lec7: CPU Scheduling Priority Preemptive and NON 2018-2109

3 Priority Scheduling
The SJF algorithm is a special case of the general priority scheduling algorithm. A priority is associated with each process, and the CPU is allocated to the process with the highest priority. Equal-priority processes are scheduled in FCFS order. An SJF algorithm is simply a priority algorithm where the priority (p) is the inverse of the next CPU burst. The larger the CPU burst, the lower the priority, and vice versa.
Note that we discuss scheduling in terms of high priority and low priority. Priorities are generally indicated by some fixed range of numbers, such as 0 to 7 or 0 to 4,095. However, there is no general agreement on whether 0 is the highest or lowest priority. Some systems use low numbers to represent low priority; others use low numbers for high priority. This difference can lead to confusion. In this text, we assume that low numbers represent high priority.
As an example, consider the following set of processes, assumed to have arrived at time 0 in the order P1, P2, ···, P5, with the length of the CPU burst given in milliseconds:

P1 P2 P4 P1 P3
Process Burst Time Priority
P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2
Using priority scheduling, we would schedule these processes according to the following Gantt chart:

The average waiting time is 8.2 milliseconds.?Priorities can be defined either internally or externally. Internally defined