使用OpenMP进行Fortran并行编程

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"OpenMP是用于并行编程的一种标准API，尤其在Fortran语言中广泛应用。这份文档由Miguel Hermanns编写，主要介绍了如何使用OpenMP在Fortran 95中进行并行编程，涵盖了OpenMP的基础概念、各种并行构造以及同步原语等内容。" OpenMP Fortran Application Program Interface (API) 是一个库，它允许程序员通过添加特定的编译器指示来使Fortran代码并行化。OpenMP为并行编程提供了一种简洁且高效的接口，特别是在科学计算和高性能计算领域。 1.1 基本介绍历史回顾：OpenMP起源于1998年，由几家主要的计算机制造商和软件供应商联合创建，旨在为共享内存并行计算提供一个跨平台的标准。它的目标是简化多线程编程，使得并行化成为可能，而无需深入理解底层硬件细节。参与者：OpenMP的参与者包括了程序员、编译器开发者、工具提供商等，他们共同维护和更新OpenMP规范，确保其在不同平台上的一致性和兼容性。文档目的：此文档旨在为Fortran开发者提供一个关于OpenMP的指南，帮助他们理解和使用OpenMP指令进行并行编程。 1.2 基本概念 OpenMP的指示通常以预处理器指令的形式出现，如`!$OMP`，它们在源代码中作为注释，但会被编译器识别处理。Sentinels（哨兵）是这些指示的标识，用于开启或关闭并行区域。 1.2.1 并行区域构造：`!$OMP PARALLEL`指示开始一个并行区域，其中的代码将在多个线程中执行，而`!$OMP END PARALLEL`则标志着并行区域的结束。 2. OpenMP构造 2.1 工作共享构造：这些构造将任务分配给并行区域中的线程。 - `!$OMP DO` / `!$OMP END DO`用于循环并行化，每个线程处理循环的一部分。 - `!$OMP SECTIONS` / `!$OMP END SECTIONS`用于将代码块分配给不同的线程。 - `!$OMP SINGLE` / `!$OMP END SINGLE`确保代码只在一个线程上执行，通常用在初始化或控制流决策中。 - `!$OMP WORKSHARE` / `!$OMP END WORKSHARE`将工作负载分发到所有线程。 2.2 组合并行工作共享构造：这些构造结合了并行和工作共享，如并行循环。 - `!$OMP PARALLEL DO` / `!$OMP END PARALLEL DO`启动一个并行化的循环。 - `!$OMP PARALLEL SECTIONS` / `!$OMP END PARALLEL SECTIONS`将代码块并行化为多个部分。 - `!$OMP PARALLEL WORKSHARE` / `!$OMP END PARALLEL WORKSHARE`与`WORKSHARE`类似，但在并行环境中运行。 2.3 同步构造：这些构造用于控制线程间的同步，防止数据竞争。 - `!$OMP MASTER` / `!$OMP END MASTER`定义一个只有主线程执行的代码段。 - `!$OMP CRITICAL` / `!$OMP END CRITICAL`用于保护临界区，确保同一时间只有一个线程能执行该段代码。 - `!$OMP BARRIER`强制所有线程在继续执行前等待其他线程到达屏障点。 - `!$OMP TASKWAIT`等待所有已启动的任务完成。此外，还有其他高级特性，如动态调度、任务（Task）、队列管理、原子操作和并行环境的设置等。理解并掌握这些OpenMP构造是实现高效并行程序的关键，它们可以帮助充分利用多核处理器的计算能力，显著提升程序性能。

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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