An operating system schedules preemption events to guarantee that equal priority processes receive round-robin service, discussed earlier. These events are scheduled at context switches, and ensure that a process cannot run continuously for more than a time determined by the granularity of preemption. Thus if a process becomes current at time `t', then a preemption event is scheduled for time `t + QUANTUM', where QUANTUM is a constant whose value determines the granularity of preemption. A preemption event is cancelled if a process relinquishes control of the CPU before its time expires.
An operating system has to be careful in selecting a suitable granularity
of preemption.
Setting it too low,
say
0.01 seconds,
would be inefficient since the processor would spent too much time rescheduling.
Setting it too high,
say
100 seconds, would allow compute-bound jobs
to `hog' the computer.
In Xinu the granularity of preemption is 1 second.
Older versions of Unix also used this granularity but BSD uses 0.1
second.
Few processes,
however,
use their allocated quantums,
since they make frequent calls to I/O and other routines that call
resched.