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<html>
<head>
<title>
OPENMP - C++ Examples of Parallel Programming with OpenMP
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
OPENMP <br> C++ Examples of Parallel Programming with OpenMP
</h1>
<hr>
<p>
<b>OPENMP</b>
is a directory of C++ programs which
illustrate the use of the OpenMP application program interface
for carrying out parallel computations in a shared memory environment.
</p>
<p>
The directives
allow the user to mark areas of the code, such as <b>do</b>,
<b>while</b> or <b>for</b> loops, which are suitable for parallel
processing. The directives appear as a special kind of
comment, so the program can be compiled and run in serial mode.
However, the user can tell the compiler to "notice" the special
directives, in which case a version of the program will be created
that runs in parallel.
</p>
<p>
Thus the same program can easily be run in serial or parallel mode
on a given computer, or run on a computer that does not have
<b>OpenMP</b> at all.
</p>
<p>
<b>OpenMP</b> is suitable for a <i>shared memory</i> parallel system,
that is, a situation in which there is a single memory space, and
multiple processors. If memory is shared, then typically the number
of processors will be small, and they will all be on the same
physical machine.
</p>
<p>
By contrast, in a <i>distributed memory</i> system, items of data
are closely associated with a particular processor. There may
be a very large number of processors, and they may be more loosely
coupled and even on different machines. Such a system will need
to be handled with <a href = "../mpi_c/mpi_c.html">MPI</a> or some
other message passing interface.
</p>
<p>
<b>OpenMP</b> descended in part from the old Cray microtasking
directives, so if you've lived long enough to remember those, you
will recognize some features.
</p>
<h3 align = "center">
C++ Issues
</h3>
<p>
OpenMP includes a number of functions whose type must be declared in
any program that uses them. A user program calling OpenMP must have
the statement
<pre><b>
# include <omp.h>
</b></pre>
</p>
<h3 align = "center">
"Pretend" Parallelism
</h3>
<p>
<b>OpenMP</b> allows you to "request" any number of threads of execution.
This is a request, and it's not always a wise request. If your system has
four processors available, and they're not busy doing other things, or serving
other users, maybe 4 threads is what you want. But you can't guarantee
you'll get the undivided use of those processors. Moreover, if you run the
same program using 1 thread and 4 threads, you may find that using 4 threads
<i>slows you down</i>, either because you don't actually have 4 processors,
(so the system has the overhead of pretending to run in parallel), or because
the processors you have are also busy doing other things.
</p>
<p>
For this reason, it's wise to run the program at least once in single thread mode,
so you have a benchmark against which to measure the speedup you got (or didn't
get!) versus the speedup you hoped for.
</p>
<h3 align = "center">
Compiler Support
</h3>
<p>
The compiler you use must recognize the OpenMP directives in order to produce
code that will run in parallel. Here are some of the compilers available that
support OpenMP:
<ul>
<li>
The GNU compilers <b>gcc/g++</b> (version 4.2 or later)
and <b>gfortran</b> (version 2.0 or later).
</li>
<li>
The IBM compilers <b>xlc</b> and <b>xlf</b>.
</li>
<li>
The Intel compilers <b>icc</b>, <b>icpc</b> and <b>ifort</b>.
</li>
<li>
The Microsoft Visual C++ compiler.
</li>
<li>
The Portland Group C/C++ and Fortran compilers.
</li>
<li>
Sun Studio C/C++ and Fortran95 compilers.
</li>
</ul>
</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>OPENMP</b> examples are available in
<a href = "../../c_src/openmp/openmp.html">a C version</a> and
<a href = "../../cpp_src/openmp/openmp.html">a C++ version</a> and
<a href = "../../f77_src/openmp/openmp.html">a FORTRAN77 version</a> and
<a href = "../../f_src/openmp/openmp.html">a FORTRAN90 version</a>.
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../cpp_src/dijkstra_openmp/dijkstra_openmp.html">
DIJKSTRA_OPENMP</a>,
a C++ program which
uses OpenMP to parallelize a simple example of Dijkstra's
minimum distance algorithm for graphs.
</p>
<p>
<a href = "../../cpp_src/fft_openmp/fft_openmp.html">
FFT_OPENMP</a>,
a C++ program which
demonstrates the computation of a Fast Fourier Transform
in parallel, using OpenMP.
</p>
<p>
<a href = "../../cpp_src/heated_plate_openmp/heated_plate_openmp.html">
HEATED_PLATE_OPENMP</a>,
a C++ program which
solves the steady (time independent) heat equation in a 2D
rectangular region, using OpenMP to run in parallel.
</p>
<p>
<a href = "../../cpp_src/hello_openmp/hello_openmp.html">
HELLO_OPENMP</a>,
a C++ program which
prints out "Hello, world!" using the OpenMP parallel programming environment.
</p>
<p>
<a href = "../../cpp_src/md_openmp/md_openmp.html">
MD_OPENMP</a>,
a C++ program which
carries out a molecular dynamics simulation using OpenMP.
<p>
<p>
<a href = "../../cpp_src/multitask_openmp/multitask_openmp.html">
MULTITASK_OPENMP</a>,
a C++ 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 = "../../cpp_src/mxv_openmp/mxv_openmp.html">
MXM_OPENMP</a>,
a C++ program which
computes a dense matrix product C=A*B,
using OpenMP for parallel execution.
<p>
<p>
<a href = "../../cpp_src/mxv_openmp/mxv_openmp.html">
MXV_OPENMP</a>,
a C++ program which
compares the performance of the matrix multiplication problem
y=A*x, with and without parallelization by OpenMP.
<p>
<p>
<a href = "../../cpp_src/openmp_stubs/openmp_stubs.html">
OPENMP_STUBS</a>,
a C++ library which
implements a "stub" version of OpenMP, so that an OpenMP program
can be compiled, linked and executed on a system that does not
have OpenMP installed.
</p>
<p>
<a href = "../../cpp_src/poisson_openmp/poisson_openmp.html">
POISSON_OPENMP</a>,
a C++ 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 = "../../cpp_src/prime_openmp/prime_openmp.html">
PRIME_OPENMP</a>,
a C++ program which
counts the number of primes between 1 and N, using OpenMP for parallel execution.
</p>
<p>
<a href = "../../c_src/pthreads/pthreads.html">
PTHREADS</a>,
C programs which
illustrate the use of the POSIX thread library to carry out
parallel program execution.
</p>
<p>
<a href = "../../cpp_src/quad_openmp/quad_openmp.html">
QUAD_OPENMP</a>,
a C++ program which
approximates an integral using a quadrature rule, and carries out the
computation in parallel using OpenMP.
</p>
<p>
<a href = "../../cpp_src/random_openmp/random_openmp.html">
RANDOM_OPENMP</a>,
a C++ program which
illustrates how a parallel program using OpenMP can generate multiple
distinct streams of random numbers.
</p>
<p>
<a href = "../../cpp_src/satisfy_openmp/satisfy_openmp.html">
SATISFY_OPENMP</a>,
a C++ 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 = "../../cpp_src/schedule_openmp/schedule_openmp.html">
SCHEDULE_OPENMP</a>,
a C++ program which
demonstrates the default, static, and dynamic methods of "scheduling"
loop iterations in OpenMP to avoid work imbalance.
<p>
<p>
<a href = "../../cpp_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 = "../../cpp_src/ziggurat_openmp/ziggurat_openmp.html">
ZIGGURAT_OPENMP</a>,
a C++ 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">
Examples and Tests:
</h3>
<p>
<b>COMPUTE_PI</b> shows how information can be shared. Several processors cooperate
to estimate the value of pi.
<ul>
<li>
<a href = "compute_pi.cpp">compute_pi.cpp</a>,
the source code;
</li>
<li>
<a href = "compute_pi_local.sh">compute_pi_local.sh</a>,
BASH commands to compile, link, load and run the program
using 1 and 4 threads.
</li>
<li>
<a href = "compute_pi_local_output.txt">compute_pi_local_output.txt</a>,
the output file using 1 thread;
</li>
</ul>
</p>
<p>
<b>DOT_PRODUCT</b> compares the computation of a vector dot product
in sequential mode, and using OpenMP. Typically, the overhead of using
parallel processing outweighs the advantage for small vector sizes N.
The code demonstrates this fact by using a number of values of N,
and by running both sequential and OpenMP versions of the calculation.
<ul>
<li>
<a href = "dot_product.cpp">dot_product.cpp</a>,
the source code;
</li>
<li>
<a href = "dot_product_local.sh">dot_product_local.sh</a>,
BASH commands to compile, link, load and run the program.
</li>
<li>
<a href = "dot_product_local_output.txt">dot_product_local_output.txt</a>,
the output file;
</li>
</ul>
</p>
<p>
<b>HELMHOLTZ</b> is a program that solves
the Helmholtz equation on a rectangular grid, using Jacobi iteration
with overrelaxation;
<ul>
<li>
<a href = "helmholtz.cpp">helmholtz.cpp</a>,
the source code;
</li>
<li>
<a href = "helmholtz_local.sh">helmholtz_local.sh</a>,
BASH commands to compile, link, load and run the program
using 4 threads.
</li>
<li>
<a href = "helmholtz_local_output.txt">helmholtz_local_output.txt</a>,
the output file;
</li>
</ul>
</p>
<p>
<b>MXM</b> is a simple exercise in timing the computation of a matrix-matrix product.
<ul>
<li>
<a href = "mxm.cpp">mxm.cpp</a>,
the source code;
</li>
<li>
<a href = "mxm_local.sh">mxm_local.sh</a>,
BASH commands to compile, link, load and run the program
using 1 and 4 threads.
</li>
<li>
<a href = "mxm_output_local.txt">mxm_output_local.txt</a>,
the output file using 1 thread;
</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 14 May 2011.
</i>
</body>
</html>