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<H2><A NAME="SECTION00041000000000000000"></A>
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<A NAME="sec:desc"></A>
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<BR>
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Descriptor data structure
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</H2>
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All the general matrix informations and elements to be
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exchanged among processes are stored within a data structure of the
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type descdata<TT>psb_desc_type</TT>.
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Every structure of this type is associated with a discretization
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pattern and enables data communications and other operations that are
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necessary for implementing the various algorithms of interest to us.
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<P>
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The data structure itself <code>psb_desc_type</code> can be treated as an
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opaque object handled via the tools routines of
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Sec. <A HREF="node44.html#sec:toolsrout">6</A> and <A HREF="node15.html#sec:dataquery">3.5</A>;
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nevertheless we include here a description for the curious
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reader.
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<P>
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First we describe the <code>psb_indx_map</code> type. This is a data
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structure that keeps track of a certain number of basic issues such
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as:
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<UL>
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<LI>The value of the communication/MPI context;
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</LI>
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<LI>The number of indices in the index space, i.e. global number of
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rows and columns of a sparse matrix;
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</LI>
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<LI>The local set of indices, including:
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<UL>
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<LI>The number of local indices (and local rows);
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</LI>
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<LI>The number of halo indices (and therefore local columns);
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</LI>
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<LI>The global indices corresponding to the local ones.
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</LI>
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</UL>
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</LI>
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</UL>
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There are many different schemes for storing these data; therefore
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there are a number of types extending the base one, and the descriptor
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structure holds a polymorphic object whose dynamic type can be any of
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the extended types.
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The methods associated with this data type answer the following
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queries:
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<UL>
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<LI>For a given set of local indices, find the corresponding indices
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in the global numbering;
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</LI>
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<LI>For a given set of global indices, find the corresponding
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indices in the local numbering, if any, or return an invalid
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</LI>
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<LI>Add a global index to the set of halo indices;
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</LI>
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<LI>Find the process owner of each member of a set of global
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indices.
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</LI>
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</UL>
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All methods but the last are purely local; the last method potentially
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requires communication among processes, and thus is a synchronous
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method. The choice of a specific dynamic type for the index map is
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made at the time the descriptor is initially allocated, according to
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the mode of initialization (see also <A HREF="node44.html#sec:toolsrout">6</A>).
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<P>
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The descriptor contents are as follows:
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<DL>
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<DT><STRONG><B>indxmap</B></STRONG></DT>
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<DD>A polymorphic variable of a type that is any
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extension of the indx_map type described above.
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<BR></DD>
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<DT><STRONG><B>halo_index</B></STRONG></DT>
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<DD>A list of the halo and boundary elements for
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the current process to be exchanged with other processes; for each
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processes with which it is necessary to communicate:
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<OL>
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<LI>Process identifier;
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</LI>
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<LI>Number of points to be received;
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</LI>
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<LI>Indices of points to be received;
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</LI>
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<LI>Number of points to be sent;
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</LI>
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<LI>Indices of points to be sent;
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</LI>
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</OL>
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The list may contain an arbitrary number of groups; its end is marked
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by a -1.
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<BR>
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Specified as: an allocatable integer array of rank one.
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</DD>
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<DT><STRONG><B>ext_index</B></STRONG></DT>
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<DD>A list of element indices to be exchanged to
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implement the mapping between a base descriptor and a descriptor
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with overlap.
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</DD>
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<DT><STRONG><B>ovrlap_index</B></STRONG></DT>
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<DD>A list of the overlap elements for the
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current process, organized in groups like the previous vector:
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<OL>
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<LI>Process identifier;
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</LI>
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<LI>Number of points to be received;
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</LI>
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<LI>Indices of points to be received;
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</LI>
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<LI>Number of points to be sent;
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</LI>
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<LI>Indices of points to be sent;
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</LI>
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</OL>
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The list may contain an arbitrary number of groups; its end is marked
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by a -1.
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<BR>
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Specified as: an allocatable integer array of rank one.
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</DD>
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<DT><STRONG><B>ovr_mst_idx</B></STRONG></DT>
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<DD>A list to retrieve the value of each
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overlap element from the respective master process.
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<BR>
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Specified as: an allocatable integer array of rank one.
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</DD>
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<DT><STRONG><B>ovrlap_elem</B></STRONG></DT>
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<DD>For all overlap points belonging to th
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ecurrent process:
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<OL>
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<LI>Overlap point index;
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</LI>
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<LI>Number of processes sharing that overlap points;
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</LI>
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<LI>Index of a ``master'' process:
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</LI>
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</OL>
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Specified as: an allocatable integer array of rank two.
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</DD>
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<DT><STRONG><B>bnd_elem</B></STRONG></DT>
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<DD>A list of all boundary points, i.e. points
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that have a connection with other processes.
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</DD>
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</DL>
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The Fortran 2003 declaration for <code>psb_desc_type</code> structures is
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as follows:
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<DIV ALIGN="CENTER"><A NAME="fig:desctype"></A><A NAME="637"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3:</STRONG>
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The PSBLAS defined data type that
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contains the communication descriptor.</CAPTION>
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<TR><TD>
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<BR>
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<DIV ALIGN="CENTER">
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<!-- MATH
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$\fbox{\TheSbox}$
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-->
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<IMG
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WIDTH="535" HEIGHT="218" ALIGN="MIDDLE" BORDER="0"
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SRC="img20.png"
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</DIV></TD></TR>
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</TABLE>
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</DIV>
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<P>
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A communication descriptor associated with a sparse matrix has a
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state, which can take the following values:
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<DL>
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<DT><STRONG>Build:</STRONG></DT>
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<DD>State entered after the first allocation, and before the
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first assembly; in this state it is possible to add communication
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requirements among different processes.
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</DD>
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<DT><STRONG>Assembled:</STRONG></DT>
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<DD>State entered after the assembly; computations using
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the associated sparse matrix, such as matrix-vector products, are
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only possible in this state.
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</DD>
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</DL>
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<P>
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<BR><HR>
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<!--Table of Child-Links-->
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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
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<UL>
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<LI><A NAME="tex2html287"
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HREF="node10.html">Named Constants</A>
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