Scheduling (computing): Difference between revisions

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===={{Anchor|SHORT-TERM}}Short-term scheduling====

The ''short-term scheduler'' (also known as the ''CPU scheduler'') decides which of the ready, in-memory processes is to be executed (allocated a CPU) after a clock [[interrupt]], an I/O interrupt, an operating [[system call]] or another form of [[Signal programming(IPC)|signal]]. Thus the short-term scheduler makes scheduling decisions much more frequently than the long-term or mid-term schedulers{{snd}} A scheduling decision will at a minimum have to be made after every time slice, and these are very short. This scheduler can be [[Preemption (computing)|preemptive]], implying that it is capable of forcibly removing processes from a CPU when it decides to allocate that CPU to another process, or non-preemptive (also known as ''voluntary'' or ''co-operative''), in which case the scheduler is unable to ''force'' processes off the CPU.

A preemptive scheduler relies upon a [[programmable interval timer]] which invokes an [[interrupt handler]] that runs in [[kernel mode]] and implements the scheduling function.

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===Linux===

{{See also|Linux kernel#Scheduling}}

==== Linux 1.2 ====

Linux 1.2 used a [[round-robin scheduling]] policy.<ref name=":0">{{Cite web |last=Jones |first=M. |date=2018-09-18 |orig-date=first published on 2009-12-14 |title=Inside the Linux 2.6 Completely Fair Scheduler |url=https://developer.ibm.com/tutorials/l-completely-fair-scheduler/ |access-date=2024-02-07 |website=developer.ibm.com}}</ref>

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===NetBSD===

[[NetBSD]] uses a multilevel feedback queue with priorities ranging from 0–223. 0–63 are reserved for time-shared threads (default, SCHED_OTHER policy), 64–95 for user threads which entered [[kernel space]], 96-12896–128 for kernel threads, 128–191 for user real-time threads (SCHED_FIFO and SCHED_RR policies), and 192–223 for [[Interrupt|software interrupts]].

===Solaris===