Editing Scheduling (computing) (section)

===Classic Mac OS and macOS===
Mac OS 9 uses cooperative scheduling for threads, where one process controls multiple cooperative threads, and also provides preemptive scheduling for multiprocessing tasks. The kernel schedules multiprocessing tasks using a preemptive scheduling algorithm. All Process Manager processes run within a special multiprocessing task, called the ''blue task''. Those processes are scheduled cooperatively, using a [[round-robin scheduling]] algorithm; a process yields control of the processor to another process by explicitly calling a [[blocking function]] such as <code>WaitNextEvent</code>. Each process has its own copy of the [[Thread Manager]] that schedules that process's threads cooperatively; a thread yields control of the processor to another thread by calling <code>YieldToAnyThread</code> or <code>YieldToThread</code>.<ref name=appletn2028>{{cite web|url=https://developer.apple.com/library/archive/technotes/tn/tn2028.html |title= Technical Note TN2028: Threading Architectures |website=developer.apple.com |access-date=2019-01-15}}</ref>

macOS uses a multilevel feedback queue, with four priority bands for threads{{snd}}normal, system high priority, kernel mode only, and real-time.<ref>{{cite web|url=https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/KernelProgramming/scheduler/scheduler.html |title= Mach Scheduling and Thread Interfaces |website=developer.apple.com |access-date=2019-01-15}}</ref> Threads are scheduled preemptively; macOS also supports cooperatively scheduled threads in its implementation of the Thread Manager in [[Carbon (API)|Carbon]].<ref name=appletn2028/>