1. Uniprocessor Process Management & Process Scheduling
CS 314 Operating Systems
Uniprocessor Process Management &
Process Scheduling
Department of Computer Science
Southern Illinois University Edwardsville
Fall, 2018
Dr. Hiroshi Fujinoki
Process_Management/000

2. CS 314 Operating Systems Three Different Layers of Process Scheduling
 Long-term Scheduling
 Medium-term Scheduling
 Short-term Scheduling
The scheduler invoked when a new process is started
by a human user or by another process already active
The scheduler that assigns a (the) processor to the processes
ready to run in the memory
The scheduler that is performed to the ready processes to temporarily
swap them out of the memory (to make them temporarily inactive)
Process_Management/001

3. CS 314 Operating Systems Long-Term Scheduling Your program must be in
memory for the processor
to execute it
Computer System
Processor
Multitasking requires
multiple processes
ready (waiting) in the
memory
User(s)
Main Memory
Two reasons your request to start a new process can be rejected:
 Your system does not have enough memory
to hold more process (not enough memory)
Process_Management/002

4. CS 314 Operating Systems Long-Term Scheduling Your program must be in
memory for the processor
to execute it
Computer System
Processor
Long-term
Schedule Queue
Multitasking requires
multiple processes
ready (waiting) in the
memory
User(s)
Main Memory
Two reasons your request to start a new process can be rejected:
 Your system does not have enough memory
to hold more process (not enough memory)
Process_Management/003

5. CS 314 Operating Systems Long-Term Scheduling Computer System
Processor
User(s)
Process
Completed
Main Memory
Two reasons your request to start a new process can be rejected:
 Your system does not have enough memory
to hold more process
Process_Management/004

6. CS 314 Operating Systems Long-Term Scheduling Computer System
Processor
User(s)
Main Memory
Two reasons your request to start a new process can be rejected:
 Degree of multitasking exceeds its limit and thrashing
is happening (not enough processor resource)
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7. CS 314 Operating Systems Long-Term Scheduling Computer System
Processor
User(s)
Main Memory
Process_Management/006

8. CS 314 Operating Systems Degree of Multitasking and Thrashing n
Process 1 7 10 5 n
Processor is used for
n seconds m
Processor is NOT used
for m seconds
Process 2 5 2 4 3
Process 3 10
This is “short-term scheduling”
Number of processes ready in the memory
Process 1 7 10 5
Utilization = 17/32 = 53.1% 5 1 1 2 2 3 3
Utilization = 29/34 = 85.2%
Process 2 2 3 5 2 3
Process 3
Utilization = 42/44 = 95.5%
Process 4
= “Degree of multitasking”
Process_Management/007

9. CS 314 Operating Systems  Multiprogramming and Timesharing OS
Multiprogramming & Timesharing
User Programs A
Computer System B
CPU C D
Program Loader E D C B A
user
Memory
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10. CS 314 Operating Systems Degree of Multitasking Context Switching
Service time
(Time Slice)
All registers in the
processor must be
saved
user
Process_Management/009

11. CS 314 Operating Systems Degree of Multitasking 
If we try to keep the response time
for each user same, the service time
(time slice) for each user must shrink
user
Overhead for Context Switching
user
user
user
user
user
Process_Management/010

12. CS 314 Operating Systems Degree of Multitasking
Most of the processor cycles are used
For just switching between processes
but not for processing them
Degree of Multitasking
(High)
Throughput
Long-term scheduling controls
degree of multitasking so that
the slowdown will not happen
Slowdown
Process_Management/011

13. CS 314 Operating Systems Three Different Layers of Process Scheduling
New
user
Ready
System has enough
resources
Blocked
System does not have
enough resources
Processor
Running
Selected for
execution
System has enough
resources
Another process is
selected for execution
I/O access is
completed
Blocked
Process is making
I/O accesses
Completed
Process_Management/012

14. CS 314 Operating Systems Three Different Layers of Process Scheduling
Short-term Scheduling
Long-term Scheduling
New
user
Processor
Ready
Blocked
Running
Blocked
System does not have
enough resources
Completed
Medium-term Scheduling
Process_Management/013

15.   CS 314 Operating Systems Short-term Scheduling
Major existing short-term scheduling algorithms: 
 Preemptive
Non-Preemptive
 FCFS (a.k.a. “FIFO”)
 Round-Robin
 SJF (Shortest Job First) (a.k.a. “SPN”)
 HRRN (Highest Response Rate Next)
 SRTF (Shortest Remaining Time First)
 Feed-back 
Priority-based
(Highest Priority First)
Process_Management/014

16. CS 314 Operating Systems Short-term Scheduling
The scheduler schedules assignments of the processor to each process to
optimize some performance factor(s)
 Throughput
 Response time
 Resource utilization
(especially processor utilization)
 Fairness
(we can measure this as “customer satisfaction”)
 Turnaround time
Each short-term scheduling algorithm is good at optimizing
a performance factor, but it is hard to optimize all the factors
Process_Management/015

17. CS 314 Operating Systems Uniprocessor Process Management
Core of process management
Process scheduling
Process scheduling =
Decision-making for when to start (and stop) processes
Why is process scheduling important?
Because it has significant impact to the following performance factors:
 Throughput
 Response time
 Resource utilization
 Fairness
 Turnaround time
(especially processor utilization)
(we can measure this as “customer satisfaction”)
Process_Management/016

18. CS 314 Operating Systems Throughput Throughput
= Number of jobs completed in unit time
P14 9 P2 4 P8 P3 P9 P6 P4 P1 25 P5 15
P10
P13 P7
P12
P11
If all tasks
submitted
here
5 jobs / Minute
6 jobs / Minute
P14 9
P13 15
P12
P11
P10 P9 4 P8 P7 P6 P5 P4 P3 P2 P1 25 25 29 33 42 57 61
111
102 74 93 70 78
126
135
Process_Management/017

19. CS 314 Operating Systems Response Time Response Time Two questions:
=The time since submission until the first response appears
Two questions:
 How should “average response
time” be defined?
P14 9 P2 4 P8 P3 P9 P6 P4 P1 25 P5 15
P10
P13 P7
P12
P11
 How can we improve “average
response time (two methods)”?
If all tasks
submitted
here
P14 9
P13 15
P12
P11
P10 P9 4 P8 P7 P6 P5 P4 P3 P2 P1 25
111
102 74 61 33 25 29 42 93 57 70 78
126
P2 waits for 25 sec.
Process_Management/018

20. CS 314 Operating Systems Response Time Response Time
=The time since submission until the first response appears
If all tasks
submitted
here
P14 9
111
102 74 61 33 25 29 42 93 57 70 78
126
P13 15
P12
P11
P10 P9 4 P8 P7 P6 P5 P4 P3 P2 P1
Average Response Time
= 901/14 = 64.35 P1 25
P13 15
P10 P5
P14 9
P12
P11 P7 P4 P9 4 P8 P6 P3 P2 8 12 16 20 29 38 47 56 65 80 95
110
Average Response Time
= 580/14 = 41.28
Average Response Time
Optimized 
What did we loose ?
Process_Management/019

21. CS 314 Operating Systems Resource Utilization (processor utilization)
Processor is used for
n seconds m
Processor is NOT used
for m seconds
Process 1 7 10 5
Process 2 12 2 3 6
Process 3 10 2 3 7 10 5
Process 1 12 2 3 6 10 2 3
Process 3
Process 2 73 51
Utilization = 51/73 = 69.9%
Process_Management/020

22. CS 314 Operating Systems Turnaround Time Turnaround Time
= The time since submission to completion for each process
P14 9 P2 4 P8 P3 P9 P6 P4 P1 25 P5 15
P10
P13 P7
P12
P11
If all tasks
submitted
here P1 25 P2 4 P3 4 P4 9 P5 15 P6 4 P7 9 P8 4 P9 4
P10 15
P11 9
P12 9
P13 15
P14 9
111
102 74 61 33 25 29 42 93 57 70 78
126
135
Process_Management/021

23. CS 314 Operating Systems Fairness
Reasonable resource allocation to each task (or process)
Meaning of “fairness” depends on each system
 If some tasks are high-priority tasks
Assigning exactly the same amount of resource to all tasks
Will result in unfairness
 If all the tasks have exactly the same priority
Each task should be assigned the same amount of resource
 If some tasks have lower priority
Assigning more resource to high-priority tasks
Will result in starvation for low-priority tasks
Process_Management/022