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<html>
<head>
<title>
SCHEDULE_OPENMP - Scheduling Loop Iterations in OpenMP
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
SCHEDULE_OPENMP <br> Scheduling Loop Iterations in OpenMP
</h1>
<hr>
<p>
<b>SCHEDULE_OPENMP</b>
is a FORTRAN90 program which
demonstrates the use of default, static and dynamic scheduling
of loop iterations in OpenMP.
</p>
<p>
By default, when OpenMP, executing with T threads, encounters a
parallel loop of N iterations, it assigns the first N/T iterations
to thread 0, the second chunk of N/T iterations to thread 1 and so on.
</p>
<p>
Sometimes this simple, default "scheduling" of the iterations is
not ideal. It may be the case that iterations of high index
represent more work. In that case, the lower numbered threads
will finish early and have nothing to do.
</p>
<p>
The <b>static</b> schedule clause modifies the iteration assignment
procedure by essentially "dealing out" the iterations. The clause
<b>schedule(static,5)</b>, for instance, indicates that the N iterations
are to be dealt out in groups of 5, until all are assigned. This
schedule might divide up the work more evenly.
</p>
<p>
In more complicated situations, where the work involved with each
iteration can vary dramatically, the <b>dynamic</b> schedule clause
allows the user to parcel out a small number of iterations initially,
and then to wait until a thread is finished that work, at which
point it is given another group of iterations. The format of this
clause is <b>schedule(dynamic,7)</b> where here 7 iterations are
assigned to each thread initially, and the remaining work is assigned,
7 iterations at a time, to threads that finish what they have already
been assigned.
</p>
<h3 align = "center">
Example:
</h3>
<p>
For simplicity, we assume that we have a loop of 16 iterations,
which has been parallelized by OpenMP, and that we are about
to execute that loop using 2 threads.
</p>
<p>
In <b>default scheduling</b>
<ul>
<li>
thread 1 is assigned to do iterations 1 to 8;
</li>
<li>
thread 2 is assigned to do iterations 9 to 16.
</li>
</ul>
</p>
<p>
In <b>static scheduling</b>, using a "chunksize" of 4:
<ul>
<li>
thread 1 is assigned to do iterations 1 to 4 and 9 to 12.
</li>
<li>
thread 2 is assigned to do iterations 5 to 8 and 13 to 16.
</li>
</ul>
</p>
<p>
In <b>dynamic scheduling</b>, using a "chunksize" of 3:
<ul>
<li>
thread 1 is assigned to do iterations 1 to 3.
</li>
<li>
thread 2 is assigned to do iterations 4 to 6.
</li>
</ul>
The next chunk is iterations 7 to 9, and will be assigned to
whichever thread finishes its current work first, and so on
until all work is completed.
</p>
<h3 align = "center">
Usage:
</h3>
<p>
In the BASH shell, the program could be run with 2 threads using the commands:
<pre>
export OMP_NUM_THREADS=2
./schedule_openmp
</pre>
</p>
<h3 align = "center">
Licensing:
</h3>
<p>
The computer code and data files described and made available on this web page
are distributed under
<a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
</p>
<h3 align = "center">
Languages:
</h3>
<p>
<b>SCHEDULE_OPENMP</b> is available in
<a href = "../../c_src/schedule_openmp/schedule_openmp.html">a C version</a> and
<a href = "../../cpp_src/schedule_openmp/schedule_openmp.html">a C++ version</a> and
<a href = "../../f77_src/schedule_openmp/schedule_openmp.html">a FORTRAN77 version</a> and
<a href = "../../f_src/schedule_openmp/schedule_openmp.html">a FORTRAN90 version</a>.
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../f_src/dijkstra_openmp/dijkstra_openmp.html">
DIJKSTRA_OPENMP</a>,
a FORTRAN90 program which
uses OpenMP to parallelize a simple example of Dijkstra's
minimum distance algorithm for graphs.
</p>
<p>
<a href = "../../f_src/fft_openmp/fft_openmp.html">
FFT_OPENMP</a>,
a FORTRAN90 program which
demonstrates the computation of a Fast Fourier Transform
in parallel, using OpenMP.
</p>
<p>
<a href = "../../f_src/heated_plate_openmp/heated_plate_openmp.html">
HEATED_PLATE_OPENMP</a>,
a FORTRAN90 program which
solves the steady (time independent) heat equation in a 2D
rectangular region, using OpenMP to run in parallel.
</p>
<p>
<a href = "../../f_src/hello_openmp/hello_openmp.html">
HELLO_OPENMP</a>,
a FORTRAN90 program which
prints out "Hello, world!" using the OpenMP parallel programming environment.
</p>
<p>
<a href = "../../f_src/md_openmp/md_openmp.html">
MD_OPENMP</a>,
a FORTRAN90 program which
carries out a molecular dynamics simulation in parallel,
using OpenMP.
<p>
<p>
<a href = "../../f_src/multitask_openmp/multitask_openmp.html">
MULTITASK_OPENMP</a>,
a FORTRAN90 program which
demonstrates how to "multitask", that is, to execute several unrelated
and distinct tasks simultaneously, using OpenMP for parallel execution.
</p>
<p>
<a href = "../../f_src/mxv_openmp/mxv_openmp.html">
MXM_OPENMP</a>,
a FORTRAN90 program which
computes a dense matrix product C=A*B,
using OpenMP for parallel execution.
<p>
<p>
<a href = "../../f_src/mxv_openmp/mxv_openmp.html">
MXV_OPENMP</a>,
a FORTRAN90 program which
compares the performance of plain vanilla Fortran and the FORTRAN90
intrinsic routine MATMUL, for the matrix multiplication problem
y=A*x, with and without parallelization by OpenMP.
<p>
<p>
<a href = "../../f_src/openmp/openmp.html">
OPENMP</a>,
FORTRAN90 programs which
illustrate the use of the OpenMP application program interface
for carrying out parallel computations in a shared memory environment.
</p>
<p>
<a href = "../../f_src/poisson_openmp/poisson_openmp.html">
POISSON_OPENMP</a>,
a FORTRAN90 program which
computes an approximate solution to the Poisson equation in a rectangle,
using the Jacobi iteration to solve the linear system, and OpenMP to
carry out the Jacobi iteration in parallel.
</p>
<p>
<a href = "../../f_src/prime_openmp/prime_openmp.html">
PRIME_OPENMP</a>,
a FORTRAN90 program which
counts the number of primes between 1 and N, using OpenMP for parallel execution.
</p>
<p>
<a href = "../../f_src/quad_openmp/quad_openmp.html">
QUAD_OPENMP</a>,
a FORTRAN90 program which
approximates an integral using a quadrature rule, and carries out the
computation in parallel using OpenMP.
</p>
<p>
<a href = "../../f_src/random_openmp/random_openmp.html">
RANDOM_OPENMP</a>,
a FORTRAN90 program which
illustrates how a parallel program using OpenMP can generate multiple
distinct streams of random numbers.
</p>
<p>
<a href = "../../f_src/satisfy_openmp/satisfy_openmp.html">
SATISFY_OPENMP</a>,
a FORTRAN90 program which
demonstrates, for a particular circuit, an exhaustive search
for solutions of the circuit satisfy problem,
using OpenMP for parallel execution.
<p>
<p>
<a href = "../../f_src/sgefa_openmp/sgefa_openmp.html">
SGEFA_OPENMP</a>,
a C++ program which
reimplements the SGEFA/SGESL linear algebra routines from
LINPACK for use with OpenMP.
</p>
<p>
<a href = "../../f_src/ziggurat_openmp/ziggurat_openmp.html">
ZIGGURAT_OPENMP</a>,
a FORTRAN90 program which
demonstrates how the ZIGGURAT library can be used to generate random numbers
in an OpenMP parallel program.
</p>
<h3 align = "center">
Reference:
</h3>
<p>
<ol>
<li>
Peter Arbenz, Wesley Petersen,<br>
Introduction to Parallel Computing - A practical guide with examples in C,<br>
Oxford University Press,<br>
ISBN: 0-19-851576-6,<br>
LC: QA76.58.P47.
</li>
<li>
Rohit Chandra, Leonardo Dagum, Dave Kohr, Dror Maydan,
Jeff McDonald, Ramesh Menon,<br>
Parallel Programming in OpenMP,<br>
Morgan Kaufmann, 2001,<br>
ISBN: 1-55860-671-8,<br>
LC: QA76.642.P32.
</li>
<li>
Barbara Chapman, Gabriele Jost, Ruud vanderPas, David Kuck,<br>
Using OpenMP: Portable Shared Memory Parallel Processing,<br>
MIT Press, 2007,<br>
ISBN13: 978-0262533027.
</li>
<li>
<a href = "http://www.openmp.org/">The OpenMP web site</a>
</li>
<li>
OpenMP Architecture Review Board,<br>
OpenMP Application Program Interface,<br>
Version 3.0,<br>
May 2008.
</li>
</ol>
</p>
<h3 align = "center">
Source Code:
</h3>
<p>
<ul>
<li>
<a href = "schedule_openmp.f">schedule_openmp.f</a>, the source code.
</li>
</ul>
</p>
<h3 align = "center">
Examples and Tests:
</h3>
<p>
<b>SCHEDULE_LOCAL_GFORTRAN</b> compiles, links, loads and runs the program
using 2 threads, the local interactive environment,
and the GFORTRAN C++ compiler.
<ul>
<li>
<a href = "schedule_local_gfortran.sh">schedule_local_gfortran.sh</a>,
BASH commands.
</li>
<li>
<a href = "schedule_local_gfortran_output.txt">schedule_local_gfortran_output.txt</a>,
the output file;
</li>
</ul>
</p>
<p>
<b>SCHEDULE_LOCAL_IFORT</b> compiles, links, loads and runs the program
using 2 threads, the local interactive environment,
and the IFORT C++ compiler.
<ul>
<li>
<a href = "schedule_local_ifort.sh">schedule_local_ifort.sh</a>,
BASH commands.
</li>
<li>
<a href = "schedule_local_ifort_output.txt">schedule_local_ifort_output.txt</a>,
the output file;
</li>
</ul>
</p>
<p>
<b>SCHEDULE_ITHACA_GFORTRAN</b> compiles, links, loads and runs the program
using 2 threads, using the PBS queueing system on Virginia
Tech's Ithaca cluster, and the GFORTRAN C++ compiler.
<ul>
<li>
<a href = "schedule_ithaca_gfortran.sh">schedule_ithaca_gfortran.sh</a>,
BASH commands.
</li>
<li>
<a href = "schedule_ithaca_gfortran_output.txt">schedule_ithaca_gfortran_output.txt</a>,
the output file;
</li>
</ul>
</p>
<p>
<b>SCHEDULE_ITHACA_IFORT</b> compiles, links, loads and runs the program
using 2 threads, using the PBS queueing system on Virginia
Tech's Ithaca cluster,
and the IFORT C++ compiler.
<ul>
<li>
<a href = "schedule_ithaca_ifort.sh">schedule_ithaca_ifort.sh</a>,
BASH commands.
</li>
<li>
<a href = "schedule_ithaca_ifort_output.txt">schedule_ithaca_ifort_output.txt</a>,
the output file;
</li>
</ul>
</p>
<p>
You can go up one level to <a href = "../cpp_src.html">
the C++ source codes</a>.
</p>
<hr>
<i>
Last revised on 10 July 2010.
</i>
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