/openmp (Enable OpenMP Support)

Causes the compiler to process #pragma omp directives in support of OpenMP.

Syntax

/openmp
/openmp:experimental
/openmp:llvm

Remarks

#pragma omp is used to specify Directives and Clauses. If /openmp isn’t specified in a compilation, the compiler ignores OpenMP clauses and directives. OpenMP Function calls are processed by the compiler even if /openmp isn’t specified.

The C++ compiler currently supports the OpenMP 2.0 standard. However, Visual Studio 2019 also now offers SIMD functionality. To use SIMD, compile by using the /openmp:experimental option. This option enables both the usual OpenMP features, and OpenMP SIMD features not available when using the /openmp switch.

Starting in Visual Studio 2019 version 16.9, you can use the experimental /openmp:llvm option instead of /openmp to target the LLVM OpenMP runtime. Support currently isn’t available for production code, since the required libomp DLLs aren’t redistributable. The option supports the same OpenMP 2.0 directives as /openmp . And, it supports all the SIMD directives supported by the /openmp:experimental option. It also supports unsigned integer indices in parallel for loops according to the OpenMP 3.0 standard. For more information, see Improved OpenMP Support for C++ in Visual Studio.

Currently, the /openmp:llvm option only works on the x64 architecture. The option isn’t compatible with /clr or /ZW .

Applications compiled by using both /openmp and /clr can only be run in a single application domain process. Multiple application domains aren’t supported. That is, when the module constructor ( .cctor ) is run, it detects if the process is compiled using /openmp , and if the app is loaded into a non-default runtime. For more information, see appdomain , /clr (Common Language Runtime Compilation), and Initialization of Mixed Assemblies.

If you attempt to load an app compiled using both /openmp and * /clr* into a non-default application domain, a TypeInitializationException exception is thrown outside the debugger, and a OpenMPWithMultipleAppdomainsException exception is thrown in the debugger.

These exceptions can also be raised in the following situations:

If your application is compiled using /clr but not /openmp , and is loaded into a non-default application domain, where the process includes an app compiled using /openmp .

If you pass your /clr app to a utility, such as regasm.exe, which loads its target assemblies into a non-default application domain.

The common language runtime’s code access security doesn’t work in OpenMP regions. If you apply a CLR code access security attribute outside a parallel region, it won’t be in effect in the parallel region.

Microsoft doesn’t recommend that you write /openmp apps that allow partially trusted callers. Don’t use AllowPartiallyTrustedCallersAttribute, or any CLR code access security attributes.

To set this compiler option in the Visual Studio development environment

Open the project’s Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

Expand the Configuration Properties > C/C++ > Language property page.

Modify the OpenMP Support property.

To set this compiler option programmatically

Example

The following sample shows some of the effects of thread pool startup versus using the thread pool after it has started. Assuming an x64, single core, dual processor, the thread pool takes about 16 ms to start up. After that, there’s little extra cost for the thread pool.

When you compile using /openmp , the second call to test2 never runs any longer than if you compile using /openmp- , as there’s no thread pool startup. At a million iterations, the /openmp version is faster than the /openmp- version for the second call to test2. At 25 iterations, both /openmp- and /openmp versions register less than the clock granularity.

If you have only one loop in your application and it runs in less than 15 ms (adjusted for the approximate overhead on your machine), /openmp may not be appropriate. If it’s higher, you may want to consider using /openmp .

/openmp (Включить поддержку OpenMP) /openmp (Enable OpenMP Support)

Заставляет компилятор обрабатывать #pragma omp директивы для поддержки OpenMP. Causes the compiler to process #pragma omp directives in support of OpenMP.

Синтаксис Syntax

/openmp
/openmp:experimental
/openmp:llvm

Remarks Remarks

#pragma omp используется для указания директив и предложений. #pragma omp is used to specify Directives and Clauses. Если параметр /openmp не указан при компиляции, компилятор игнорирует предложения и директивы OpenMP. If /openmp isn’t specified in a compilation, the compiler ignores OpenMP clauses and directives. Вызовы функций OpenMP обрабатываются компилятором, даже если /openmp не указаны. OpenMP Function calls are processed by the compiler even if /openmp isn’t specified.

Компилятор C++ в настоящее время поддерживает стандарт OpenMP 2,0. The C++ compiler currently supports the OpenMP 2.0 standard. Однако теперь Visual Studio 2019 также предлагает функции SIMD. However, Visual Studio 2019 also now offers SIMD functionality. Чтобы использовать SIMD, Скомпилируйте с помощью /openmp:experimental параметра. To use SIMD, compile by using the /openmp:experimental option. Этот параметр включает как обычные функции OpenMP, так и функции OpenMP SIMD, недоступные при использовании /openmp параметра. This option enables both the usual OpenMP features, and OpenMP SIMD features not available when using the /openmp switch.

Начиная с Visual Studio 2019 версии 16,9 можно использовать экспериментальный /openmp:llvm вариант вместо /openmp для целевой среды выполнения OpenMP LLVM. Starting in Visual Studio 2019 version 16.9, you can use the experimental /openmp:llvm option instead of /openmp to target the LLVM OpenMP runtime. В настоящее время поддержка в рабочем коде недоступна, так как требуемые библиотеки DLL либомп не являются распространяемыми. Support currently isn’t available for production code, since the required libomp DLLs aren’t redistributable. Параметр поддерживает те же директивы OpenMP 2,0, что и /openmp . The option supports the same OpenMP 2.0 directives as /openmp . И поддерживают все директивы SIMD, поддерживаемые /openmp:experimental параметром. And, it supports all the SIMD directives supported by the /openmp:experimental option. Он также поддерживает параллельные индексы целых чисел без знака в соответствии со стандартом OpenMP 3,0. It also supports unsigned integer indices in parallel for loops according to the OpenMP 3.0 standard. Дополнительные сведения см. в статье Улучшенная поддержка OpenMP для C++ в Visual Studio. For more information, see Improved OpenMP Support for C++ in Visual Studio.

В настоящее время /openmp:llvm параметр работает только в архитектуре x64. Currently, the /openmp:llvm option only works on the x64 architecture. Параметр несовместим с /clr или /ZW . The option isn’t compatible with /clr or /ZW .

Приложения, скомпилированные с помощью /openmp и, /clr могут выполняться только в одном процессе домена приложения. Applications compiled by using both /openmp and /clr can only be run in a single application domain process. Несколько доменов приложений не поддерживаются. Multiple application domains aren’t supported. Это значит, что при запуске конструктора модуля ( .cctor ) он обнаруживает, компилируется ли процесс с помощью /openmp , и если приложение загружается в среду выполнения, не используемую по умолчанию. That is, when the module constructor ( .cctor ) is run, it detects if the process is compiled using /openmp , and if the app is loaded into a non-default runtime. Дополнительные сведения см. в статьях appdomain , /clr (компиляция среды CLR)и Инициализация смешанных сборок. For more information, see appdomain , /clr (Common Language Runtime Compilation), and Initialization of Mixed Assemblies.

При попытке загрузить приложение, скомпилированное с помощью /openmp и, и * /clr* в домен приложения, не заданный по умолчанию, TypeInitializationException исключение создается вне отладчика, а OpenMPWithMultipleAppdomainsException в отладчике создается исключение. If you attempt to load an app compiled using both /openmp and * /clr* into a non-default application domain, a TypeInitializationException exception is thrown outside the debugger, and a OpenMPWithMultipleAppdomainsException exception is thrown in the debugger.

Эти исключения также могут возникать в следующих ситуациях. These exceptions can also be raised in the following situations:

Значение, если приложение компилируется с использованием /clr , но не /openmp и загружается в домен приложения, не заданный по умолчанию, где процесс включает приложение, скомпилированное с помощью /openmp . If your application is compiled using /clr but not /openmp , and is loaded into a non-default application domain, where the process includes an app compiled using /openmp .

При передаче /clr приложения в служебную программу, например regasm.exe, которая загружает целевые сборки в домен приложения, не используемый по умолчанию. If you pass your /clr app to a utility, such as regasm.exe, which loads its target assemblies into a non-default application domain.

Управление доступом для кода среды CLR не работает в регионах OpenMP. The common language runtime’s code access security doesn’t work in OpenMP regions. Если атрибут управления доступом для кода CLR применяется за пределами параллельной области, он не будет действовать в параллельной области. If you apply a CLR code access security attribute outside a parallel region, it won’t be in effect in the parallel region.

Корпорация Майкрософт не рекомендует создавать /openmp приложения, допускающие частично доверенные вызывающие объекты. Microsoft doesn’t recommend that you write /openmp apps that allow partially trusted callers. Не используйте AllowPartiallyTrustedCallersAttribute или любые атрибуты управления доступом для кода CLR. Don’t use AllowPartiallyTrustedCallersAttribute, or any CLR code access security attributes.

Установка данного параметра компилятора в среде разработки Visual Studio To set this compiler option in the Visual Studio development environment

Откройте диалоговое окно Страницы свойств проекта. Open the project’s Property Pages dialog box. Подробнее см. в статье Настройка компилятора C++ и свойства сборки в Visual Studio. For details, see Set C++ compiler and build properties in Visual Studio.

Разверните страницу Свойства > языка C/C++ > язык . Expand the Configuration Properties > C/C++ > Language property page.

Измените свойство поддержки OpenMP . Modify the OpenMP Support property.

Установка данного параметра компилятора программным способом To set this compiler option programmatically

Пример Example

В следующем примере показаны некоторые эффекты запуска пула потоков по сравнению с использованием пула потоков после его запуска. The following sample shows some of the effects of thread pool startup versus using the thread pool after it has started. При условии, что 64-разрядная, одноядерная, Двухъядерный процессор, пул потоков занимает около 16 мс для запуска. Assuming an x64, single core, dual processor, the thread pool takes about 16 ms to start up. После этого пул потоков будет немного излишним. After that, there’s little extra cost for the thread pool.

При компиляции с помощью /openmp второй вызов test2 никогда не выполняется дольше, чем при компиляции с помощью /openmp- , так как отсутствует запуск пула потоков. When you compile using /openmp , the second call to test2 never runs any longer than if you compile using /openmp- , as there’s no thread pool startup. В миллионах итераций /openmp версия выполняется быстрее, чем /openmp- версия второго вызова test2. At a million iterations, the /openmp version is faster than the /openmp- version for the second call to test2. При 25 итерациях обе /openmp- /openmp версии регистрируются меньше, чем степень гранулярности часов. At 25 iterations, both /openmp- and /openmp versions register less than the clock granularity.

Если в приложении имеется только один цикл и оно выполняется менее чем на 15 мс (оно корректируется на приблизительную нагрузку на компьютер), /openmp может быть неприемлемо. If you have only one loop in your application and it runs in less than 15 ms (adjusted for the approximate overhead on your machine), /openmp may not be appropriate. Если это более высокое значение, можно использовать /openmp . If it’s higher, you may want to consider using /openmp .

Running openmp on windows

After a long thirst for parallelizing highly regular loops in matrix-oriented numerical programming, OpenMP was introduced by OpenMP Architecture Review Board (ARB) on 1997. In the subsequent releases, the enthusiastic OpenMP team added many features to it including the task parallelizing, support for accelerators, user-defined reductions and lot more. The latest OpenMP 5.0 release was made in 2018 November.

Open Multi-processing (OpenMP) is a technique of parallelizing a section(s) of C/C++/Fortran code. OpenMP is also seen as an extension to C/C++/Fortran languages by adding the parallelizing features to them. In general, OpenMP uses a portable, scalable model that gives programmers a simple and flexible interface for developing parallel applications for platforms that ranges from the normal desktop computer to the high-end supercomputers.

THREAD Vs PROCESS
A process is created by the OS to execute a program with given resources(memory, registers); generally, different processes do not share their memory with another. A thread is a subset of a process, and it shares the resources of its parent process but has its own stack to keep track of function calls. Multiple threads of a process will have access to the same memory.

Parallel Memory Architectures
Before getting deep into OpenMP, let’s revive the basic parallel memory architectures.
These are divided into three categories;

Shared memory:
OpenMP comes under the shared memory concept. In this, different CPU’s (processors) will have access to the same memory location. Since all CPU’s connect to the same memory, memory access should be handled carefully.

Steps for Installation of OpenMP

STEP 1: Check the GCC version of the compiler

GCC provides support for OpenMP starting from its version 4.2.0. So if the system has GCC compiler with the version higher than 4.2.0, then it must have OpenMP features configured with it.

If the system doesn’t have the GCC compiler, we can use the following command

For more detailed support for installation, we can refer here
STEP 2: Configuring OpenMP
We can check whether the OpenMP features are configured into our compiler or not, using the command

If OpenMP is not featured in the compiler, we can configure it use using the command

STEP 3: Setting the number of threads
In OpenMP, Before running the code, we can initialise the number of threads to be executed using the following command. Here, we set the number of threads to be getting executed to be 8 threads.