Ch10: Job and CPU Scheduling
 Introduction
 Scheduling Levels
 Scheduling Objectives
 Scheduling Criteria
 Preemptive vs. Non-preemptive Scheduling
 Priorities
 Deadlock Scheduling
 Processor Scheduling
 FIFO
 RR
 SJF
 SRT
Introduction
What is schedule?
A program of events or appointments expected in a given time.
What is Scheduling:
Setting an order and time for planned events.
Processor scheduling:
Is the problem of determining when processors should be assigned, and to
which processes.
Scheduling Levels
 High-level Scheduling/ job Scheduling/ Admission Scheduling: it
determines which jobs shall be allowed to compete actively for the
resources of the system.
 Intermediate-level Scheduling: It determines which processes shall be
allowed to compete for the CPU. Ex: suspend & resume.
 Low-level Scheduling (Dispatching): It determines which ready
processes will be assigned the CPU when it next becomes available,
and actually assigns the CPU to this process. It dispatches the CPU to
the process.
Scheduling objectives
A scheduling discipline should:
 Be fair.
 Maximize throughput.
 Maximize # of interactive users receiving acceptable response times.
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 Minimize overhead:
Overhead is commonly viewed as wasted resource.
 Balance resource use.
 Avoid indefinite postponement.
This is
best accomplished by aging, as a process waits for a resource, its
priority should grow.
 Enforce priorities.
 Give preference to processes holding key resources.
 Give better service to processes exhibiting desirable behavior.
Ex: low paging rates.
 Degrade gracefully under heavy loads.
Scheduling Criteria
To realize the scheduling objectives, a scheduling mechanism should
consider:
 CPU utilization: we want to keep the CPU busy as much as possible.
 Throughput: # of processes completed per unit of time.
 Turnaround time: the interval from the time of submission to the time
of completion.
 Waiting time: the amount of time a process spends waiting in the
ready queue.
 Response time: the time from submission of a request until the first
response.
Preemtive Vs. Non-preeptive Scheduling
Preemptive Vs. Non-peermptive
The CPU can be
taken a way.
Once a process has been given
the CPU, the CPU can’t be taken a
way from that process.
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Priorities
 Static Vs. Dynamic
Static priority: don’t change.
Dynamic priority: responsive to change.
 Purchased Priorities
A user with a rush job may be willing to pay a premium for a higher
level of service.
Deadline Scheduling
Certain jobs are scheduled to be completed by a specific time or
deadline.
Processor Scheduling Algorithms
 First-In-First-Out Scheduling (FIFO)
Processors are dispatched to their arrival time on the ready queue.
Sometimes it is called First-Come-First-Served (FCFS)
FIFO Characteristics
 Simple.
 Non-preemptive, once a process has the CPU, it runs to
completion
 Fair in general, but short jobs must wait,
important jobs also
wait.
 More predictable, small variance in response time.
 Not useful in scheduling interactive users.
 Usually it is embedded in other schemes.
Round Robin Scheduling (RR)
 Processes are dispatched FIFO, but are given a limited amount of
CPU time slice (quantum).
Time slice: A time interval during which a time-sharing system is processing one
particular computer program. Also known as time quantum.
 If a process does not complete before its CPU time expires, the CPU
is preempted and given to the next waiting process.
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Ready list
A
C
B
CPU
A
Completion
Preemption
Quantum Size
 Should the quantum be large or small?
When very large FIFO
When very small Context switching overhead
 Consider the optimal value of the quantum that yields good response
time.
How?
It should be large enough, so that the vast majority of interactive requests
require less time than the duration of the quantum.
Shortest-Job-First Scheduling (SJF)
 It is a non preemptive scheduling discipline in which the waiting job
with the smallest estimated run-time-to-completion is run next.
 Reduces average waiting time over FIFO.
 Unpredictable, waiting time has a large variance.
 How to estimate how long a job will run?
Users may mislead you.
Shortest-Remaining-Time Scheduling (SRT)
 It is a preemptive counter part of SJF and is useful in time sharing.
 The process with the smallest estimated run-time-to-completion is run
next, including new arrivals.
 its quantum before completion.
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q= 8
q= 16
FCFS
Ex: FCFS
Process
Arrival time
P1
Burst time/ milliseconds(cpu
execution time)
24
P2
3
0
P3
3
0
0
Gantt Chart
p1
p2
0
24
p3
27 30
The average waiting time is
(WT(P1) + WT(P2) + WT(P3))/3
= (0+24+27)/3
= 51/3
= 17 milliseconds
Suppose the processes arrive in the order P2,P3,P1
p2
0
p3
3
p1
6
The average waiting time is now
Ex: RR
30
(6+0+3)/3=3 milliseconds
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q= 4
Process
Burst time
24
P1
3
P2
3
P3
Gantt Chart
P1
0
P2
4
P3
7
P1
10
P1
14
P1
18
P1
22
P1
26
30
The average waiting time is : (WT(P1) + WT(P2) + WT(P3))/3
= (6+4+7)/3
= 17/3 = 5.66
Ex: SJF
Process
Burst time
Arrival time
P1
6
0
P2
8
0
P3
7
0
P4
3
0
Gantt Chart
P4
P1
P3
P2
0
3
9
16
24
The average waiting time is : (WT(P1) + WT(P2) + WT(P3) + WT(P4))/4
= (3+16+9+0)/4
=7
Ex: SRT
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Process
Arrival time
Burst time
P1
0
8
P2
1
4
P3
2
9
P4
3
5
Gantt Chart
P1
0
P2
1
P4
5
P1
10
 The average waiting time is
P1 P2
P3
P4
(10-1)+(1-1)+(17-2)+(5-3)/4
=26/4 =6.5
 A non-preemptive SJF will result 7.75
 Turn average turnaround time is
P1 P2 P3 P4
(17+4+24+7)/4
= 13
 Average response time is
(0+0+15+2)/4 = 4.25
Priority
Process
Burst time
Priority
P1
10
3
P2
1
1
P3
2
3
P4
1
4
P5
5
2
Gantt Chart
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P3
17
26
P2
P5
P1
0
1
6
The average waiting time is 8.2 milliseconds
P3
16
P4
18
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Disk Management
 Operation of Moving-Head Disk Storage.
 Why scheduling is necessary?
 Desirable Characteristics of scheduling polices.
 Basic Disk Scheduling Policies.
Operation of Moving-Head Disk Storage
Cylinder
(Track)
Read-Write
head
Sectors
Spindle
A platter or magnetic disk
A disk access consists of three significant times:
1. Seek time: the time it takes to move the head to a specific cylinder.
2. Latency time (rotational time): the time it takes to rotate the disk from
its current position to a position adjacent to the read-write head.
3. Transmission time: the time that needs the head to go over the data to
read or write.
Why scheduling is necessary?
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 The generated requests by processes in a multiprogramming system is
much faster then the ability of the head to service them.
 Disk scheduling is the process of minimizing time spent seeking
records.
Desirable Characteristics of scheduling polices
 Throughput
 Mean response time
 Variance in response time (Predictability)
Basic Disk Scheduling Policies
 FCFS no ordering on the queue.
 SSTF (Shortest-Seek-Time-First)
Disk arm is positioned next at the request that minimizes arm movement.
 SCAN
Operates like SSTF except that it chooses the request that results in the
shortest seek distance in a preferred direction.
 N-stop Scan
Disk arm sweeps back and forth as in scan, but all requests that arrive
during a sweep in one direction are batched and reordered for optimum
service during the return sweep.
 C-SCAN (Circular Scan)
Disk arm moves unidirectional across the disk surface toward the inner
track. When there are no more requests for service ahead of the arm, it
jumps back to service the request nearest the outer track and proceeds
inward again.
 Eshenbach scheme
Disk arm movement is circular as in C-SCAN, but with several important
exceptions. Every cylinder is serviced for exactly one full track of
information whether or not there is a request for that cylinder. Requests are
reordered for service within a cylinder to take advantage of rotational
position.
Example
 Consider for example, an ordered disk queue with requests
involving tracks 98, 183, 37, 122, 14, 124, 65, and 67
start at track 53.
FCFS Disk Scheduling
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Queue= 98, 183, 37, 122, 14, 124, 65, 67
head starts at 53
0
14
37
53
65 67
98
122 124
183
199
122 124
183
199
SSTF Disk Scheduling
Queue= 98, 183, 37, 122, 14, 124, 65, 67
head starts at 53
0
14
37
53
65 67
98
SCAN Disk Scheduling
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Queue= 8, 183, 37, 122, 14, 124, 65, 67
head starts at 53
0
14
37
53
65 67
98
122 124
183
199
122 124
183
199
C-SCAN Disk Scheduling
Queue= 8, 183, 37, 122, 14, 124, 65, 67
head starts at 53
0
14
37
53
65 67
98
C-LOOK Disk Scheduling
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Queue= 8, 183, 37, 122, 14, 124, 65, 67
head starts at 53
0
14
37
53
65 67
98
122 124
183
199
Scheduling Criteria
 CPU utilization – keep the CPU as busy as possible
 Throughput – number of processes that complete their execution per time unit
 Turnaround time – the duration between job submission time and job completion
time.
 Waiting time – amount of time a process has been waiting in the ready queue
 Response time – amount of time it takes from when a request was submitted until
the first response is produced, not output (for timesharing environment)
 Deadlines – cyclic RT-tasks must finish each cycle before its deadline
Optimization Criteria
 Maximize CPU utilization
 Maximize throughput
 Minimize turnaround time
 Minimize waiting time
 Minimize response time
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