Operating Systems Lecture 3

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Operating SystemsLecture 3

• Multiprogramming, Multithreading,
  Multiprocessing, and Multitasking
• M. Naghibzadeh
• Reference M. Naghibzadeh, Operating System
  Concepts and Techniques, iUniverse publisher,
  2005.
• To order www.iUniverse.com, www.barnesandnoble.
  com, or www.amazon.com

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Definitions

• A program is a set of instructions which is
  prepared to perform a specific assignment if
  executed by a computer.
• A program need not be online it could be stored
  on a flash memory and placed in ones pocket.
• A program is not an active entity. It is
  completely passive.

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Definitions

• The operating system creates a process 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 like a human who is born to
  live his life.

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Interoduction

• In a single-programming environment there exist
  at the most one process at any given time 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.
• This means, if a process has asked for a data to
  be entered by 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 facilities
  are not used in an efficient manner.

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Process States in Single-Programming

• With single-programming, right after a process is
  born the system starts executing its
  corresponding programs instruction.
• The instruction execution continues until the
  process needs to read some data from an input
  device or wants to write some results on an
  output device.
• 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 instructions.
• 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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Processs life cycle

• Figure 1 The life cycle of processes 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.
• 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 our 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
• 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 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 system productivity.
  If CPU wait time is represented by w in
  single-programming environment, the CPU wait time
  decreases to approximately for a system
  running n processes simultaneously.
• Example If w .9 then 0.59 meaning
  that if we have five processes running
  simultaneously, the CPU utilization is increased
  by (0.41-.10)100 310.
• By increasing the CPU utilization other devices
  utilization is also increased.

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Process State Transition Diagram

• The life cycle of a process in multiprogramming
  is not the same as singleprogramming.
• A process may be ready to use the CPU to run its
  program while the CPU is running another program.
• The basic states are thus Ready, Running, and
  Wait/Blocked. Wait refers to a state in which the
  process is waiting for a device or an event and
  Blocked is for the case the process is waiting
  for its I/O to be completed by an I/O processor.

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Processs life cycle

• 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. 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.

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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 and, as a result, overall user gain
  increases.
• 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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Process Deficiencies

• A process is created to run a program to perform
  a duty.
• What if we need to perform two or more similar
  duties?
• One approach is to create more than one exact
  same processes each assigned to handle one of
  the two duties.
• This is a correct solution, but it spawns two
  major problems
• As the numbers of duties increase, the number of
  processes increases too, and very soon we will
  either run out of main memory or, in the case of
  virtual memory, we may reach an inefficient state
  of main memory.
• By increasing the number of processes, the number
  of objects that compete for computer resources
  increases, too. It will led to an undesirable
  state in which many processes cannot complete
  their duty because they do not get the chance to
  use the resources needed.
• Thread is introduced to solve these problems

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Thread

• Thread refers to a path through a programs
  instructions during its execution.
• Multithreading methodology allows more than one
  thread of execution for every process.
• Now, if we need to perform two or more similar
  duties we can crate one process and from which
  create more than one thread each assigned to
  handle one of the duties.
• All threads of a single process share the same
• address space
• global data
• files for storing and/or reading information
• resources that are assigned to their
  corresponding process.

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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

• UNIX Process states
• Windows thread states
• What is meant by the degree of multiprogramming
• Why we have a transition from the Running state
  to the Ready state in the state transition
  diagrams.
• How the state transition diagram is changed if
  the wait/blocked state is broken into two states.

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• Thank you!