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<H2><A NAME="SECTION00041000000000000000"></A>
<A NAME="sec:desc"></A>
<BR>
Descriptor data structure
</H2>
All the general matrix informations and elements to be
exchanged among processes are stored within a data structure of the
type descdata<TT>psb_desc_type</TT>. 
Every structure of this type is associated with a discretization
pattern and enables data communications and other operations that are
necessary for implementing the various algorithms of interest to us. 

<P>
The data structure itself <code>psb_desc_type</code> can be treated as an
opaque object handled via the   tools routines of
Sec.&nbsp;<A HREF="node66.html#sec:toolsrout">6</A> or the query routines detailed below;
nevertheless we include here a  description for the curious 
reader. 

<P>
First we describe the <code>psb_indx_map</code> type. This is a data
structure that keeps track of a certain number of basic issues such
as:

<UL>
<LI>The value of the communication/MPI context;
</LI>
<LI>The number of indices in the index space, i.e. global number of
  rows and columns of a sparse matrix;
</LI>
<LI>The local set of indices, including:

<UL>
<LI>The number of local indices (and local rows);
</LI>
<LI>The number of halo indices (and therefore local columns); 
</LI>
<LI>The global indices corresponding to the local ones. 
</LI>
</UL>
</LI>
</UL>
There are many different schemes for storing these data; therefore
there are a number of types extending the base one, and the descriptor
structure holds a polymorphic object whose dynamic type can be any of
the extended  types. 
The methods associated with this data type answer the following
queries:

<UL>
<LI>For a given set of local indices, find the corresponding indices
  in the global numbering;
</LI>
<LI>For a given set of global indices, find the corresponding
  indices in the local numbering, if any, or return an invalid 
</LI>
<LI>Add a global index to the set of halo indices;
</LI>
<LI>Find the process  owner of each member of a set of global
  indices.
</LI>
</UL>
All methods but the last are purely local; the last method potentially
requires communication among processes, and thus is a synchronous
method. The choice of a specific dynamic type for the index map is
made at the time the descriptor is initially allocated, according to
the mode of initialization (see also&nbsp;<A HREF="node66.html#sec:toolsrout">6</A>).

<P>
The descriptor contents are as follows:
<DL>
<DT><STRONG><B>indxmap</B></STRONG></DT>
<DD>A polymorphic variable of a type that is any
  extension of the indx_map type described above. 
<BR></DD>
<DT><STRONG><B>halo_index</B></STRONG></DT>
<DD>A list of the halo and boundary elements for
the current process to be exchanged with other processes; for each
processes with which it is necessary to communicate:

<OL>
<LI>Process identifier;
</LI>
<LI>Number of points to be received;
</LI>
<LI>Indices of points to be received;
</LI>
<LI>Number of points to be sent;
</LI>
<LI>Indices of points to be sent;
</LI>
</OL>
The list may contain an arbitrary number of groups; its end is marked
by a -1.
<BR>
Specified as: an allocatable integer array of rank one.
</DD>
<DT><STRONG><B>ext_index</B></STRONG></DT>
<DD>A list of element indices to be exchanged to
  implement the mapping between a base descriptor and a descriptor
  with overlap. 
</DD>
<DT><STRONG><B>ovrlap_index</B></STRONG></DT>
<DD>A list of the overlap elements for the
current process, organized in groups like the previous vector:

<OL>
<LI>Process identifier;
</LI>
<LI>Number of points to be received;
</LI>
<LI>Indices of points to be received;
</LI>
<LI>Number of points to be sent;
</LI>
<LI>Indices of points to be sent;
</LI>
</OL>
The list may contain an arbitrary number of groups; its end is marked
by a -1.
<BR>
Specified as: an allocatable integer array  of rank one.
</DD>
<DT><STRONG><B>ovr_mst_idx</B></STRONG></DT>
<DD>A list to retrieve the value of each
  overlap element from the respective master process.
<BR>
Specified as: an allocatable integer array of rank one.
</DD>
<DT><STRONG><B>ovrlap_elem</B></STRONG></DT>
<DD>For all overlap points belonging to th
ecurrent process:

<OL>
<LI>Overlap point index;
</LI>
<LI>Number of processes sharing that overlap points;
</LI>
<LI>Index of a ``master'' process: 
</LI>
</OL>
Specified as: an allocatable integer array of rank two.
</DD>
<DT><STRONG><B>bnd_elem</B></STRONG></DT>
<DD>A list of all boundary points, i.e. points
  that have a connection with other processes.
</DD>
</DL>
The Fortran&nbsp;2003 declaration  for <code>psb_desc_type</code> structures is 
as follows:

<DIV ALIGN="CENTER"><A NAME="fig:desctype"></A><A NAME="858"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3:</STRONG>
The PSBLAS defined data type that
    contains the communication descriptor.</CAPTION>
<TR><TD><DIV ALIGN="CENTER">
</DIV><TABLE  WIDTH="90%">
<TR><TD>
<PRE> 
type psb_desc_type 
    class(psb_indx_map), allocatable :: indxmap
    integer, allocatable  :: halo_index(:)
    integer, allocatable  :: ext_index(:)
    integer, allocatable  :: ovrlap_index(:)
    integer, allocatable  :: ovrlap_elem(:,:)
    integer, allocatable  :: ovr_mst_idx(:)
    integer, allocatable  :: bnd_elem(:)
end type psb_desc_type
</PRE></TD></TR>
</TABLE>
<DIV ALIGN="CENTER">
</DIV></TD></TR>
</TABLE>
</DIV>

<P>
A communication descriptor associated with a sparse  matrix has a
state, which can take the following values:
<DL>
<DT><STRONG>Build:</STRONG></DT>
<DD>State entered after the first allocation, and before the
  first assembly; in this state it is possible to add communication
  requirements among different processes. 
</DD>
<DT><STRONG>Assembled:</STRONG></DT>
<DD>State entered after the assembly; computations using
  the associated sparse matrix, such as matrix-vector products, are
  only possible   in this state.
</DD>
</DL>

<P>
<BR><HR>
<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

<UL>
<LI><A NAME="tex2html340"
  HREF="node11.html">Descriptor Methods</A>
<LI><A NAME="tex2html341"
  HREF="node12.html">get_local_rows -- Get number of local rows</A>
<LI><A NAME="tex2html342"
  HREF="node13.html">get_local_cols -- Get number of local cols</A>
<LI><A NAME="tex2html343"
  HREF="node14.html">get_global_rows -- Get number of global rows</A>
<LI><A NAME="tex2html344"
  HREF="node15.html">get_global_cols -- Get number of global cols</A>
<LI><A NAME="tex2html345"
  HREF="node16.html">get_context -- Get communication context</A>
<LI><A NAME="tex2html346"
  HREF="node17.html">Clone -- clone current object</A>
<LI><A NAME="tex2html347"
  HREF="node18.html">psb_cd_get_large_threshold -- Get threshold for
  index mapping switch</A>
<LI><A NAME="tex2html348"
  HREF="node19.html">psb_cd_set_large_threshold -- Set threshold for
  index mapping switch</A>
<LI><A NAME="tex2html349"
  HREF="node20.html">Named Constants</A>
</UL>
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