OpenACC (for ''open accelerators'') is a programming standard for

parallel computing Parallel computing is a type of computation in which many calculations or processes are carried out simultaneously. Large problems can often be divided into smaller ones, which can then be solved at the same time. There are several different fo ...

developed by

Cray Cray Inc., a subsidiary of Hewlett Packard Enterprise, is an American supercomputer manufacturer headquartered in Seattle, Washington. It also manufactures systems for data storage and analytics. Several Cray supercomputer systems are listed ...

, CAPS,

Nvidia Nvidia CorporationOfficially written as NVIDIA and stylized in its logo as VIDIA with the lowercase "n" the same height as the uppercase "VIDIA"; formerly stylized as VIDIA with a large italicized lowercase "n" on products from the mid 1990s to ...

and PGI. The standard is designed to simplify parallel programming of

heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

CPU/

GPU A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mobi ...

systems. As in

OpenMP OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared-memory multiprocessing programming in C, C++, and Fortran, on many platforms, instruction-set architectures and operating syste ...

, the programmer can annotate C,

C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...

and Fortran

source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the wo ...

to identify the areas that should be accelerated using

compiler directives In computer programming, a directive or pragma (from "pragmatic") is a language construct that specifies how a compiler (or other Translator (computing), translator) should process its input. Directives are not part of the Formal grammar, grammar ...

and additional functions. Like OpenMP 4.0 and newer, OpenACC can target both the CPU and

architectures and launch computational code on them. OpenACC members have worked as members of the OpenMP standard group to merge into OpenMP specification to create a common specification which extends OpenMP to support accelerators in a future release of OpenMP. These efforts resulted in a technical report for comment and discussion timed to include the annual

Supercomputing Conference SC (formerly Supercomputing), the International Conference for High Performance Computing, Networking, Storage and Analysis, is the annual conference established in 1988 by the Association for Computing Machinery and the IEEE Computer Society. In ...

(November 2012,

Salt Lake City Salt Lake City (often shortened to Salt Lake and abbreviated as SLC) is the Capital (political), capital and List of cities and towns in Utah, most populous city of Utah, United States. It is the county seat, seat of Salt Lake County, Utah, Sal ...

) and to address non-Nvidia accelerator support with input from hardware vendors who participate in OpenMP. At ISC’12 OpenACC was demonstrated to work on

AMD Advanced Micro Devices, Inc. (AMD) is an American multinational semiconductor company based in Santa Clara, California, that develops computer processors and related technologies for business and consumer markets. While it initially manufactur ...

and

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...

accelerators, without performance data. On November 12, 2012, at the SC12 conference, a draft of the OpenACC version 2.0 specification was presented. New suggested capabilities include new controls over data movement (such as better handling of

unstructured data Unstructured data (or unstructured information) is information that either does not have a pre-defined data model or is not organized in a pre-defined manner. Unstructured information is typically text-heavy, but may contain data such as dates, num ...

and improvements in support for non-contiguous memory), and support for explicit function calls and separate compilation (allowing the creation and reuse of libraries of accelerated code). OpenACC 2.0 was officially released in June 2013. Version 2.5 of the specification was released in October 2015, while version 2.6 was released in November 2017. The latest version of specification, version 2.7, was released in November 2018.

Compiler support

Support of OpenACC is available in commercial compilers from PGI (from version 12.6), and (for Cray hardware only) Cray. OpenUH is an

Open64 Open64 is a free, open-source, optimizing compiler for the Itanium and x86-64 microprocessor architectures. It derives from the SGI compilers for the MIPS R10000 processor, called ''MIPSPro''. It was initially released in 2000 as GNU GPL s ...

based open source OpenACC compiler supporting C and FORTRAN, developed by HPCTools group from

University of Houston The University of Houston (UH) is a Public university, public research university in Houston, Texas. Founded in 1927, UH is a member of the University of Houston System and the List of universities in Texas by enrollment, university in Texas ...

. OpenARC is an open source C compiler developed at

Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a U.S. multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT–Battelle as a federally funded research and ...

to support all features in the OpenACC 1.0 specification. An experimental open source compiler, accULL, is developed by the

University of La Laguna The University of La Laguna (ULL; Spanish: ''Universidad de La Laguna'') is a public research university situated in San Cristóbal de La Laguna, on the island of Tenerife, Spain. It is the oldest university in the Canary Islands. The university ...

(

C language C (''pronounced like the letter c'') is a general-purpose computer programming language. It was created in the 1970s by Dennis Ritchie, and remains very widely used and influential. By design, C's features cleanly reflect the capabilities o ...

only). Omni Compiler is an open source compiler developed at HPCS Laboratory. of

University of Tsukuba is a public university, public research university located in Tsukuba, Ibaraki Prefecture, Ibaraki, Japan. It is a top 10 Designated National University, and was ranked Type A by the Japanese government as part of the Top Global University Pro ...

and Programming Environment Research Team of RIKEN Center for Computational Science, Japan, supported OpenACC, and combining XcalableMP and OpenACC. IPMACC is an open source C compiler developed by

University of Victoria The University of Victoria (UVic or Victoria) is a public research university located in the municipalities of Oak Bay and Saanich, British Columbia, Canada. The university traces its roots to Victoria College, the first post-secondary instit ...

that translates OpenACC to CUDA, OpenCL, and ISPC. Currently, only following directives are supported: ''data'', ''kernels'', ''loop'', and ''cache''. GCC support for OpenACC was slow in coming. A GPU-targeting implementation from Samsung was announced in September 2013; this translated OpenACC 1.1-annotated code to

OpenCL OpenCL (Open Computing Language) is a framework for writing programs that execute across heterogeneous platforms consisting of central processing units (CPUs), graphics processing units (GPUs), digital signal processors (DSPs), field-progra ...

. The announcement of a "real" implementation followed two months later, this time from NVIDIA and based on OpenACC 2.0. This sparked some controversy, as the implementation would only target NVIDIA's own PTX assembly language, for which no open source assembler or runtime was available. Experimental support for OpenACC/PTX did end up in GCC as of version 5.1. GCC6 and GCC7 release series include a much improved implementation of the OpenACC 2.0a specification. GCC 9.1 offers nearly complete OpenACC 2.5 support.

Usage

In a way similar to

3.x on homogeneous system or the earlier

OpenHMPP OpenHMPP (HMPP for Hybrid Multicore Parallel Programming) - programming standard for heterogeneous computing. Based on a set of compiler directives, standard is a programming model designed to handle hardware accelerators without the complexity a ...

, the primary mode of programming in OpenACC is directives. The specifications also include a

runtime library In computer programming, a runtime library is a set of low-level routines used by a compiler to invoke some of the behaviors of a runtime environment, by inserting calls to the runtime library into compiled executable binary. The runtime enviro ...

defining several support functions. To exploit them, user should include "openacc.h" in C or "openacc_lib.h" in Fortran; and then call ''acc_init()'' function.

Directives

OpenACC defines an extensive list of pragmas (directives), for example: #pragma acc parallel #pragma acc kernels Both are used to define parallel computation kernels to be executed on the accelerator, using distinct semantics #pragma acc data Is the main directive to define and copy data to and from the accelerator. #pragma acc loop Is used to define the type of parallelism in a parallel or kernels region. #pragma acc cache #pragma acc update #pragma acc declare #pragma acc wait

Runtime API

There are some runtime

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

functions defined too: acc_get_num_devices(), acc_set_device_type(), acc_get_device_type(), acc_set_device_num(), acc_get_device_num(), acc_async_test(), acc_async_test_all(), acc_async_wait(), acc_async_wait_all(), acc_init(), acc_shutdown(), acc_on_device(), acc_malloc(), acc_free(). OpenACC generally takes care of work organisation for the target device however this can be overridden through the use of gangs and workers. A gang consists of workers and operates over a number of processing elements (as with a workgroup in OpenCL).

References

External links