1. Operating Systems
Lecture 15

2. © Copyright Virtual University of Pakistan
Agenda for Today
Review of previous lecture
Shortest-job-first
Round-Robin scheduling
Multi-level queues scheduling
Multi-level feedback queues scheduling
Recap of lecture
21 November 2018
© Copyright Virtual University of Pakistan

3. © Copyright Virtual University of Pakistan
Review of Lecture 14
Short-term scheduler
Dispatcher
Reasons for invoking scheduler
Preemptive and non-preemptive schedulers
Optimization criteria
FCFS
21 November 2018
© Copyright Virtual University of Pakistan

4. Shortest-Job-First (SJF) Scheduling
Process with the shortest CPU burst is scheduled first.
Non-preemptive – once CPU given to a process it cannot be preempted until completes its CPU burst.
21 November 2018
© Copyright Virtual University of Pakistan

5. Shortest-Job-First (SJF) Scheduling
Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt it—Shortest-Remaining-Time-First (SRTF).
SJF is optimal non-preemptive scheduling algorithm – gives minimum average waiting time for a given set of processes.
21 November 2018
© Copyright Virtual University of Pakistan

6. © Copyright Virtual University of Pakistan
Non-Preemptive SJF
Process Arrival Time Burst Time P P P P
Gantt chart
Average waiting time = ( )/4 = 4 P1 P3 P2 7 16 P4 12
21 November 2018
© Copyright Virtual University of Pakistan

7. © Copyright Virtual University of Pakistan
Preemptive SJF
Process Arrival Time Burst Time P P P P
Gantt chart
Average waiting time = ( )/4 = 3 P3 P2 4 2 11 P4 5 7 P1 16
21 November 2018
© Copyright Virtual University of Pakistan

8. SJF—CPU Burst of a Process
Can only estimate the length of the next CPU burst.
Can be done by using the length of previous CPU bursts, using exponential averaging.
21 November 2018
© Copyright Virtual University of Pakistan

9. Exponential Averaging
Estimation based on historical data
tn = Length of nth CPU burst
n+1 = Estimate for n+1st CPU burst
, 0 ≤  ≤1
n+1 = tn + (1- ) n
21 November 2018
© Copyright Virtual University of Pakistan

10. Exponential Averaging
Plugging in value for n, we get
n+1 = tn + (1- )[tn-1 + (1- )n-1]
= tn + (1- )tn-1 + (1- )2n-1
Continuing like this results in
n+1 =  tn+ (1 - )  tn-1 + …
+ (1 - )j  tn-j + …
+ (1 - )n+1 0
21 November 2018
© Copyright Virtual University of Pakistan

11. Exponential Averaging
If  = 0
n+1 = n
Actual previous bursts do not count
If  = 1
n+1 = tn
Estimates do not count
21 November 2018
© Copyright Virtual University of Pakistan

12. Exponential Averaging
Typical value used for  is ½. With this value, our n+1st estimate is
Hence the name exponential averaging
n+1 = tn/2 + tn-1/22 + tn-2/23 + tn-3/24 + …
21 November 2018
© Copyright Virtual University of Pakistan

13. © Copyright Virtual University of Pakistan
Priority Scheduling
A priority number (integer) is associated with each process
The CPU is allocated to the process with the highest priority (smallest integer  highest priority).
Preemptive
Non-preemptive
21 November 2018
© Copyright Virtual University of Pakistan

14. © Copyright Virtual University of Pakistan
Priority Scheduling
SJF is a priority scheduling where priority is the predicted next CPU burst time.
Problem  Starvation – low priority processes may never execute.
Solution  Aging – as time progresses increase the priority of the process.
21 November 2018
© Copyright Virtual University of Pakistan

15. © Copyright Virtual University of Pakistan
Recap of Lecture
Shortest-Job-First (SJF)
Shortest-Remaining-Time-First (SRTF)
Exponential averaging
Priority scheduling
Recap of lecture
21 November 2018
© Copyright Virtual University of Pakistan