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Wenzel Jakob
100 lines
4.2 KiB
HTML
100 lines
4.2 KiB
HTML
<HTML>
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<BODY>
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<H2>Overview</H2>
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Example that uses parallel_do to do parallel preorder traversal of a sparse graph.
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<P>
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Each vertex in the graph is called a "cell".
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Each cell has a value.
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The value is a matrix.
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Some of the cells have operators
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that compute the cell's value, using other cell's values as input.
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A cell that uses the value of cell x is called a successor of x.
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</P><P>
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The algorithm works as follows.
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<OL>
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<LI> Compute the set of cells that have no inputs. This set is called <TT>root_set</TT>.
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<LI> Each cell has an associated field <TT>ref_count</TT> that is an atomic integer.
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Initialize <TT>ref_count</TT> to the number of inputs for the Cell.
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<LI> Update each cell in <TT>root_set</TT>, by applying a <TT>parallel_do</TT> to a <TT>root_set</TT>
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<LI> After updating a cell, for each of its successors
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<OL>
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<LI> Atomically decrement the successor's <TT>ref_count</TT>
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<LI> If the count became zero, add the cell to the set of cells to be updated,
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by calling <TT>parallel_do_feeder_impl::add</TT>.
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</OL>
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</OL>
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</P><P>
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The times printed are for the traversal and update,
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and do not include time for computing the root_set.
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</P><P>
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The example is using custom synchronization via <TT>ref_count</TT> atomic variable.
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Correctness checking tools might not take this into account, and report data races
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between different tasks that are actually synchronized.
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</P>
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<B>NOTE: </B>It is important to understand that this example is unlikely to show speedup
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if the cell values are changed to type "float". The reason is twofold.
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<UL>
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<LI> The smaller value type causes each Cell to be significantly smaller than a cache line,
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which leads to false sharing conflicts.
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<LI> The time to update the cells becomes very small, and consequently the overhead of
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parallel_do swamps the useful work.
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</UL>
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<H2>Files</H2>
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<DL>
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<DT><A HREF="main.cpp">main.cpp</A>
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<DD>Main program which parses command line options and runs the algorithm with different numbers of threads.
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<DT><A HREF="parallel_preorder.cpp">parallel_preorder.cpp</A>
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<DD>Implementation of the parallel preorder traversal algorithm.
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<DT><A HREF="Graph.h">Graph.h</A>
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<DD>Interfaces of the Graph and Cell classes.
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<DT><A HREF="Graph.cpp">Graph.cpp</A>
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<DD>Implementations of the Graph and Cell classes.
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<DT><A HREF="Matrix.h">Matrix.h</A>
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<DD>The Matrix class definition.
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<DT><A HREF="Makefile">Makefile</A>
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<DD>Makefile for building example.
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</DL>
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<H2>Directories</H2>
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<DL>
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<DT><A HREF="msvs">msvs</A>
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<DD>Contains Microsoft* Visual Studio* 2010 workspace for building and running the example (Windows* systems only).
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<DT><A HREF="xcode">xcode</A>
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<DD>Contains Xcode* IDE workspace for building and running the example (OS X* systems only).
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</DL>
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<H2>To Build</H2>
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General build directions can be found <A HREF=../../index.html#build>here</A>.
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<H2>Usage</H2>
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<DL>
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<DT><TT>parallel_preorder <I>-h</I></TT>
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<DD>Prints the help for command line options
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<DT><TT>parallel_preorder [<I>n-of-threads</I>=value] [<I>n-of-nodes</I>=value] [<I>n-of-traversals</I>=value] [<I>silent</I>] </TT>
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<DT><TT>parallel_preorder [<I>n-of-threads</I> [<I>n-of-nodes</I> [<I>n-of-traversals</I>]]] [<I>silent</I>] </TT>
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<DD><I>n-of-threads</I> is the number of threads to use; a range of the form <I>low</I>[:<I>high</I>], where low and optional high are non-negative integers or 'auto' for the TBB default.<BR>
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<I>n-of-nodes</I> is a number of nodes in the graph. Default value is 1000.<BR>
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<I>n-of-traversals</I> is the number of times to evaluate the graph. Default value is 500.<BR>
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<I>silent</I> - no output except elapsed time.<BR>
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<DT>To run a short version of this example, e.g., for use with Intel® Parallel Inspector:
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<DD>Build a <I>debug</I> version of the example
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(see the <A HREF=../../index.html#build>build directions</A>).
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<BR>Run it with the desired number of threads and smaller number of traversals, e.g., <TT>parallel_preorder 4 1000 5</TT>.
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</DL>
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<HR>
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<A HREF="../index.html">Up to parent directory</A>
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<p></p>
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Copyright © 2005-2016 Intel Corporation. All Rights Reserved.
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<P></P>
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Intel is a registered trademark or trademark of Intel Corporation
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or its subsidiaries in the United States and other countries.
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<p></p>
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* Other names and brands may be claimed as the property of others.
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