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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>Point to Point Communications</title>	<meta name="Generator" content="ATutor">	<meta name="Keywords" content=""></head><body> <p>The MPI Standard implements message passing with a Point-to-Point communication protocol.&nbsp; The Point-to-Point protocol&nbsp; implies that there is
one source process and one destination process for the data to be transmitted.</p>

<img SRC="point2point1.gif" height=236 width=442>

<p>MPI Version 1.2 supports two sided communication, where a function call must be made at each processor in order for data to be transferred. MPI version 2.0 will support one sided communication, where a function call can be made on one processor to send/fetch data to/from a remote process. 
The remote process does not need to issue a function call and, therefore, may not know about the data transfer.  The potential benefit is lower overhead and faster communication.</p>

<p>For two sided point-to-point communication, each process is aware of the data transfer. The source process initiates the sending of the message to the destination process and the destination process initiates the receive of the message from the source.&nbsp; The order of which is posted first, the send or receive, is variable.</p>

<h3>Completion Criteria</h3>

When MPI communications are discussed, we will talk about the completion criteria of the communication. <em><a href="../glossary.html#Completion+criteria" target="body" class="at-term">Completion criteria</a></em> is when the memory locations used in the message transfer can be safely accessed.  This is a general definition that applies to both send and receive operations.  To define the completion criteria of different functions, it is useful to divide the definition into two groups:</p>
<ol>
<li>
Completion criteria for a routine that sends data<br /><br/>

A send operation meets its completion criteria when the variable or array elements being sent sent can be reused after completion.&nbsp; This means that the value(s) to be sent has been copied out of the memory location; either sent over the network to the remote process or copied to a buffer.</li>

<li>
Completion criteria for a routine that receives data<br/>
<br />

A receive operation meets its completion criteria when the
variable or array elements receiving the data can be used.&nbsp; This means that the value(s) to be received have been placed into the memory location of the variable specified in the receive function call; either received
directly from the network or copied from a system buffer.</li>
</ol></body></html>
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