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<h1 class="topictitle1">Message Passing Protocol</h1>
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<p>In an Intel® Threading Building Blocks (Intel® TBB) flow graph, edges dynamically switch between a push and
pull protocol for passing messages. An Intel® TBB flow graph G = ( V, S, L ),
where V is the set of nodes, S is the set of edges that are currently using a
push protocol, and L is the set of edges that are currently using a pull
protocol. For each edge (Vi, Vj), Vi is the predecessor / sender and Vj is the
successor / receiver. When in the push set S, messages over an edge are
initiated by the sender, which tries to
<em>put</em> to the receiver. When in the pull set, messages are initiated
by the receiver, which tries to
<em>get</em> from the sender.
</p>
<p>If a message attempt across an edge fails, the edge is moved to the
other set. For example, if a put across the edge (Vi, Vj) fails, the edge is
removed from the push set S and placed in the pull set L. This dynamic
push/pull protocol is the key to performance in a non-preemptive tasking
library such as Intel® TBB, where simply repeating failed sends or receives is
not an efficient option. The following graphic summarizes this dynamic
protocol.
</p>
<div class="fignone" id="fig4_flow_graph"><a name="fig4_flow_graph"><!-- --></a><span class="figcap">The dynamic push / pull protocol.</span>
<br><div class="imagecenter"><img src="../Resources/flow-graph.jpg" height="196" width="442" alt="The dynamic push / pull protocol." align="center"></div><br>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="../../reference/flow_graph.htm">Flow Graph</a></div>
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