OpenMP Fortran编程基础教程

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“这是一份关于OpenMP的入门教程，旨在帮助初学者理解并掌握OpenMP在Fortran 95中的应用。文档由Miguel Hermanns编写，详细介绍了OpenMP的基础知识和主要构造，包括工作共享、同步和并行区域等概念。” OpenMP（Open Multi-Processing）是一种并行编程模型，它为共享内存多处理器系统提供了一种简单而有效的接口。在Fortran 95中使用OpenMP，开发者可以将串行代码转化为并行代码，从而充分利用多核处理器的计算能力。 1. OpenMP Fortran Application Program Interface (API) OpenMP API为Fortran程序员提供了若干关键的编译器指令和运行时库函数，以实现并行化。这些指令以预处理宏的形式存在，通常以`!$OMP`开头。 1.1 基本概念 - 历史背景：OpenMP的发展始于1998年，由多家计算机硬件和软件公司共同发起，旨在简化并行编程。 - 参与者：OpenMP标准由OpenMP Architecture Review Board (ARB)维护，成员包括各种硬件和软件供应商，以及学术界。 - 文档目的：这份文档旨在为初学者提供一个清晰的OpenMP入门指南，解释其基本原理和使用方法。 1.2 基本要素 - 指令哨兵：OpenMP指令通常与特定的哨兵一起使用，如`!$OMP`，用于指示编译器进行并行处理。 - 并行区域：`!$OMP PARALLEL`是并行区域的开始，`!$OMP END PARALLEL`则表示并行区域结束。在并行区域内，任务可以被多个线程并行执行。 2. OpenMP构造 - 工作共享构造：这些构造允许将任务分配给线程组。 - 循环并行化：`!$OMP DO`和`!$OMP END DO`用于将循环并行化，线程们各自处理循环的一部分。 - 部分并行化：`!$OMP SECTIONS`和`!$OMP END SECTIONS`用于将程序分为多个部分，每个部分由单独的线程执行。 - 单例：`!$OMP SINGLE`确保只有一个线程执行特定的代码块。 - 工作共享：`!$OMP WORKSHARE`和`!$OMP END WORKSHARE`使得所有线程并行执行同一块代码。 - 组合并行工作共享构造：将并行和工作共享结合，例如： - 并行循环：`!$OMP PARALLEL DO`和`!$OMP END PARALLEL DO`创建一个并行的循环结构。 - 并行部分：`!$OMP PARALLEL SECTIONS`和`!$OMP END PARALLEL SECTIONS`用于并行执行代码的多个部分。 - 并行工作共享：`!$OMP PARALLEL WORKSHARE`和`!$OMP END PARALLEL WORKSHARE`将并行和工作共享特性结合。 - 同步构造：确保线程间的正确同步，避免数据竞争。 - 主线程：`!$OMP MASTER`指定一个线程作为主线程，`!$OMP END MASTER`恢复到并行模式。 - 其他同步指令如`!$OMP BARRIER`用于所有线程等待，确保所有线程到达某个点后再继续执行。通过这个OpenMP入门教程，学习者将能够理解和应用OpenMP的关键特性，从而编写出高效的并行Fortran程序，利用多核处理器的计算潜力。同时，这份文档也提供了实际示例和解释，使学习过程更加直观和实用。

Chapter 2

OpenMP constructs

If only the previous parallel region constructor would exist, then the only possible thing to

do would be that all the threads perform exactly the same task, but this is not the aim of

parallelism. Therefore, OpenMP deﬁnes additional constructs which allow to distribute

a given task over the diﬀerent threads and achieve in this way a real parallel working

program.

Four diﬀerent groups of OpenMP directives or constructs exist. Each group has a

diﬀerent aim and the selection of one directive or another inside the same group depends

on the nature of the problem to be solved. Therefore, it is good to understand the

principles of each of these directives in order to perform the correct choices.

2.1 Work-sharing constructs

The ﬁrst group of OpenMP directives looks forward to divide a given work into pieces

and to give one or more of these pieces to each parallel running thread. In this way the

work, which would be done by a single thread in a serial program, is distributed over a

team of threads achieving a faster running program

All work-sharing constructs must be placed inside dynamic extends of parallel regions

in order to be eﬀective. If this is not the case, the work-sharing construct will still

work, but a team with only one thread will be used. The reason is that a work-sharing

construct is not able of creating new threads; a task reserved to the

!$OMP PARALLEL/!$OMP

END PARALLEL

directive-pair.

The following restrictions need to be taken into account when using a work-sharing

construct:

• Work-sharing constructs must be encountered by all threads in a team or by none

at all.

Obviously, this is only true, if the team of threads is executed on more than one processor, which is

the case of SMP machines. Otherwise, when using a single processor, the computational overhead due to

the OpenMP directives as well as due to the need of executing several threads in a sequential way lead

to slower parallel programs than the corresponding serial versions!

2.1. Work-sharing constructs 11

The way in which the work is distributed, and in general how the work-sharing con-

struct has to behave, can be controlled with the aid of clauses linked to the opening-

directive

!$OMP DO. The syntax is as follows:

!$OMP DO clause1 clause2 ...

...

!$OMP END DO end_clause

Only the following clauses of those presented and explained in chapter 3 are allowed

in the opening-directive

!$OMP DO:

•

PRIVATE(list): see page 37.

•

FIRSTPRIVATE(list): see page 40.

•

LASTPRIVATE(list): see page 41.

•

REDUCTION(operator:list): see page 43.

•

SCHEDULE(type, chunk): see page 48.

•

ORDERED: see page 52.

Additionally to the opening-directive clauses, it is possible to add to the closing-

directive the

NOWAIT clause in order to avoid the implied synchronization. Also implied

to the closing-directive is an updating of the shared variables aﬀected by the do-loop.

When the

NOWAIT clause is added, this implied updating is also suppressed. Therefore,

care has to be taken, if after the do-loop the modiﬁed variables have to be used. In such

a case it is necessary to add an implied or an explicit updating of the shared variables, for

example using the

!$OMP FLUSH directive. This side eﬀect also happens in other directives,

although it will not be mentioned explicitly again. Therefore, it is convenient to read the

information regarding the

!$OMP FLUSH directive in page 27 and the NOWAIT clause in

page 47 for further information. Also the concepts explained in chapter 3 are of use to

understand the impact of suppressing implied updates of the variables.

Since the work of a parallelized do-loop is distributed over a team of threads running

in parallel, it has no sense that one or more of these threads can branch into or out of

the block of code enclosed inside the directive-pair

!$OMP DO/!$OMP END DO, for example

using a

GOTO command. Therefore, this possibility is directly forbidden by the OpenMP

speciﬁcation.

Since each thread is executing part of the iterations of the do-loop and the updates of

the modiﬁcations made to the variables are not ensured until the end of the work-sharing

construct, the following example will not work correctly using the

!$OMP DO/!$OMP END

directive-pair for its parallelization:

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