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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head>	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />	<style type="text/css">	body { font-family: Verdana, Arial, Helvetica, sans-serif;}	a.at-term {	font-style: italic; }	</style>	<title>Introduction to Virtual Topologies</title>	<meta name="Generator" content="ATutor">	<meta name="Keywords" content=""></head><body> <p>You learned from the MPI example programs that the default structure of the processes in a communicator is a one dimensional array. Starting with the process with Rank 0, each process is given an incrementally larger rank number and has two neighbors. The first and last process, of course, only have one neighbor.</p>

<img SRC="virtop1.gif" height=140 width=462>

<p>The MPI implementation is then responsible for mapping the logical processes to the physical processors. Two observations can be made of this arrangement:</p>

<ol>
<li>
How does the system know the optimal mapping of logical processes to physical
processors?</li>

<li>
Many real world problems are not described naturally with 1-D structures</li>
</ol>

<p>To address these issues, and many more, a set of routines are provided that allow the creation of a "Virtual Topology". This effectively allows you to create a new mapping for your logical processes. Thus, if your problem is naturally described by a tree or a 2-D mesh, you can
construct a tree or 2-D mesh topology.</p>

<p>Virtual topologies provide the following potential benefits:</p>

<ul>
<li>Convenient process naming</li>

<li>Naming scheme to fit the communication pattern</li>

<li>Simplifies writing of code</li>

<li>Can allow MPI to optimize communications</li>

<li>Rationale: access to useful topology routines</li>
</ul>

<h3>How to use a Virtual Topology</h3>

<p>There are several steps that must be taken to establish a new virtual topology.</p>

<ul>
<li>
Creating a topology produces a new communicator which will be used in MPI
routine calls</li>

<li>
MPI provides "mapping functions" to convert processor rank to the coordinate
system defined by the new topology scheme</li>
</ul></body></html>
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