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# Concurrency (computer science)

In computer science, **concurrency** is a property of systems in which several computations are executing simultaneously, and potentially interacting with each other. The computations may be executing on multiple cores in the same chip, preemptively time-shared threads on the same processor, or executed on physically separated processors.
A number of mathematical models have been developed for general concurrent computation including Petri nets, process calculi, the Parallel Random Access Machine model, the Actor model and the Reo Coordination Language.

## Contents

## Issues

Because computations in a concurrent system can interact with each other while they are executing, the number of possible execution paths in the system can be extremely large, and the resulting outcome can be indeterminate. Concurrent use of shared resources can be a source of indeterminacy leading to issues such as deadlock, and starvation.^{[1]}

The design of concurrent systems often entails finding reliable techniques for coordinating their execution, data exchange, memory allocation, and execution scheduling to minimize response time and maximize throughput.^{[2]}

## Theory

Concurrency theory has been an active field of research in theoretical computer science. One of the first proposals was Carl Adam Petri's seminal work on Petri Nets in the early 1960s. In the years since, a wide variety of formalisms have been developed for modeling and reasoning about concurrency.

### Models

A number of formalisms for modeling and understanding concurrent systems have been developed, including:^{[3]}

- The Parallel Random Access Machine
^{[4]} - The Actor model
- Computational bridging models such as the BSP model
- Petri nets
- Process calculi
- CSP model

- Tuple spaces, e.g., Linda
- SCOOP (Simple Concurrent Object-Oriented Programming)
- Reo Coordination Language

Some of these models of concurrency are primarily intended to support reasoning and specification, while others can be used through the entire development cycle, including design, implementation, proof, testing and simulation of concurrent systems. Some of these are based on message passing, while others have different mechanisms for concurrency.

The proliferation of different models of concurrency has motivated some researchers to develop ways to unify these different theoretical models. For example, Lee and Sangiovanni-Vincentelli have demonstrated that a so-called "tagged-signal" model can be used to provide a common framework for defining the denotational semantics of a variety of different models of concurrency,^{[5]} while Nielsen, Sassone, and Winskel have demonstrated that category theory can be used to provide a similar unified understanding of different models.^{[6]}

The Concurrency Representation Theorem in the Actor model provides a fairly general way to represent concurrent systems that are closed in the sense that they do not receive communications from outside. (Other concurrency systems, e.g., process calculi can be modeled in the Actor model using a two-phase commit protocol.^{[7]}) The mathematical denotation denoted by a closed system `S` is constructed increasingly better approximations from an initial behavior called `⊥ _{S}` using a behavior approximating function

`to construct a denotation (meaning ) for`

**progression**_{S}`S`as follows:

^{[8]}

**Denote**_{S}≡ ⊔_{i∈ω}**progression**_{S}^{i}(⊥_{S})

In this way, `S` can be mathematically characterized in terms of all its possible behaviors.

### Logics

Various types of temporal logic^{[9]} can be used to help reason about concurrent systems. Some of these logics, such as linear temporal logic and computational tree logic, allow assertions to be made about the sequences of states that a concurrent system can pass through. Others, such as action computational tree logic, Hennessy-Milner logic, and Lamport's temporal logic of actions, build their assertions from sequences of *actions* (changes in state). The principal application of these logics is in writing specifications for concurrent systems.^{[1]}

## Practice

This section does not cite any references or sources. (April 2007) |

*correctness*,

*performance*and

*robustness*. Concurrent systems such as Operating systems and Database management systems are generally designed to operate indefinitely, including automatic recovery from failure, and not terminate unexpectedly (see Concurrency control). Some concurrent systems implement a form of transparent concurrency, in which concurrent computational entities may compete for and share a single resource, but the complexities of this competition and sharing are shielded from the programmer.

Because they use shared resources, concurrent systems in general require the inclusion of some kind of arbiter somewhere in their implementation (often in the underlying hardware), to control access to those resources. The use of arbiters introduces the possibility of indeterminacy in concurrent computation which has major implications for practice including correctness and performance. For example arbitration introduces unbounded nondeterminism which raises issues with model checking because it causes explosion in the state space and can even cause models to have an infinite number of states.

Some concurrent programming models include coprocesses and deterministic concurrency. In these models, threads of control explicitly yield their timeslices, either to the system or to another process.

## See also

- Client–server network nodes
- Cluster nodes
- Concurrency control
- Concurrent computing
- Concurrent object-oriented programming
- Concurrency pattern
- Chu space
- Distributed systemnodes
- Go
- Gordon Pask
- OpenMP
- Parallel Computing
- Partitioned global address space
- Processes
- Ptolemy Project
- Sheaf (mathematics)
- Threads
- X10 (programming language)

## References

- ^
^{a}^{b}Cleaveland, Rance; Scott Smolka (December 1996). "Strategic Directions in Concurrency Research".*ACM Computing Surveys***28**(4): 607. doi:10.1145/242223.242252. **^**Campbell, Colin; Johnson, Ralph; Miller, Ade; Toub, Stephen (August 2010).*Parallel Programming with Microsoft .NET*. Microsoft Press. ISBN 978-0-7356-5159-3.**^**Filman, Robert; Daniel Friedman (1984).*Coordinated Computing - Tools and Techniques for Distributed Software*. McGraw-Hill. ISBN 0-07-022439-0.**^**Keller, Jörg; Christoph Keßler; Jesper Träff (2001).*Practical PRAM Programming*. John Wiley and Sons.**^**Lee, Edward; Alberto Sangiovanni-Vincentelli (December 1998). "A Framework for Comparing Models of Computation".*IEEE Transactions on CAD***17**(12): 1217–1229. doi:10.1109/43.736561.**^**Mogens Nielsen; Vladimiro Sassone and Glynn Winskel (1993). "Relationships Between Models of Concurrency".*REX School/Symposium*.**^**Frederick Knabe. A Distributed Protocol for Channel-Based Communication with Choice PARLE 1992.**^**William Clinger (June 1981). "Foundations of Actor Semantics". Mathematics Doctoral Dissertation. MIT.**^**Roscoe, Colin (2001).*Modal and Temporal Properties of Processes*. Springer. ISBN 0-387-98717-7.

## Further reading

- Lynch, Nancy A. (1996).
*Distributed Algorithms*. Morgan Kauffman. ISBN 1-55860-348-4. - Tanenbaum, Andrew S.; Van Steen, Maarten (2002).
*Distributed Systems: Principles and Paradigms*. Prentice Hall. ISBN 0-13-088893-1. - Kurki-Suonio, Reino (2005).
*A Practical Theory of Reactive Systems*. Springer. ISBN 3-540-23342-3. - Garg, Vijay K. (2002).
*Elements of Distributed Computing*. Wiley-IEEE Press. ISBN 0-471-03600-5. - Magee, Jeff; Kramer, Jeff (2006).
*Concurrency: State Models and Java Programming*. Wiley. ISBN 0-470-09355-2.