Operating Understanding Systems, Fifth Edition
Chapter 4
Processor Management
At a Glance
Instructors Manual Table of Contents
Overview
Objectives
Teaching Tips
Quick Quizzes
Class Discussion Topics
Additional Projects
Additional Resources
Key Terms
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Lecture Notes
Overview
This chapter explains how the Processor Manager manages and allocates a single central
processing unit (CPU) to execute the incoming jobs. The chapter begins by presenting an
overview of single-user systems and providing a foundation of terms and concepts used in
processor management. Multi-core technologies are then briefly reviewed, followed by a
discussion and comparison of job scheduling and process scheduling. The role of the
Process Scheduler is explained in more detail, including a presentation of job and process
status concepts, the process control block (PCB) data structure, and queuing. Process
scheduling policies are presented next with an explanation of the advantages and
disadvantages of preemptive versus nonpreemptive process scheduling algorithms. The
goals of process scheduling policies are included. Six common process scheduling
algorithms are explained in detail, followed by a description of four cases exemplifying
how jobs move through the system. Finally, the chapter discusses the role of internal
interrupts and the tasks performed by the interrupt handler.
Learning Objectives
After completing this chapter, the student should be able to describe:
The difference between job scheduling and process scheduling, and how they relate
The advantages and disadvantages of process scheduling algorithms that are preemptive
versus those that are nonpreemptive
The goals of process scheduling policies in single-core CPUs
Up to six different process scheduling algorithms
The role of internal interrupts and the tasks performed by the interrupt handler
Teaching Tips
Overview
1. Provide students with an introduction to processor management in single-user and
multi-user (multiprogramming) systems. Explain the two states in a single-user system
(busy and idle). Discuss the role of the CPU for both types of systems.
2. Make sure students understand the difference between a job and a process. Provide
explanations of the terms program or job, process, thread, processor, interrupt, higher
priority, and context switch using the examples on pages 110-111 in the text. These
terms and concepts provide a foundation of understanding for the remaining chapter
contents.
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About Multi-Core Technologies
1. Note that the term processor and core are considered equivalent and the processor(s) is
located on a single silicon chip. Provide students with an introduction to multi-core
technologies. Define multi-core (two or more processors) and explain what dual-core
and quad-core mean.
2. Explain how multi-core engineering surfaced due to issues with nano-sized transistors
and their ultra-close-placement on a computer chip. Be sure students understand that
multi-core engineering helps resolve the current leakage and heat problems, in addition
to allowing for the opportunity to permit multiple calculations simultaneously.
3. Note that for the Processor Manager, multi-core processing is more complex to manage
than single-core processing. Multi-core processing will be covered in more detail in
Chapter 6. This chapter focuses on single-core processing.
Teaching
Tip
For a brief article providing an introduction to the multi-core concept and a
discussion on factors to consider when determining how to take advantage of
multi-core technology, refer your students to the following article:
www.dell.com/downloads/global/power/ps2q05-20050103-Fruehe.pdf
Job Scheduling Versus Process Scheduling
1. Outline the two Processor Manager submanagers: the Job Scheduler and the Process
Scheduler. Note the hierarchy between them and provide a brief overview of each.
2. Point out that the Job Scheduler initiates each job based on certain criteria.
3. Outline the most important functions of the Job Scheduler.
4. Be sure to mention the goals of the Job Scheduler in terms of striving for efficient use
of system resources and keeping most components of the computer system busy most of
the time.
Process Scheduler
1. Point out that once the Job Scheduler has placed a job on the READY queue, the
Process Scheduler takes over. The Process Scheduler is the lower-level scheduler in the
submanager hierarchy. Outline important functions of the Process Scheduler, such as:
a. Determining which jobs will get the CPU, when, and for how long
b. Deciding when processing should be interrupted
c. Determining which queues the job should be moved to during its execution
d. Recognizing when a job has concluded and should be terminated
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2. Discuss common programming traits. Most programs alternate between CPU and I/O
cycles, and the frequency and CPU cycle often vary. Explain that these traits lead to
general tendencies of I/O-bound jobs and CPU-bound jobs in terms of CPU cycles and
I/O cycles.
3. Explain, using Figure 4.1 on page 114 of the text, that the total effect of all CPU cycles,
from both I/O-bound and CPU-bound jobs, approximates a Poisson distribution curve.
That is, a greater number of jobs request short CPU cycles (the frequency peaks close to
the low end of the CPU cycle axis), and fewer jobs request long CPU cycles.
4. Point out that a middle-level scheduler (third layer) is used in a highly interactive
environment. It handles system job overloading by removing active jobs from memory
to reduce the degree of multiprogramming and allows jobs to be completed faster. This
in turn results in faster job completion overall.
5. Note that in a single-user environment, there is no distinction between job and process
scheduling because only one job is active at a time. Therefore, it receives dedicated
system resources for the duration of the job.
Job and Process Status
1. Outline the different states that a job takes as it moves through the system (HOLD,
READY, WAITING, RUNNING, FINISHED). Note that these states are also referred
to as job status or process status.
2. Explain, using Figure 4.2 on page 114 of the text, how the job status changes when a
user submits a job to the system via batch or interactive mode.
3. Use the bullet points on page 115 in the text to describe every possible transition from
one job or process state to another that is initiated by either the Job Scheduler or the
Process Scheduler.
Process Control Blocks
1. Provide students with an overview of Process Control Blocks (PCB), which contain all
of the data about the job needed by the operating system to manage the processing of
the job. Job data includes the following: what it is; where it is going; how much of its
processing has been completed; where it is stored; and how much it has spent in using
resources.
2. Describe briefly the contents of the PCB, such as process identification, process status,
process state, and accounting.
3. Use Figure 4.3 on page 116 of the text as visual reinforcement of the PCB contents.
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PCBs and Queuing
1. Discuss the queuing process. Mention that queues use PCBs to track jobs. Be sure to
emphasize that the PCBs are linked to form queues, not the jobs. Queues must be
managed by process scheduling policies and algorithms.
2. Use Figure 4.4 on page 118 in the text to show queuing paths from HOLD to
FINISHED. Point out that the Job and Processor Schedulers release resources when the
job leaves the RUNNING state.
Process Scheduling Policies
1. Briefly describe the multiprogramming environment. In this case, there are more jobs to
be executed than could possibly run at one time. Outline the three limitations of a
system that the operating system must resolve before scheduling all jobs in a multiprogramming environment. As stated in the text on page 118, they are: (1) there are a
finite number of resources (such as disk drives, printers, and tape drives); (2) some
resources, once theyre allocated, cant be shared with another job (such as printers);
and (3) some resources require operator intervention; that is, they cant be reassigned
automatically from job to job (such as tape drives).
2. Discuss different criteria of a good process scheduling policy. A policy should
maximize throughput, minimize response time, minimize turnaround time, minimize
waiting time, maximize CPU efficiency, and ensure fairness for all jobs. Notice that
some of these criteria contradict each other. Point out that the final decision rests with
the system designer, who must determine which criteria are most important for that
specific system.
3. Introduce the concepts of a timing mechanism and interrupts. These items are used by a
processor in order to remove unacceptable imbalances in the READY and I/O queues.
4. Explain the preemptive scheduling policy and nonpreemptive scheduling policy. Point
out that a preemptive scheduling policy is a strategy that interrupts the processing of a
job and transfers the CPU to another job. It is widely used in time-sharing
environments. The nonpreemptive scheduling policy functions without external
interrupts.
Teaching
Tip
Refer to the following Web site to learn more about process scheduling policies:
http://docs.hp.com/en/5965-4642/ch01s13.html
Teaching
Tip
To learn more about the optimality of a scheduling policy for processing a job
stream, download the article from the following link:
http://portal.acm.org/citation.cfm?id=2080.357395
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Quick Quiz 1
1. Which of the following are the functions of a Process Scheduler? (Choose all that
apply.)
a. Initiates the job based on certain criteria
b. Decides when processing should be interrupted
c. Puts jobs in a sequence such that they use all system resources as fully as possible
d. Determines which queues the job should be moved to during its execution
Answer: b and d
2. The total effect of all CPU cycles, from both I/O-bound and CPU-bound jobs,
approximates which of the following distribution curves?
a. Gaussian distribution
b. Poisson distribution
c. Lorentzian Distribution
d. Random Distribution
Answer: b
3. The ____________________ contains all of the data about the job needed by the
operating system to manage the processing of the job.
Answer: Process Control Block
Process Scheduling Algorithms
1. Remind students that a Process Scheduler allocates the CPU and moves jobs through
the system. List the six different process scheduling algorithms. Point out that the early
systems used nonpreemptive policies to move batch jobs. Most current systems, with
their emphasis on interactive use and response time, use preemptive policies that take
care of the immediate requests of interactive users.
First-Come, First-Served
1. Explain the first-come, first-served (FCFS) algorithm. Point out that it is nonpreemptive
and handles jobs according to their arrival time. Outline FCFS advantages and
disadvantages. For example, it is simple to implement and good for batch systems;
however, turnaround time is unpredictable.
2. Use the examples shown in Figure 4.5 and Figure 4.6 on page 121 of the text to
demonstrate how this algorithm works.
Shortest Job Next
1. Explain the shortest job next (SJN) algorithm. Point out that it is nonpreemptive and
handles jobs based on the length of their CPU cycle time.
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2. Outline SJN advantages and disadvantages. For example, it is simple to implement in
batch systems; however, it does not work in interactive systems. Also, it is optimal only
when all jobs are available at the same time and the CPU estimates are available and
accurate.
3. Use the example shown in Figure 4.7 on page 122 of the text to demonstrate how this
algorithm works.
Priority Scheduling
1. Explain the priority scheduling algorithm. Point out that it is nonpreemptive and gives
preferential treatment to important jobs. That is, programs with the highest priority are
processed first and they are not interrupted until CPU cycles are completed or a natural
wait occurs.
2. Point out that a FCFS policy is used if two or more jobs have equal priority in the
READY queue.
3. Note that the system administrator or the process manager may use different methods
for assigning priorities.
4. Outline different criteria the Processor Manager uses to determine priorities, such as
memory requirements, number and type of peripheral devices, total CPU time, and
amount of time already spent in the system.
Shortest Remaining Time
1. Explain the shortest remaining time (SRT) algorithm. Point out that it is a preemptive
version of the SJN algorithm, and the processor is allocated to jobs that are closest to
completion.
2. Outline SRT advantages and disadvantages. For example, it ensures fast completion of
short jobs; however, it involves more overhead than SJN due to context switching. Also,
this algorithm cannot be implemented in an interactive system because it requires
advance knowledge of the CPU time required to finish each job.
3. Use the examples shown in Figure 4.8 and Figure 4.9 on page 125 to demonstrate how
this algorithm works. Also discuss the situation in which the turnaround time is greater
in SJN than in SRT. Note that a precise comparison of SRT and SJN would have to
include the time required to do context the switching in the SRT algorithm.
Round Robin
1. Explain the round robin algorithm. Point out that it is preemptive, used extensively in
interactive systems, and based on a predetermined slice of time called a time quantum.
Mention that this algorithm ensures that the CPU is equally shared among all active
processes so that the CPU is not monopolized by any one job.
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2. Discuss job processing within the round robin algorithm and explain the different
situations possible, such as when the jobs CPU cycle is shorter or longer than the time
quantum.
3. Use the example shown in Figure 4.10 on page 127 of the text to demonstrate how this
algorithm works.
4. Explain the effect of the size of the time quantum on efficiency, in relation to the
average CPU cycle. Use the example shown in Figure 4.11 on page 128 of the text to
show how this works.
5. Outline the two general rules of thumb for selecting the proper time quantum. First, it
should be long enough to allow 80 percent of CPU cycles to run to completion. Second,
it should be at least 100 times longer than the time required to perform one context
switch. Point out that these rules are flexible and depend on the particular system.
Multiple-Level Queues
1. Provide a brief overview of multiple-level queues. Point out that this concept works in
conjunction with several other schemes. It is found in systems where jobs can be
grouped according to a common characteristic.
2. Provide examples of systems that employ multiple-level queues. One example is that of
a priority-based system with different queues for each priority level. A second example
is when another kind of system might gather all of the CPU-bound jobs in one queue
and all I/O-bound jobs in another. The Process Scheduler then alternately selects jobs
from each queue to keep the system balanced. A third example is a hybrid environment
that supports both batch and interactive jobs.
3. Briefly mention the four primary methods to handle the movement of a job before
moving to a detailed discussion of each case.
Case 1: No Movement Between Queues
1. Discuss the pros and cons of this job movement policy. For example, it is quite simple
and rewards high-priority jobs. The processor is allocated to the jobs in the high-priority
queue using the first-come, first-serve algorithm. Low-priority jobs get the processor
only when the high-priority queues are empty.
2. Note that this policy can be justified if there are relatively few users with high-priority
jobs, so the top queues quickly empty out, allowing the processor to spend a fair amount
of time running the low-priority jobs.
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Case 2: Movement Between Queues
1. Discuss the pros and cons of this job movement policy between queues. For example,
the processor adjusts priorities assigned to each job. High-priority jobs are initially
given a favorable assignment; however, they are later adjusted to be like all other jobs.
When a time quantum interrupt occurs, the job is preempted and moved to the end of
the next lower queue. A job may also have its priority increased; for example, when it
issues an I/O request before its time quantum has expired.
2. Note that this policy is considered quite fair in a system in which the jobs are handled
according to their computing cycle characteristics: CPU-bound or I/O-bound.
Case 3: Variable Time Quantum Per Queue
1. Note that this case represents a variation of Case 2 above (the movement between
queues policy), and it allows for faster turnaround of CPU-bound jobs. Use the example
in the text on page 130, in which each of the queues is given a time quantum twice as
long as the previous queue. As discussed in the text, the highest queue might have a
time quantum of 100 milliseconds. Therefore, the second-highest queue would have a
time quantum of 200 milliseconds; the third would have 400 milliseconds, and so on. If
there are enough queues, the lowest one might have a relatively long time quantum of
three seconds or more.
2. The advantage of this policy is that a CPU-bound job executes longer and is provided
longer time periods in which to run. Thus, this policy improves a CPU-bound jobs
chance of finishing faster.
Case 4: Aging
1. Aging is used to ensure that jobs in the lower-level queues will eventually complete
their execution. The system keeps track of a jobs wait time, and if a job becomes too
old waiting for the CPU, the system moves the job to the next highest queue. This
movement continues until the old job reaches the top available queue. In some severe
cases, the job may move drastically to the highest queue for processing.
2. The advantage of this job movement policy is that it guards against the problem of
indefinite postponement of unwieldy jobs. As discussed in the text on page 131,
indefinite postponement means that a jobs execution is delayed for an undefined
amount of time because it is repeatedly preempted so other jobs can be processed.
Eventually the situation could lead to the old jobs starvation. Indefinite postponement
is a major problem. This problem of indefinite postponement is discussed further in
Chapter 5.
Teaching
Tip
Various articles related to process scheduling algorithms can be found by
entering process scheduling algorithm in the search toolbar at the following Web
site:
http://portal.acm.org
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A Word About Interrupts
1. Briefly discuss various types of interrupts, such as page interrupts to accommodate job
requests in memory management, time quantum expiration interrupts, I/O interrupts,
internal interrupts (synchronous interrupts), illegal arithmetic operations (attempts to
divide by zero), and illegal job instructions.
2. Briefly explain the interrupt handler, which is used to control the program that handles
the interruption sequence of events. Outline its functions and sequence of steps when
addressing a nonrecoverable error.
Teaching
Tip
More on the topic of context switching may be found at the following Web site:
www.linfo.org/context_switch.html
Quick Quiz 2
1. Which of the following are preemptive scheduling algorithms? (Choose all that apply.)
a. FCFS
b. SRT
c. SJN
d. Round robin
Answer: b and d
2. Which of the following algorithms employ the concept of a time quantum that is given
to each job to ensure that the CPU is equally shared among all active processes?
a. SRT
b. SJN
c. Round robin
d. Priority scheduling
Answer: c
3. The control program that handles the interruption sequence of events is called the
____________________.
Answer: interrupt handler
Class Discussion Topics
1. Have students discuss the pros and cons of the various process management algorithms
presented in this chapter. Given a choice, which algorithm would they prefer to
implement and why?
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2. Have students discuss the different criteria for designing a process scheduling policy.
Do they think that it is possible to optimize the design of a process scheduling policy to
suit any system? If so, how? If not, why? Ask them to provide some examples to
corroborate their ideas.
Additional Projects
1. Have students compile a list of the important characteristics of processors commonly
employed in todays personal computers.
2. Have students research how to write the algorithms for shortest job next (SJN) and
shortest remaining time (SRT) policies.
Additional Resources
1. Microsoft.com:
www.microsoft.com
2. Osdata.com:
www.osdata.com/kind/history.htm
3. Intel Multi-Core:
www.intel.com/multi-core/
4. AMD Quad-Core Resource Center:
http://multicore.amd.com/us-en/AMD-Multi-Core/Resources.aspx
5. Multi-core Enterprise Technology at Dell:
www1.euro.dell.com/content/topics/topic.aspx/emea/topics/products/pedge/en/
multi_core?c=uk&cs=ukbsdt1&l=en&s=bsd
6. Freescale's Multi-core Platform:
www.freescale.com/webapp/sps/site/overview.jsp?nodeId=0162468rH3bTdG25E4&tid
=NEV20070625MULTIC
Key Terms
Aging: a policy used to ensure that jobs that have been in the system for a long time in
the lower-level queues will eventually complete their execution.
Context switching: the acts of saving a jobs processing information in its PCB so the
job can be swapped out of memory, and of loading the processing information from the
PCB of another job into the appropriate registers so the CPU can process it. Context
switching occurs in all preemptive policies.
CPU-bound: a job that will perform a great deal of nonstop processing before issuing
an interrupt.
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First-come, first-served (FCFS): a nonpreemptive process scheduling policy (or
algorithm) that handles jobs according to their arrival time.
High-level scheduler: a synonym for the Job Scheduler.
I/O-bound: a job that requires a large number of input/output operations, resulting in
much free time for the CPU.
Indefinite postponement: signifies that a jobs execution is delayed indefinitely
because it is repeatedly preempted so other jobs can be processed.
Interrupt: a hardware signal that suspends execution of a program and activates the
execution of a special program known as the interrupt handler.
Interrupt handler: the program that controls what action should be taken by the
operating system when a sequence of events is interrupted.
Job Scheduler: the high-level scheduler of the Processor Manager that selects jobs
from a queue of incoming jobs based on each jobs characteristics.
Job status: the condition of a job as it moves through the system from the beginning to
the end of its execution.
Low-level scheduler: a synonym for the Process Scheduler.
Middle-level scheduler: a scheduler used by the Processor Manager to manage
processes that have been interrupted because they have exceeded their allocated CPU
time slice.
Multiple-level queues: a process scheduling scheme (used with other scheduling
algorithms) that groups jobs according to a common characteristic.
Multiprogramming: a technique that allows a single processor to process several
programs residing simultaneously in main memory and interleaving their execution by
overlapping I/O requests with CPU requests.
Natural wait: a common term used to identify an I/O request from a program in a
multiprogramming environment that would cause a process to wait naturally before
resuming execution.
Nonpreemptive scheduling policy: a job scheduling strategy that functions without
external interrupts so that once a job captures the processor and begins execution, it
remains in the running state uninterrupted until it issues an I/O request or its finished.
Preemptive scheduling policy: any process scheduling strategy that, based on
predetermined policies, interrupts the processing of a job and transfers the CPU to
another job. It is widely used in time-sharing environments.
Priority scheduling: a nonpreemptive process scheduling policy (or algorithm) that
allows for the execution of high-priority jobs before low-priority jobs.
Process: an instance of execution of a program that is identifiable and controllable by
the operating system.
Process Control Block (PCB): a data structure that contains information about the
current status and characteristics of a process.
Process Scheduler: the low-level scheduler of the Processor Manager that establishes
the order in which processes in the READY queue will be served by the CPU.
Process scheduling algorithm: an algorithm used by the Job Scheduler to allocate the
CPU and move jobs through the system.
Process scheduling policy: any policy used by the Processor Manager to select the
order in which incoming jobs will be executed.
Process status: information stored in the jobs PCB that indicates the current position
of the job and the resources responsible for that status.
Processor: (1) a synonym for the CPU, or (2) any component in a computing system
capable of performing a sequence of activities.
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Program: an interactive unit, such as a file stored on a disk.
Queue: a linked list of PCBs that indicates the order in which jobs or processes will be
serviced.
Response time: a measure of the efficiency of an interactive system that tracks the
speed with which the system will respond to a users command.
Round robin: a preemptive process scheduling policy (or algorithm) that allocates to
each job one unit of processing time per turn to ensure that the CPU is equally shared
among all active processes and isnt monopolized by any one job.
Shortest job next (SJN): a nonpreemptive process scheduling policy (or algorithm)
that selects the waiting job with the shortest CPU cycle time.
Shortest remaining time (SRT): a preemptive process scheduling policy (or
algorithm) similar to the SJN algorithm that allocates the processor to the job closest to
completion.
Task: (1) the term used to describe a process, or (2) the basic unit of concurrent
programming languages that defines a sequence of instructions that may be executed in
parallel with other similar units.
Thread: a portion of a program that can run independently of other portions.
Multithreaded applications programs can have several threads running at one time with
the same or different priorities.
Time quantum: a period of time assigned to a process for execution before it is
preempted.
Turnaround time: a measure of a systems efficiency that tracks the time required to
execute a job and return output to the user.