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[[File:Thread pool.svg|thumb|400px|A sample [[thread pool]] (green boxes) with [[task queue]]s of waiting '''tasks''' (blue) and completed '''tasks''' (yellow), in the sense of task as "unit of work".]]
In [[computers|computing]], a '''task''' is a unit of [[execution (computing)|execution]] or a unit of work. The term is ambiguous;
==Terminology==
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* A process and the procedures that run the process.
* A set of actions designed to achieve a particular result. A task is performed on a set of targets on a specific schedule.
* A unit of computation. In a parallel job, two or more concurrent tasks work together through message passing and shared memory. Although it is common to allocate one task per physical or logical processor, the terms "task" and "processor" are not interchangeable.
* An activity that has business value, is initiated by a user, and is performed by software.
In [[z/OS]] specifically, it is defined precisely as:<ref>[http://www-01.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zglossary.doc/zglossary.html Glossary of z/OS terms and abbreviations]: [http://www-01.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zglossary.doc/zglossary.html#t T]</ref>
* "In a multiprogramming or multiprocessing environment, one or more sequences of instructions treated by a control program as an element of work to be accomplished by a computer."
The term task in OS/360 through z/OS is roughly equivalent to light-weight process; the tasks in a job step share an address space. However, in [[MVS/ESA]] through z/OS, a task or [[Service Request Block]] (SRB) may have access to other address spaces via its access list.
===Linux kernel===
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{{quote |The serial model has the ability to process tasks of one job in an independent manner similar to the functioning of the [[IBM 709]].<ref>{{cite book |title=Simultaneous multiprogramming of electronic computers |year=1961 |author=James Larrimore McKenney |page=[https://books.google.com/books?id=ThYcAQAAMAAJ&dq=%22tasks+of+one+job%22 154]}}</ref>}}
The term was popularized with the introduction of [[OS/360 and successors|OS/360]] (announced 1964), which featured [[OS/360 and successors#MFT|Multiprogramming with a Fixed number of Tasks]] (MFT) and [[OS/360 and successors#MVT|Multiprogramming with a Variable number of Tasks]] (MVT). In this case tasks were identified with light-weight processes, a job consisted of a number of tasks, and, later, tasks could have sub-tasks (in modern terminology, [[child process]]es).
Today the term "task" is used very ambiguously. For example, the [[Windows Task Manager]] manages (running) ''processes'', while [[Windows Task Scheduler]] schedules ''programs'' to execute in future, what is traditionally known as a [[job scheduler]], and uses the <code>.job</code> extension. By contrast, the term "[[task queue]]" is commonly used in the sense of "units of work".
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