Rensselaer Polytechnic Institute
CSC 432 – Operating Systems
David Goldschmidt, Ph.D.

The short-term scheduler decides which
process the CPU executes next

The dispatcher gives control of the CPU to
the process selected by the CPU scheduler:
 Performs context switch
 Switches to user mode
 Jumps to the proper location in the user
program to resume program execution
the dispatcher operates here

CPU scheduling requires an algorithm to
determine which process to dispatch next

Scheduling algorithms include:
 First-Come, First-Served (FCFS)
 Shortest-Job-First (SJF)
 Round-Robin (RR)
 Priority
 Multilevel Queue (MQ)

Preemptive scheduling preempts a running
process before its
time slice expires
process
process
process
process
 Or it preempts a process
because its time slice has expired

Non-preemptive scheduling gives a process
exclusive uninterrupted access to the CPU
for the entirety of its execution

Compare scheduling algorithms by measuring
 CPU utilization – keep CPU as busy as possible
 Throughput – maximize the number of processes
that complete their execution per unit time
 Turnaround time – minimize the elapsed time to
fully execute a particular process
 Waiting time – minimize the elapsed time a
process waits in the ready queue

FCFS dispatches processes
in the order they enter
the ready queue
Process
CPU Burst Time
P1
24 ms
P2
3 ms
P3
3 ms
 FCFS is non-preemptive
P1
P2
0
24
time
P3
27
30

SJF dispatches processes by
selecting the process with
the lowest CPU burst time
Process
CPU Burst Time
P1
24 ms
P2
3 ms
P3
3 ms
 SJF is non-preemptive (and predictive)
P2
0
P3
3
P1
6
30
time

What happens
when processes
arrive at different
arrival times?
Process
Arrival Time
CPU Burst Time
P1
0
7 ms
P2
2 ms
4 ms
P3
4 ms
1 ms
P4
5 ms
4 ms
 Calculate the turnaround times and wait times
for each process
 Calculate the average turnaround time and
average wait time

Same as SJF, but a
new process may
preempt the
running process
P1
0
P2
2
P3
4
Process
Arrival Time
CPU Burst Time
P1
0
7 ms
P2
2 ms
4 ms
P3
4 ms
1 ms
P4
5 ms
4 ms
P2
5
P4
7
time
P1
11
16

SJF is the optimal solution
 The problem with SJF is the inability to
know the required CPU burst times
 Apply a prediction algorithm that uses
previous CPU burst times
 Algorithm uses exponential averaging:
▪ tn = actual length of the nth CPU burst
▪ τn+1 = predicted value for the next CPU burst
▪ τn+1 = α tn + (1 – α) τn , where 0 < α < 1
α is ½
α is ¾
α is ½
α is ¼

Apply the FCFS, SJF, and Preemptive SJF
scheduling algorithms to this table:
Process
Arrival Time
CPU Burst Time
P1
0
45 ms
P2
0
5 ms
P3
20 ms
15 ms
P4
60 ms
25 ms
recalculate using
context switch
time tcs = 20 μs
 Calculate the wait and turnaround times of
each process, as well as overall averages