Parallel programming model

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 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 conditions, and mechanisms such as locks, semaphores and monitors can be used to avoid these. Conventional multi-core processors directly support shared memory, which many parallel programming languages and libraries, such as Cilk, OpenMP and Threading Building Blocks, are designed to exploit.

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, Limbo and Go. In contrast, the actor model uses asynchronous message passing and has been employed in the design of languages such as D, Scala and SALSA.

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

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 allows non-dependent functions to be executed in parallel.[6] However, this kind of parallelism is difficult to manage[7] and functional languages such as Concurrent Haskell and Concurrent ML provide features to manage parallelism explicitly and correctly.

A parallel program is composed of simultaneously executing processes. Problem decomposition relates to the way in which the constituent processes are formulated.[8][9]

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, task parallelism is usually classified as MIMD/MPMD or MISD.

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 Flynn's taxonomy, data parallelism is usually classified as MIMD/SPMD or SIMD.

Stream parallelism, also known as pipeline parallelism, focuses on dividing a computation into a sequence of stages, where each stage processes a portion of the input data. Each stage operates independently and concurrently, and the output of one stage serves as the input to the next stage. Stream parallelism is particularly suitable for applications with continuous data streams or pipelined computations.

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.