psblas3/docs/html/node9.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">

<!--Converted with LaTeX2HTML 2002-2-1 (1.71)
original version by:  Nikos Drakos, CBLU, University of Leeds
* revised and updated by:  Marcus Hennecke, Ross Moore, Herb Swan
* with significant contributions from:
  Jens Lippmann, Marek Rouchal, Martin Wilck and others -->
<HTML>
<HEAD>
<TITLE>Descriptor data structure</TITLE>
<META NAME="description" CONTENT="Descriptor data structure">
<META NAME="keywords" CONTENT="userhtml">
<META NAME="resource-type" CONTENT="document">
<META NAME="distribution" CONTENT="global">

<META NAME="Generator" CONTENT="LaTeX2HTML v2002-2-1">
<META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css">

<LINK REL="STYLESHEET" HREF="userhtml.css">

<LINK REL="next" HREF="node11.html">
<LINK REL="previous" HREF="node8.html">
<LINK REL="up" HREF="node8.html">
<LINK REL="next" HREF="node10.html">
</HEAD>

<BODY >
<!--Navigation Panel-->
<A NAME="tex2html281"
  HREF="node10.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
 SRC="file:/usr/share/latex2html/icons/next.png"></A> 
<A NAME="tex2html277"
  HREF="node8.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
 SRC="file:/usr/share/latex2html/icons/up.png"></A> 
<A NAME="tex2html271"
  HREF="node8.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
 SRC="file:/usr/share/latex2html/icons/prev.png"></A> 
<A NAME="tex2html279"
  HREF="node1.html">
<IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents"
 SRC="file:/usr/share/latex2html/icons/contents.png"></A>  
<BR>
<B> Next:</B> <A NAME="tex2html282"
  HREF="node10.html">Named Constants</A>
<B> Up:</B> <A NAME="tex2html278"
  HREF="node8.html">Data Structures</A>
<B> Previous:</B> <A NAME="tex2html272"
  HREF="node8.html">Data Structures</A>
 &nbsp; <B>  <A NAME="tex2html280"
  HREF="node1.html">Contents</A></B> 
<BR>
<BR>
<!--End of Navigation Panel-->

<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 to a sparse matrix, it
contains data about general matrix informations and elements to be
exchanged among processes.  

<P>
It is not necessary for the user to know the internal structure of
<code>psb_desc_type</code>, it is set in a transparent mode by the tools
routines of Sec.&nbsp;<A HREF="node42.html#sec:toolsrout">6</A>, and its fields may be accessed
if necessary via  the routines of sec.&nbsp;<A HREF="node14.html#sec:dataquery">3.4</A>;
nevertheless we include a  description for the curious 
reader:   
<DL>
<DT><STRONG><B>matrix_data</B></STRONG></DT>
<DD>includes general information about matrix and
process grid, such as the communication context, the size of the
global matrix, the size of the portion of matrix stored on the current
process, and so on. 
Specified as: an allocatable integer array of dimension <code>psb_mdata_size_</code>.
</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>loc_to_glob</B></STRONG></DT>
<DD>each element <IMG
 WIDTH="10" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
 SRC="img3.png"
 ALT="$i$"> of this array contains
global identifier of the local variable <IMG
 WIDTH="10" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
 SRC="img3.png"
 ALT="$i$">.
<BR>
Specified as: an allocatable integer array of rank one.
</DD>
<DT><STRONG><B>glob_to_loc, glb_lc, hashv</B></STRONG></DT>
<DD>Contain a mapping from
  global to local indices. 
</DD>
</DL>
The Fortran&nbsp;95 definition  for <code>psb_desc_type</code> structures is 
as follows:

<DIV ALIGN="CENTER"><A NAME="fig:desctype"></A><A NAME="618"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3:</STRONG>
The PSBLAS defined data type that
    contains the communication descriptor.</CAPTION>
<TR><TD>
<BR>
   
  <DIV ALIGN="CENTER">
<!-- MATH
 $\fbox{\TheSbox}$
 -->
<IMG
 WIDTH="536" HEIGHT="218" ALIGN="MIDDLE" BORDER="0"
 SRC="img17.png"
 ALT="\fbox{\TheSbox}">
</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>
The global to local index  mapping may be stored in two different
formats: the first is simpler but more expensive, as it requires on
each process an amount of memory proportional to the global size of
the index space; the second is more complex, but only requires memory
proportional to the local index space size. The choice is made at the
time of the initialization according to a threshold; this threshold
may be queried and set using the functions in
sec.&nbsp;<A HREF="node14.html#sec:dataquery">3.4</A>. 

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

<UL>
<LI><A NAME="tex2html283"
  HREF="node10.html">Named Constants</A>
</UL>
<!--End of Table of Child-Links-->
<HR>
<!--Navigation Panel-->
<A NAME="tex2html281"
  HREF="node10.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
 SRC="file:/usr/share/latex2html/icons/next.png"></A> 
<A NAME="tex2html277"
  HREF="node8.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
 SRC="file:/usr/share/latex2html/icons/up.png"></A> 
<A NAME="tex2html271"
  HREF="node8.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
 SRC="file:/usr/share/latex2html/icons/prev.png"></A> 
<A NAME="tex2html279"
  HREF="node1.html">
<IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents"
 SRC="file:/usr/share/latex2html/icons/contents.png"></A>  
<BR>
<B> Next:</B> <A NAME="tex2html282"
  HREF="node10.html">Named Constants</A>
<B> Up:</B> <A NAME="tex2html278"
  HREF="node8.html">Data Structures</A>
<B> Previous:</B> <A NAME="tex2html272"
  HREF="node8.html">Data Structures</A>
 &nbsp; <B>  <A NAME="tex2html280"
  HREF="node1.html">Contents</A></B> 
<!--End of Navigation Panel-->

</BODY>
</HTML>