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A Round Robin preemptive scheduling example with quantum=3. Round-robin (RR) is one of the algorithms employed by process and network schedulers in computing. [1] [2] As the term is generally used, time slices (also known as time quanta) [3] are assigned to each process in equal portions and in circular order, handling all processes without priority (also known as cyclic executive).
Each queue will be assigned a priority and will have its own scheduling algorithm like Round-robin scheduling [1]: 194 or FCFS. For the process in a queue to execute, all the queues of priority higher than it should be empty, meaning the process in those high priority queues should have completed its execution.
For example, Windows NT/XP/Vista uses a multilevel feedback queue, a combination of fixed-priority preemptive scheduling, round-robin, and first in, first out algorithms. In this system, threads can dynamically increase or decrease in priority depending on if it has been serviced already, or if it has been waiting extensively.
That scheduler could be a Round-robin scheduler. The other scheduler is the higher-level scheduler whose only concern is to swap in and swap out processes from memory. It does its scheduling much less often than the lower-level scheduler since swapping takes so much time.
The proportional fair (= and =) scheduler could be called "equal effort scheduler" or "time/spectrum Round Robin scheduler". This technique can be further parametrized by using a "memory constant" that determines the period of time over which the station data rate used in calculating the priority function is averaged.
Weighted round robin (WRR) is a network scheduler for data flows, but also used to schedule processes. Weighted round robin [ 1 ] is a generalisation of round-robin scheduling . It serves a set of queues or tasks.
If a process uses too much CPU time, it will be moved to a lower-priority queue. If a process is I/O-bound or an interactive process, it will be moved to a higher-priority queue. If a process is waiting too long in a low-priority queue and starving, it will be aged to a higher-priority queue.
In weighted round robin scheduling, the fraction of bandwidth used depend on the packet's sizes. Compared with WFQ scheduler that has complexity of O(log(n)) ( n is the number of active flows/queues ), the complexity of DRR is O(1) , if the quantum Q i {\displaystyle Q_{i}} is larger than the maximum packet size of this flow.