Scheduling (computing): Difference between revisions

Revision as of 22:04, 22 December 2023

Revision as of 17:44, 30 January 2024

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* [[Job shop scheduling]]{{snd}} there are {{mvar|n}} jobs and {{mvar|m}} identical stations. Each job should be executed on a single machine. This is usually regarded as an online problem.

* [[Open-shop scheduling]]{{snd}} there are {{mvar|n}} jobs and {{mvar|m}} different stations. Each job should spend some time at each station, in a free order.

* [[Flow -shop scheduling]]{{snd}} there are {{mvar|n}} jobs and {{mvar|m}} different stations. Each job should spend some time at each station, in a pre-determined order.

===Manual scheduling===

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==Operating system process scheduler implementations==

The algorithm used may be as simple as [[Roundround-robin scheduling|round-robin]] in which each process is given equal time (for instance 1 ms, usually between 1 ms and 100 ms) in a cycling list. So, process A executes for 1 ms, then process B, then process C, then back to process A.

More advanced algorithms take into account process priority, or the importance of the process. This allows some processes to use more time than other processes. The kernel always uses whatever resources it needs to ensure proper functioning of the system, and so can be said to have infinite priority. In [[Symmetric multiprocessing|SMP]] systems, [[processor affinity]] is considered to increase overall system performance, even if it may cause a process itself to run more slowly. This generally improves performance by reducing [[cache thrashing]].

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===Windows===

Very early [[MS-DOS]] and Microsoft Windows systems were non-multitasking, and as such did not feature a scheduler. [[Windows 3.1x]] used a non-preemptive scheduler, meaning that it did not interrupt programs. It relied on the program to end or tell the OS that it didn't need the processor so that it could move on to another process. This is usually called cooperative multitasking. Windows 95 introduced a rudimentary preemptive scheduler; however, for legacy support opted to let 16 -bit applications run without preemption.<ref>{{webarchive |url=https://web.archive.org/web/*/www.jgcampbell.com/caos/html/node13.html |date=* |title=Early Windows }}</ref>

[[Windows NT]]-based operating systems use a multilevel feedback queue. 32 priority levels are defined, 0 through to 31, with priorities 0 through 15 being ''normal'' priorities and priorities 16 through 31 being soft real-time priorities, requiring privileges to assign. 0 is reserved for the Operating System. User interfaces and APIs work with priority classes for the process and the threads in the process, which are then combined by the system into the absolute priority level.