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Fixed a sentence that did not correspond with the preceding sentence or commonly agreed upon terms in computing. Concurrency is not the simultaneous execution of multiple processes, it is the appearance of simultaneous execution. In reality, (and somewhat oversimplified still), the processor is switching between processes rapidly but not actually executing them at the exact same time. It just appears that way. Tags: Mobile edit Mobile web edit |
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[[Computer multitasking|Multitasking]] is a method to allow multiple processes to share [[Central processing unit|processors]] (CPUs) and other system resources. Each CPU (core) executes a single process at a time. However, multitasking allows each processor to [[context switch|switch]] between tasks that are being executed without having to wait for each task to finish ([[Preemption (computing)|preemption]]). Depending on the operating system implementation, switches could be performed when tasks initiate and wait for completion of [[input/output]] operations, when a task voluntarily yields the CPU, on hardware [[interrupt]]s, and when the operating system scheduler decides that a process has expired its fair share of CPU time (e.g, by the [[Completely Fair Scheduler]] of the [[Linux kernel]]).
A common form of multitasking is provided by CPU's [[time-sharing]] that is a method for interleaving the execution of users' processes and threads, and even of independent kernel tasks – although the latter feature is feasible only in preemptive [[Kernel (operating system)|kernels]] such as [[Linux kernel|Linux]]. Preemption has an important side effect for interactive processes that are given higher priority with respect to CPU bound processes, therefore users are immediately assigned computing resources at the simple pressing of a key or when moving a mouse. Furthermore, applications like video and music reproduction are given some kind of real-time priority, preempting any other lower priority process. In time-sharing systems, [[context switch]]es are performed rapidly, which makes it seem like multiple processes are being executed simultaneously on the same processor. This seemingly-simultaneous execution of multiple processes is called [[Concurrency (computer science)|concurrency]].
For security and reliability, most modern [[operating system]]s prevent direct [[inter-process communication|communication]] between independent processes, providing strictly mediated and controlled inter-process communication.
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