Transcript OperatingSystems

Operating System Concepts and Techniques
Lecture 2
Multiprogramming, Multithreading,
Multiprocessing, and Multitasking
M. Naghibzadeh
Reference
M. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011.
To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com
Definitions
Program: a set of instructions which is
prepared to perform a specific
assignment if executed by a computer.
Program need not be connected; it
could be stored on a flash memory and
placed in one’s pocket.
Program is not an active entity. It is
completely passive.
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Definitions…
Process: is created by operating system from
a program

To do so, it has to perform many activities, like
assigning a name, allocating space, (partially)
loading the corresponding program, etc.
Roughly speaking, A process is an active
program
A process is created to run a program by
using computer facilities such as CPU; like a
human who is born to live his life
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Single programming
Single-programming: there exist at the most one user
process at any given time in the computer; thus there is
usually one ongoing activity at a time



That is, from many devices within the computer often one device
is active at any given time
If a process has asked for a data from the user, the system has
to wait until this data is entered before being able to proceed
If this data entry takes one second the CPU could have done
millions of instructions if it did not have to wait
With single-programming the computer resources are not
used efficiently
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Process States in Single-Programming
With single-programming, right after a process is born the
system starts running it
It continues until the process has to wait for data, output results,
or wait for an event
There are special purpose processors called Input/Output (I/O)
processors for transferring data from input devices to main
memory and from main memory to output devices

It is understandable that such a processor will perform the specific
task better than a general-purpose processor, i.e., CPU
While an I/O operation is in progress, the CPU has to wait and
do nothing. After the I/O operation is completed, the CPU will
resume the execution of the program
This cycle, of going through process states of running and
input/output, may be repeated over and over, until the job is
completed or, for some reason, the process is aborted
The life cycle of a process in a single-programming environment
is shown in Figure 1
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Process’s life cycle
Process
birth
Blocked for
I/O
Running
Input/Output
I/O completed
Process
Termination
Figure 1: The life cycle of a process in single-programming environments
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Processor wait ratio
If the average execution time of a program with
single-programming is e and the average I/O
time is b, then the following ratio is the CPU
wait fraction (w). It is actually the fraction of
the time the CPU is idle. b
w
eb
For example, if execution time of programs is
10, of which 9 seconds is spent on I/O, then w
= 9/10 = 0.9. This means, on the average, 90%
of the CPU time is wasted.
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The Multiprogramming Concept
Multiprogramming: is a technique that allows more
than one program to be ready for execution (process)
and provides the ability to switch from one process to
another, even if the former is not completed

Of course, sometimes in the future we will have to switch
back to the first process and resume (not restart) its
computation
This technique works for both single-processor (like
some personal computers) and multiprocessor (such
as large main frame) computers.
Multiprogramming is mainly accomplished by the
operating system. The hardware provides some
specific circuitry that may be used by the operating
system in the course of facilitating multiprogramming
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Multiprogramming and PCs
Do we need multiprogramming for PCs? Yes. All PC
users like to run many applications simultaneously

Nobody runs for example an Internet explorer looking for an
information while staring at the monitor for the results for a
long time.
In this era of computer usage, every general-purpose
operating system must have the following capabilities:

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It must provide an environment to run processes in a
multiprogramming fashion.
It must act as a service provider for all common services that
are usually needed by computer users, such as copying files,
making new folders, compressing information, sending and
receiving messages from/to other computers in the network,
etc.
Its user interface must be easy to use and pleasant to work
with.
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Multiprogramming productivity
Multiprogramming increases productivity

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If CPU wait time is represented by w in singleprogramming environment, the CPU wait time
decreases to approximately wn
for a system
running n processes simultaneously.
Example: If w = .9 then wn =0.59 ; meaning that if
we have five processes running simultaneously, the
CPU utilization is increased by (0.41-.10)/0.1*100 =
310%.
By increasing the CPU utilization other device’s
utilization is also increased.
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Process State Transition Diagram
The life cycle of a process
programming has three states



in
multi-
Ready: ready to use CPU, however CPU is busy
Running: Uses CPU
Wait/Blocked
 Wait: process is waiting for a device or an event
 Blocked: process is waiting for its I/O to be completed by
an I/O processor
May switch between states many times during
its life
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Process’s life cycle
Process
Termination
Running
A process is
picked to run
Preempted for the
interest of others
Needs I/O or
circumstance
Process birth
Wait/Blocked
Ready
Running obstacle is
vanished
Figure 2: Basic process state transition diagram in multiprogramming
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Requirements of Multiprogramming
Process Switching possibility: the system must be able
to safely switch from one process to another and be
able to return in the future
 This is called context switching.
Direct Memory Access: I/O processors must be able
to directly access main memory without interference
and conflictions
The Interrupt System: I/O processors and monitoring
devices must be able to safely communicate with the
CPU to report or to get new assignments
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To context switch
Save enough information from current process
to be able to return to it and continue
Save all temporary storage information such
as registers and flags
Load/initialize the information of new process
Change PC
Need some help from Hardware to make the
switching process fast
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Interrupt
Interrupt: a signal that is sent to the CPU to
capture its attention. It may be issued by
different sources such as:

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By a fault detecting hardware that is trying to
informs the CPU of a circuitry malfunction
By a monitoring circuit within the ALU that controls
the acceptability of the data size for the operation
being performed and, consequently, the operation
result size
By a user that may press a special key like “break”
at any time
By an slave processor that wants to inform the
CPU of the completion of the assigned task
By a timer etc.
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Interrupt masking
Some interrupts are maskable by user
programs
Masked interrupts are ignored by the CPU

This is analogous to ignoring a door knock
Some interrupts are not user-maskable

It is not reasonable to let the computer continue
in the presence of a hardware failure.
However, all interrupts are maskable within
the kernel of an operating system.
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Interrupt Categorization
Interrupts are categorized into few classes
Different classes of interrupts require different
handling consideration
Each category of interrupts is represented by a
bit in interrupt vector within the CPU
Each interrupt category has a priority
When handling a higher priority interrupt
handling any lower priority interrupt is
postponed
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Program execution suspension
A non-masked interrupt causes the suspension of the
currently executing program.
Usually possible to resume the execution of the program
after the interrupt is handled.
Interrupt signals reach the CPU and are stored in the
interrupt vector. The interrupt(s) is ignored until the
currently executing machine instruction is completed.
There are very few exceptions. One example might be the
“move” instruction in some computers that is suppose to
move a big chunk, say 256 bytes, of data from one
location of main memory to another.
The CPU always looks for interrupt signals just before
fetching a new instruction to execute. If there are any
interrupt signals, the system handles them in the order of
priorities and urgencies.
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Fetch-Execute Cycle with interrupt
Fetch cycle
Handle all non-masked interrupts
Read the instruction pointed out
by the Program Counter (PC)
register from main memory and
move it to the CPU
Find out what this instruction is
Execute cycle
Move the data upon which the
instruction has to be executed from
main memory to the CPU.
Execute the instruction, i.e.,
perform what is requested by the
instruction. Perhaps this may
readjust the PC.
Adjust PC for the next instruction
Figure 4: A simplified functional model of a computer with interrupt
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Privileged instructions
Privileged instructions are those which are not
usable by assembly application programmers
Privileged instructions are executable only
within the operating system kernel
Other parts of the OS and application
programmers
may
execute
privileged
instruction by special means that are provided
by the operating system
Disable interrupt which disables all interrupts
is a sample privileged instruction
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Multiprocessing
If you think clearly, you will notice that we
should have used multiprocessing instead of
multiprogramming. This is true. Unfortunately,
the term “multiprogramming” is recognized
for this technique of the operating system and
we will stick to it
On the other hand, “multiprocessing” is used
for systems with more than one processor.
Processors, in such a system, can collectively
run many tasks simultaneously
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Multitasking
Computer users like to have many application programs simultaneously
operational. This is necessary because some application programs
require long processing times before the desired results can be
produced.
It is true that by having more than one application program operational,
the time that it takes for each process to complete its task increases.
However, the overall system productivity
These simultaneously executing programs are called tasks. Therefore,
a system with the capability of multitasking allows users to activate
more than one task, or application program, at a time. An Internet
browser that searches for some information and A word-processing
software that is activated to perform the word-processing task are
applications.
The operating system will switch between tasks based on the tasks
current states and their requirements and priorities.
Multitasking is only possible when multiprogramming is the
fundamental capability of simultaneously executing pieces of software.
Most modern operating systems, like UNIX, Linux, and Windows,
support multitasking.
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Multithreading
A multithreading operating system is one that is capable of
handling processes and threads at the same time and in which
from each process the system is able to generate more than one
thread.
In such an operating system, there must be facilities for thread
creation, deletion, switching, etc.
Such an operating system allows users to generate more than
one request to a process at a time. For example, a browser can
be made to search simultaneously for more than one topic, even
though there is only one copy of the “browser program” in main
memory.
The multiprogramming methodology and technique are essential
in the implementation of multithreading. In this new
environment, a thread becomes the smallest functional object to
which CPU (or a PU) is assigned.
Details of thread methodology and technique is discussed in
upcoming lectures.
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Summary
The ultimate goal in the design and implementation of an operating
system is to produce a handy software program that manages computer
resources in the most efficient way so as to serve computer users
correctly, reliably and fairly.
This is not achievable in single-programming environments.
Modern operating systems are built with the capabilities of
multiprogramming, multitasking, and multithreading.
Providing these capabilities requires many hardware and software
methodologies and techniques.
A good understanding of process creation, life cycle, and termination,
along with its state transition conditions is most essential in elucidating
the needs of different mechanisms within the operating system.
Some of these mechanisms, namely process switching, interrupt system
and handling, and direct memory access, are briefly explained in this
chapter.
Multithreading, as an offspring of multiprogramming, has become an
essential part of all modern operating systems.
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Find out
How many interrupt classes your computer’s
processor has
Some privileged instructions and the reasons
for being privileged
How are interrupts actually masked
The exact differences of multiprogramming
and multiprocessing
What an ultra DMA is
The exact differences between program and
process
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Any questions?
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