME="CHILD_LINKS">&#160;</A>
<LI> <A NAME="tex2html3196" HREF="node82.html#SECTION04441000000000000000">The Block Column and Row Distributions</A>
<LI> <A NAME="tex2html3197" HREF="node83.html#SECTION04442000000000000000">The Block Mapping</A>
<LI> <A NAME="tex2html3198" HREF="node84.html#SECTION04443000000000000000">Local Storage Scheme for Narrow Band Matrices</A>
<LI> <A NAME="tex2html3199" HREF="node85.html#SECTION04444000000000000000">Local Storage Schemes for Tridiagonal Matrices</A>
<LI> <A NAME="tex2html3200" HREF="node86.html#SECTION04445000000000000000">Array Descriptor for Narrow Band and Tridiagonal Matrices</A>
<LI> <A NAME="tex2html3201" HREF="node87.html#SECTION04446000000000000000">Array Descriptor for the Matrix of Right-Hand-Side Vectors</A>
<LI> <A NAME="tex2html3202" HREF="node88.html#SECTION04447000000000000000">Argument Descriptions for Band and Tridiagonal Routines</A>
<LI> <A NAME="tex2html3203" HREF="node89.html#SECTION04448000000000000000">Matrix Storage Conventions for Band and Tridiagonal Matrices</A>
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<P><ADDRESS>
<I>Susan Blackford <BR>
Tue May 13 09:21:01 EDT 1997</I>
</ADDRESS>
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