How to Use OpenMP - Revision history

Paul-kapinos-e26d@rwth-aachen.de at 07:17, 4 May 2020

2020-05-04T07:17:01Z

Daniel-schurhoff-de23@rwth-aachen.de at 13:19, 3 September 2019

2019-09-03T13:19:35Z

Nina-loseke-fd7a@rwth-aachen.de at 13:33, 20 April 2018

2018-04-20T13:33:58Z

Christian-wassermann-e30b@rwth-aachen.de: /* How to Compile OpenMP Code */

2018-04-16T15:02:02Z

How to Compile OpenMP Code

Christian-wassermann-e30b@rwth-aachen.de: /* How to Run an OpenMP Application */

2018-04-13T11:39:41Z

How to Run an OpenMP Application

Christian-wassermann-e30b@rwth-aachen.de: /* How to Compile OpenMP Code */

2018-04-13T11:39:19Z

How to Compile OpenMP Code

Christian-wassermann-e30b@rwth-aachen.de at 11:38, 13 April 2018

2018-04-13T11:38:55Z

Christian-wassermann-e30b@rwth-aachen.de at 11:22, 13 April 2018

2018-04-13T11:22:03Z

Christian-wassermann-e30b@rwth-aachen.de at 11:21, 13 April 2018

2018-04-13T11:21:11Z

134.130.1.101: /* How to Compile OpenMP Code */

2018-04-09T12:28:29Z

How to Compile OpenMP Code

@@ Line 3: / Line 3: @@
 This page will give you a general overview of how to compile and execute a program that has been [[Parallel_Programming|parallelized]] with [[OpenMP]].
-As opposed to [[How_to_Use_MPI|MPI]], you do not have to load any modules to use OpenMP.
+As opposed to [[How_to_Use_MPI|MPI]], you do not have to load any modules to use OpenMP (but your compiler must support OpenMP - most of the compilers do it).
@@ Line 11: / Line 11: @@
 == How to Compile OpenMP Code ==
-Additional compiler flags tell the compiler to enable OpenMP. Otherwise, the OpenMP pragmas in the code will be ignored by the compiler.
+Additional vendor-specific (and sometimes version-specific) compiler flags tell the compiler to enable OpenMP. Otherwise, the OpenMP pragmas in the code will be ignored by the compiler.
 Depending on which compiler you have loaded, use one of the flags below to compile your code.
@@ Line 24: / Line 24: @@
 |-
 | Oracle || <code>-xopenmp</code>
 |}
@@ Line 33: / Line 35: @@
 === Setting <code>OMP_NUM_THREADS</code> ===
-If you forget to set <code>OMP_NUM_THREADS</code> to any value, the default value of your cluster environment will be used. In most cases, the default is 1, so that your program is executed serially.
+If you forget to set <code>OMP_NUM_THREADS</code> to any value, the default value of your cluster environment will be used. In many cases, the default is ''1'', so that your program is executed serially. If this envvar is not set at all the OpenMP run time may also deciede to use up ''all'' cores of your computer which must not always be the expected outcome, so it is a good idea always to set a meaningful value.
 One way to specify the number of threads is by passing an extra argument when running the executable file. In order to start the parallel regions of the example program above with 12 threads, you'd have to type:
   $ OMP_NUM_THREADS=12 ./omp_code.exe
-This automatically sets the environment variable <code>OMP_NUM_THREADS</code> to 12, but it is reset to its default value after the execution of <code>omp_code.exe</code> finished.
+This sets the environment variable <code>OMP_NUM_THREADS</code> to ''12'' for the execution time of <code>omp_code.exe</code> only, and it is reset to its default value after the execution of <code>omp_code.exe</code> finished.
 Another way to set the number of threads is by changing your environment variable. This example will increment it up to 24 threads and override the default value:

← Older revision		Revision as of 13:19, 3 September 2019
Line 1:		Line 1:
		+	[[Category:HPC-User]]
	== Basics ==		== Basics ==

@@ Line 3: / Line 3: @@
 This page will give you a general overview of how to compile and execute a program that has been [[Parallel_Programming|parallelized]] with [[OpenMP]].
 As opposed to [[How_to_Use_MPI|MPI]], you do not have to load any modules to use OpenMP.
 == How to Compile OpenMP Code ==

@@ Line 12: / Line 12: @@
 | Compiler || Flag
 |-
-| GNU || -fopenmp
+| GNU || <code>-fopenmp</code>
 |-
-| Intel || -qopenmp
+| Intel || <code>-qopenmp</code>
 |-
-| Oracle || -xopenmp
+| Clang  || <code>-fopenmp</code>
 |}

@@ Line 30: / Line 30: @@
 One way to specify the number of threads is by passing an extra argument when running the executable file. In order to start the parallel regions of the example program above with 12 threads, you'd have to type:
   $ OMP_NUM_THREADS=12 ./omp_code.exe
-This automatically sets the environment variable <code>OMP_NUM_THREADS</code> to 12, but it is reset to its default value after the execution of "omp_code.exe" finished.
+This automatically sets the environment variable <code>OMP_NUM_THREADS</code> to 12, but it is reset to its default value after the execution of <code>omp_code.exe</code> finished.
 Another way to set the number of threads is by changing your environment variable. This example will increment it up to 24 threads and override the default value:

@@ Line 19: / Line 19: @@
 |}
-For example: if you plan to use an Intel compiler for your OpenMP code written in C, you have to type this to create an application called "omp_code.exe":
+For example: if you plan to use an Intel compiler for your OpenMP code written in C, you have to type this to create an application called <code>omp_code.exe</code>:
   $ icc -qopenmp omp_code.c -o omp_code.exe

@@ Line 1: / Line 1: @@
 == Basics ==
-This will give you a general overview of how to compile and execute a program that has been [[Parallel_Programming|parallelized]] with [[OpenMP]].
+This page will give you a general overview of how to compile and execute a program that has been [[Parallel_Programming|parallelized]] with [[OpenMP]].
 As opposed to [[How_to_Use_MPI|MPI]], you do not have to load any modules to use OpenMP.

@@ Line 21: / Line 21: @@
 For example: if you plan to use an Intel compiler for your OpenMP code written in C, you have to type this to create an application called "omp_code.exe":
-  $ icc -openmp omp_code.c -o omp_code.exe
+  $ icc -qopenmp omp_code.c -o omp_code.exe
 == How to Run an OpenMP Application ==