computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

, a parallel programming model is an

abstraction Abstraction in its main sense is a conceptual process wherein general rules and concepts are derived from the usage and classification of specific examples, literal ("real" or " concrete") signifiers, first principles, or other methods. "An a ...

parallel computer 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 f ...

architecture, with which it is convenient to express

algorithms In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...

and their composition in

programs Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of managing several related projects * Time management * Program, a part of planning Arts and entertainment Audio * Programm ...

. The value of a programming model can be judged on its ''generality'': how well a range of different problems can be expressed for a variety of different architectures, and its ''performance'': how efficiently the compiled programs can execute. The implementation of a parallel programming model can take the form of a

library A library is a collection of materials, books or media that are accessible for use and not just for display purposes. A library provides physical (hard copies) or digital access (soft copies) materials, and may be a physical location or a vi ...

invoked from a sequential language, as an extension to an existing language, or as an entirely new language. Consensus around a particular programming model is important because it leads to different parallel computers being built with support for the model, thereby facilitating

portability Portability may refer to: *Portability (social security), the portability of social security benefits * Porting, the ability of a computer program to be ported from one system to another in computer science ** Software portability, the portability ...

of software. In this sense, programming models are referred to as '' bridging'' between hardware and software.Leslie G. Valiant, "A bridging model for parallel computation", Communications of the ACM, Volume 33, Issue 8, August, 1990, pages 103–111.

Classification of parallel programming models

Classifications of parallel programming models can be divided broadly into two areas: process interaction and problem decomposition.

Process interaction

Process interaction relates to the mechanisms by which parallel processes are able to communicate with each other. The most common forms of interaction are shared memory and message passing, but interaction can also be implicit (invisible to the programmer).

Shared memory

Shared memory is an efficient means of passing data between processes. In a shared-memory model, parallel processes share a global address space that they read and write to asynchronously. Asynchronous concurrent access can lead to

race condition A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of t ...

s, and mechanisms such as locks, semaphores and monitors can be used to avoid these. Conventional

multi-core processor A multi-core processor is a microprocessor on a single integrated circuit with two or more separate processing units, called cores, each of which reads and executes program instructions. The instructions are ordinary CPU instructions (suc ...

s directly support shared memory, which many parallel programming languages and libraries, such as

Cilk Cilk, Cilk++, Cilk Plus and OpenCilk are general-purpose programming languages designed for multithreaded parallel computing. They are based on the C and C++ programming languages, which they extend with constructs to express parallel loo ...

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 sy ...

and Threading Building Blocks, are designed to exploit.

Message passing

In a message-passing model, parallel processes exchange data through passing messages to one another. These communications can be asynchronous, where a message can be sent before the receiver is ready, or synchronous, where the receiver must be ready. The Communicating sequential processes (CSP) formalisation of message passing uses synchronous communication channels to connect processes, and led to important languages such as Occam,

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and Go. In contrast, the

actor model The actor model in computer science is a mathematical model of concurrent computation that treats ''actor'' as the universal primitive of concurrent computation. In response to a message it receives, an actor can: make local decisions, create mor ...

uses asynchronous message passing and has been employed in the design of languages such as D, Scala and SALSA.

Partitioned global address space

Partitioned Global Address Space (PGAS) models provide a middle ground between shared memory and message passing. PGAS provides a global memory address space abstraction that is logically partitioned, where a portion is local to each process. Parallel processes communicate by asynchronously performing operations (e.g. reads and writes) on the global address space, in a manner reminiscent of shared memory models. However by semantically partitioning the global address space into portions with affinity to a particular processes, they allow programmers to exploit

locality of reference In computer science, locality of reference, also known as the principle of locality, is the tendency of a processor to access the same set of memory locations repetitively over a short period of time. There are two basic types of reference localit ...

and enable efficient implementation on distributed memory parallel computers. PGAS is offered by many many parallel programming languages and libraries, such as Fortran 2008,

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UPC++
and SHMEM.

Implicit interaction

In an implicit model, no process interaction is visible to the programmer and instead the compiler and/or runtime is responsible for performing it. Two examples of implicit parallelism are with domain-specific languages where the concurrency within high-level operations is prescribed, and with functional programming languages because the absence of

side-effects In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequence ...

allows non-dependent functions to be executed in parallel.Hammond, Kevin. Parallel functional programming: An introduction. In International Symposium on Parallel Symbolic Computation, p. 46. 1994. However, this kind of parallelism is difficult to manage and functional languages such as

Concurrent Haskell Concurrent Haskell extends Haskell 98 with explicit concurrency. Its two main underlying concepts are: * A primitive type MVar α implementing a bounded/single-place asynchronous channel, which is either empty or holds a value of type α. * The a ...

and Concurrent ML provide features to manage parallelism explicitly and correctly.

Problem decomposition

A parallel program is composed of simultaneously executing processes. Problem decomposition relates to the way in which the constituent processes are formulated.

Task parallelism

A task-parallel model focuses on processes, or threads of execution. These processes will often be behaviourally distinct, which emphasises the need for communication. Task parallelism is a natural way to express message-passing communication. In

Flynn's taxonomy Flynn's taxonomy is a classification of computer architectures, proposed by Michael J. Flynn in 1966 and extended in 1972. The classification system has stuck, and it has been used as a tool in design of modern processors and their functionalities ...

, task parallelism is usually classified as MIMD/

MPMD Flynn's taxonomy is a classification of computer architectures, proposed by Michael J. Flynn in 1966 and extended in 1972. The classification system has stuck, and it has been used as a tool in design of modern processors and their functionalitie ...

or MISD.

Data parallelism

A data-parallel model focuses on performing operations on a data set, typically a regularly structured array. A set of tasks will operate on this data, but independently on disjoint partitions. In

, data parallelism is usually classified as MIMD/ SPMD or

SIMD Single instruction, multiple data (SIMD) is a type of parallel processing in Flynn's taxonomy. SIMD can be internal (part of the hardware design) and it can be directly accessible through an instruction set architecture (ISA), but it should ...

Implicit parallelism

As with implicit process interaction, an implicit model of parallelism reveals nothing to the programmer as the compiler, the runtime or the hardware is responsible. For example, in compilers, automatic parallelization is the process of converting sequential code into parallel code, and in computer architecture, superscalar execution is a mechanism whereby instruction-level parallelism is exploited to perform operations in parallel.

Terminology

Parallel programming models are closely related to

models of computation In computer science, and more specifically in computability theory and computational complexity theory, a model of computation is a model which describes how an output of a mathematical function is computed given an input. A model describes ho ...

. A model of parallel computation is an

used to analyze the cost of computational processes, but it does not necessarily need to be practical, in that it can be implemented efficiently in hardware and/or software. A programming model, in contrast, does specifically imply the practical considerations of hardware and software implementation.Skillicorn, David B., and Domenico Talia, Models and languages for parallel computation, ACM Computing Surveys, 30.2 123–169 (1998), https://www.cs.utexas.edu/users/browne/CS392Cf2000/papers/ModelsOfParallelComputation-Skillicorn.pdf A parallel programming language may be based on one or a combination of programming models. For example, High Performance Fortran is based on shared-memory interactions and data-parallel problem decomposition, and Go provides mechanism for shared-memory and message-passing interaction.

Example parallel programming models