Parallel Programming in Fortran 95 using OpenMP - People

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$Intel(R) Math Kernel Library for Linux* OS User's Guide$

4 1. OpenMP Fortran Application Program Interface 1.2 The basics OpenMP represents a collection of compiler directives, library routines and environment variables meant for parallel programming in shared-memory machines. A chapter is going to be devoted to each of these elements, but before starting with the review of the available compiler directives, it is necessary to have a look at some basic aspects of OpenMP. Although named as ”basic aspects”, the information presented in this section is the fundamental part of OpenMP which allows the inclusion of OpenMP commands in programs and the creation as well as destruction of parallel running regions of code. 1.2.1 The sentinels for OpenMP directives and conditional compilation One of the aims of the OpenMP standard is to offer the possibility of using the same source code lines with an OpenMP-compliant compiler and with a normal compiler. This can only be achieved by hiding the OpenMP directives and commands in such a way, that a normal compiler is unable to see them. For that purpose the following two directive sentinels are introduced: !$OMP !$ Since the first character is an exclamation mark ”!”, a normal compiler will interpret the lines as comments and will neglect their content. But an OpenMP-compliant compiler will identify the complete sequences and will proceed as follows: !$OMP : the OpenMP-compliant compiler knows that the following information in the line is an OpenMP directive. It is possible to extend an OpenMP directive over several lines by placing the same sentinel in front of the following lines and using the standard Fortran 95 method of braking source code lines: !$OMP PARALLEL DEFAULT(NONE) SHARED(A, B) PRIVATE(C, D) & !$OMP REDUCTION(+:A) It is mandatory to include a white space between the directive sentinel !$OMP and the following OpenMP directive, otherwise the directive sentinel is not correctly identified and the line is treated as a comment. !$ : the corresponding line is said to be affected by a conditional compilation. This means that its content will only be available to the compiler in case of being OpenMP-compliant. In such a case, the two characters of the sentinel are substituted by two white spaces so that the compiler is taking into account the line. As in the previous case, it is possible to extend a source code line over several lines as follows:
1.2. The basics 5 !$ interval = L * OMP_get_thread_num() / & !$ (OMP_get_num_threads() - 1) Again, it is mandatory to include a white space between the conditional compilation directive !$ and the following source code, otherwise the conditional compilation directive is not correctly identified and the line is treated as a comment. Both sentinels can appear in any column as long as they are preceded only by white spaces; otherwise, they are interpreted as normal comments. 1.2.2 The parallel region constructor The most important directive in OpenMP is the one in charge of defining the so called parallel regions. Such a region is a block of code that is going to be executed by multiple threads running in parallel. Since a parallel region needs to be created/opened and destroyed/closed, two directives are necessary, forming a so called directive-pair: !$OMP PARALLEL/!$OMP END PARALLEL. An example of their use would be: !$OMP PARALLEL write(*,*) "Hello" !$OMP END PARALLEL Since the code enclosed between the two directives is executed by each thread, the message Hello appears in the screen as many times as threads are being used in the parallel region. Before and after the parallel region, the code is executed by only one thread, which is the normal behavior in serial programs. Therefore it is said, that in the program there are also so called serial regions. When a thread executing a serial region encounters a parallel region, it creates a team of threads, and it becomes the master thread of the team. The master thread is a member of the team as well and takes part in the computations. Each thread inside the parallel region gets a unique thread number which ranges from zero, for the master thread, up to N p − 1, where N p is the total number of threads within the team. In figure 1.1 the previous example is represented in a graphical way to clarify the ideas behind the parallel region constructor. At the beginning of the parallel region it is possible to impose clauses which fix certain aspects of the way in which the parallel region is going to work: for example the scope of variables, the number of threads, special treatments of some variables, etc. The syntaxis to use is the following one: !$OMP PARALLEL clause1 clause2 ... ... !$OMP END PARALLEL
Page 1: Parallel Programming in Fortran 95
Page 4 and 5: ii CONTENTS 3.1.4 FIRSTPRIVATE(list
Page 6 and 7: 2 1. OpenMP Fortran Application Pro
Page 14 and 15: 10 2. OpenMP constructs • Work-sh
Page 16 and 17: 12 2. OpenMP constructs real(8) ::
Page 18 and 19: 14 2. OpenMP constructs do k = 1, 1
Page 20 and 21: 16 2. OpenMP constructs !$OMP SECTI
Page 22 and 23: 18 2. OpenMP constructs thread 0 th
Page 24 and 25: 20 2. OpenMP constructs • If the
Page 26 and 27: 22 2. OpenMP constructs 2.3 Synchro
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Page 34 and 35: 30 2. OpenMP constructs 2.3.6 !$OMP
Page 36 and 37: 32 2. OpenMP constructs !$OMP END D
Page 38 and 39: 34 2. OpenMP constructs Shared memo
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Page 42 and 43: 38 3. PRIVATE, SHARED & Co. Shared
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56 4. The OpenMP run-time library T
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