2015-02-02 16:34:24 +01:00
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< H2 > Overview< / H2 >
Example that uses parallel_do to do parallel preorder traversal of a sparse graph.
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Each vertex in the graph is called a "cell".
Each cell has a value.
The value is a matrix.
Some of the cells have operators
that compute the cell's value, using other cell's values as input.
A cell that uses the value of cell x is called a successor of x.
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The algorithm works as follows.
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< LI > Compute the set of cells that have no inputs. This set is called < TT > root_set< / TT > .
< LI > Each cell has an associated field < TT > ref_count< / TT > that is an atomic integer.
Initialize < TT > ref_count< / TT > to the number of inputs for the Cell.
< LI > Update each cell in < TT > root_set< / TT > , by applying a < TT > parallel_do< / TT > to a < TT > root_set< / TT >
< LI > After updating a cell, for each of its successors
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< LI > Atomically decrement the successor's < TT > ref_count< / TT >
< LI > If the count became zero, add the cell to the set of cells to be updated,
by calling < TT > parallel_do_feeder_impl::add< / TT > .
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< / P > < P >
The times printed are for the traversal and update,
and do not include time for computing the root_set.
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The example is using custom synchronization via < TT > ref_count< / TT > atomic variable.
Correctness checking tools might not take this into account, and report data races
between different tasks that are actually synchronized.
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< B > NOTE: < / B > It is important to understand that this example is unlikely to show speedup
if the cell values are changed to type "float". The reason is twofold.
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< LI > The smaller value type causes each Cell to be significantly smaller than a cache line,
which leads to false sharing conflicts.
< LI > The time to update the cells becomes very small, and consequently the overhead of
parallel_do swamps the useful work.
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< H2 > Files< / H2 >
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< DT > < A HREF = "main.cpp" > main.cpp< / A >
< DD > Main program which parses command line options and runs the algorithm with different numbers of threads.
< DT > < A HREF = "parallel_preorder.cpp" > parallel_preorder.cpp< / A >
< DD > Implementation of the parallel preorder traversal algorithm.
< DT > < A HREF = "Graph.h" > Graph.h< / A >
< DD > Interfaces of the Graph and Cell classes.
< DT > < A HREF = "Graph.cpp" > Graph.cpp< / A >
< DD > Implementations of the Graph and Cell classes.
< DT > < A HREF = "Matrix.h" > Matrix.h< / A >
< DD > The Matrix class definition.
< DT > < A HREF = "Makefile" > Makefile< / A >
< DD > Makefile for building example.
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< H2 > Directories< / H2 >
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< DT > < A HREF = "msvs" > msvs< / A >
< DD > Contains Microsoft* Visual Studio* 2010 workspace for building and running the example (Windows* systems only).
< DT > < A HREF = "xcode" > xcode< / A >
< DD > Contains Xcode* IDE workspace for building and running the example (OS X* systems only).
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< H2 > To Build< / H2 >
General build directions can be found < A HREF = ../../index.html#build > here< / A > .
< H2 > Usage< / H2 >
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< DT > < TT > parallel_preorder < I > -h< / I > < / TT >
< DD > Prints the help for command line options
< 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 >
< DT > < TT > parallel_preorder [< I > n-of-threads< / I > [< I > n-of-nodes< / I > [< I > n-of-traversals< / I > ]]] [< I > silent< / I > ] < / TT >
< 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 >
< I > n-of-nodes< / I > is a number of nodes in the graph. Default value is 1000.< BR >
< I > n-of-traversals< / I > is the number of times to evaluate the graph. Default value is 500.< BR >
< I > silent< / I > - no output except elapsed time.< BR >
< DT > To run a short version of this example, e.g., for use with Intel® Parallel Inspector:
< DD > Build a < I > debug< / I > version of the example
(see the < A HREF = ../../index.html#build > build directions< / A > ).
< 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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< A HREF = "../index.html" > Up to parent directory< / A >
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2016-05-12 12:28:09 +01:00
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