This document discusses various CPU scheduling policies including FIFO, round robin, shortest job first, priority scheduling, and lottery scheduling. It provides details on how each policy works, its advantages and disadvantages, and examples to illustrate differences in performance between policies. The instructor is Hamza Ali for the course Analyzing and Evaluation Networks at Misurata University in the autumn 2015-2016 term.

1. Misurata University
Faculty of Information Technology
Analyzing and evaluation Networks
Performance CN 422
Instructor : HAMZA ALI
Autumn 2015-2016
Scheduling Policies
1
Modified by: Ahmad S. AL-Majouk

2. Scheduling Policies
■ FIFO (first in, first out)
■ Round robin
■ SJF (shortest job first)
■ Priority Scheduling
■ Lottery scheduling
■ This is obviously an incomplete list
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3. FIFO
FIFO is an acronym for First In, First Out, a method for organizing
and manipulating a data buffer, or data stack, where the oldest
entry, or 'bottom' of the stack, is processed first. It is analogous
to processing a queue with first-come, first-served (FCFS)
behavior: where the people leave the queue in the order in
which they arrive.
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4. FIFO
■ FIFO: assigns the CPU based on the order of
requests
❑ Nonpreemptive: A process keeps running on a CPU
until it is blocked or terminated
❑ Also known as FCFS (first come, first serve)
+ Simple
- Short jobs can get stuck behind long jobs
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5. FIFO
■
Suppose we have three jobs of equal length
A B Time
C
turnaround time of A
turnaround time of B
turnaround time of C
FIFO
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6. Round Robin
■ Round Robin (RR) periodically releases the CPU
from long-running jobs
❑ Based on timer interrupts so short jobs can get a fair
share of CPU time
❑ Preemptive : a process can be forced to leave its
running state and replaced by another running process
❑ Time slice: interval between timer interrupts
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7. Round Robin (Cont.)
■ If time slice is too long
❑ Scheduling degrades to FIFO
■ If time slice is too short
❑ Throughput suffers
❑ Context switching cost dominates
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8. FIFO vs. Round Robin
■
Suppose we have three jobs of equal length
A B C A B C Ti
m
e
A B C
turnaround time of A
turnaround time of B
turnaround time of C
Round Robin
A B Time
C
turnaround time of A
turnaround time of B
turnaround time of C
FIFO
* 8

9. FIFO vs. Round Robin
■ Round Robin
+ Shorter response time
+ Fair sharing of CPU
- Not all jobs are preemptable
- Not good for jobs of the same length (longer turnaround time)
- More precisely, not good in terms of the turnaround time, because the
turnaround of job A (last graph for example) in Robin Round state is
longer than turnaround time of the same job in FIFO state.
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Modified by: Ahmad S. Al-Majouk

10. * Instructor: Ahmad S. Al-Majouk
10
Shortest Job First (SJF)
⮚ Other name of this algorithm is Shortest-Process-Next (SPN).
Also known as STCF (shortest time to completion first).
⮚ Like FCFS, SJF is non preemptive discipline in which waiting job
(or process) with the smallest estimated run-time-to-completion is
run next.
In other words, when CPU is available, it is assigned to the process
that has smallest next CPU burst.
⮚ The SJF scheduling is especially appropriate for batch jobs for
which the run times are known in advance.
⮚ The SJF algorithm favors short jobs (or process) at the expense
of longer ones.
⮚ The obvious problem with SJF scheme is that it requires precise
knowledge of how long a job or process will run, and this information
is not usually available.
⮚ The best SJF algorithm can do is to rely on user estimates of run
times.

11. SJF Illustrated
A B Time
C
turnaround time of A
turnaround time of B
turnaround time of C
Shortest Job First
response time of A =
0
response time of
B
response time of
C
wait time of A =
0
wait time of
B
wait time of
C
* 11

12. Priority Scheduling (Multilevel Queues)
■ Priority scheduling: The process with the
highest priority runs first
■ Priority 0:
■ Priority 1:
■ Priority 2:
■ Assume that low numbers represent high priority
A
B
C
A B Ti
m
e
C
Priority
Scheduling
*
12

13. Multilevel Feedback Queues
• Multilevel feedback queues use multiple
queues with different priorities
– Round robin at each priority level
– Run highest priority jobs first
– Once those finish, run next highest priority, etc
– Jobs start in the highest priority queue
– If time slice expires, drop the job by one level
– If time slice does not expire, push the job up by one
level
* 13
Instructor: Ahmad S. Al-Majouk

14. Lottery Scheduling
■ Lottery scheduling is an adaptive scheduling
approach to address the fairness problem
❑ Each process owns some tickets
❑ On each time slice, a ticket is randomly picked
❑ On average, the allocated CPU time is proportional to
the number of tickets given to each job
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15. Lottery Scheduling (Cont.)
■ To approximate SJF, short jobs get more tickets
■ To avoid starvation, each job gets at least one
ticket
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16. Lottery Scheduling Example
• short jobs: 10 tickets each
• long jobs: 1 ticket each
# short jobs/#
long jobs
% of CPU for
each short job
% of CPU for
each long job
1/1 91% 9%
0/2 0% 50%
2/0 50% 0%
10/1 10% 1%
1/10 50% 5%
* 16
Instructor: Ahmad S. AL-Majouk