1. Priority Scheduling Example
Fixed (strict) priority scheduling
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Priority scheduling Gantt Chart
Average waiting time = 8.2 msec P2 P3 P5 1 18 16 P4 19 6 P1

2. Priority Scheduling
A priority number (integer) is associated with each process
The CPU is allocated to the process with the highest priority (usually smallest integer  highest priority)
Preemptive
Nonpreemptive
SJF is priority scheduling where priority is the inverse of predicted next CPU burst time
Equal-priority processes are scheduled in FCFS order
Problem  Starvation (indefinite blocking) – low priority processes may never execute
Solution  Aging – as time progresses increase the priority of the process

3. Internal / External Priorities
Priorities can be defined either internally to OS or externally
Internally defined priorities use some measurable quantity or quantities to compute the priority of a process. For example,
time limits
memory requirements
the number of open files
the ratio of average I/O burst to average CPU burst have been used in computing priorities
External priorities are set by criteria that are external to the operating system such as the
Importance of the process
Urgency …

4. Round Robin (RR)
The round-robin (RR) scheduling algorithm is designed especially for timesharing systems.
It is similar to FCFS scheduling, but preemption is added to switch between processes.
A small unit of time, called a time quantum (or time slice), is defined.
Adv:The RR provides acceptable response time for the processes.
DAdv:The average waiting time under the RR policy is often quite long.
Performance depends on time quantum length.

5. Example of RR with Time Quantum = 4 ms
Process Burst Time
P1 24
P2 3
P3 3
The Gantt chart is:
Typically, higher average turnaround than SJF, but better response
q should be large compared to context switch time
q usually 10ms to 100ms, context switch < 10 usec P1 P2 P3 4 7 10 14 18 22 26 30

6. Time Quantum and Context Switch Time
If the time quantum is 1 time unit, then 9 context switches will occur, slowing the execution of the process accordingly.

7. Turnaround Time Varies With The Time Quantum
Empiric Conclusion
80% of CPU bursts should be shorter than q
The average turnaround time of a set of processes does not necessarily improve as the time-quantum size increases.
In general, the average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum.