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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 to a sparse matrix, it
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contains data about general matrix informations and elements to be
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exchanged among processes.
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<P>
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It is not necessary for the user to know the internal structure of
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<code>psb_desc_type</code>, it is set in a transparent mode by the tools
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routines of Sec. <A HREF="node42.html#sec:toolsrout">6</A>, and its fields may be accessed
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if necessary via the routines of sec. <A HREF="node14.html#sec:dataquery">3.4</A>;
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nevertheless we include a description for the curious
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reader:
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<DL>
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<DT><STRONG><B>matrix_data</B></STRONG></DT>
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<DD>includes general information about matrix and
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process grid, such as the communication context, the size of the
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global matrix, the size of the portion of matrix stored on the current
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process, and so on.
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Specified as: an allocatable integer array of dimension <code>psb_mdata_size_</code>.
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</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>loc_to_glob</B></STRONG></DT>
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<DD>each element <IMG
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WIDTH="10" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
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SRC="img3.png"
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ALT="$i$"> of this array contains
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global identifier of the local variable <IMG
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WIDTH="10" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
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SRC="img3.png"
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ALT="$i$">.
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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>glob_to_loc, glb_lc, hashv</B></STRONG></DT>
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<DD>Contain a mapping from
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global to local indices.
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</DD>
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</DL>
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The Fortran 95 definition 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="618"></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="536" HEIGHT="218" ALIGN="MIDDLE" BORDER="0"
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SRC="img17.png"
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ALT="\fbox{\TheSbox}">
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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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The global to local index mapping may be stored in two different
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formats: the first is simpler but more expensive, as it requires on
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each process an amount of memory proportional to the global size of
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the index space; the second is more complex, but only requires memory
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proportional to the local index space size. The choice is made at the
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time of the initialization according to a threshold; this threshold
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may be queried and set using the functions in
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sec. <A HREF="node14.html#sec:dataquery">3.4</A>.
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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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<LI><A NAME="tex2html283"
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HREF="node10.html">Named Constants</A>
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