RR scheduling came into widespread
use with timesharing systems, beginning in the mid 1960s.
Each unit of time allocation is called a ???timeslice??? or ???quantum.??? RR schedulers can be tuned by changing
the value of the quantum. If the quantum is sufficiently large, the system effectively becomes FCFS. With
a smaller quantum, the system interleaves ready processes giving the illusion of simultaneous computation.
If the value of the quantum is too small, the overhead of context switching among the processes can hurt
overall system throughput (however measured). Most RR schedulers use a quantum value of 20 to 50 ms
(Tanenbaum, Andrew, Modern Operating Systems, Saddle River, NS, Prentice Hall, 2001).
Priority based
Scheduling based strictly on process priorities is characteristic of real-time systems. The ready process with
the highest priority is always the process chosen to execute. Usually real-time schedulers are also preemptive,
so any time a higher priority process becomes ready, a context switch occurs and the higher priority process
is dispatched.
Priority-based scheduling is also used in other contexts. For instance, a system may use priority-based scheduling,
but decrement a process??™s priority for every quantum of CPU time the process is granted.
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