Course Overview Principles of Operating Systems

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Course OverviewPrinciples of Operating Systems

• Introduction
• Computer System Structures
• Operating System Structures
• Processes
• Process Synchronization
• Deadlocks
• CPU Scheduling

• Memory Management
• Virtual Memory
• File Management
• Security
• Networking
• Distributed Systems
• Case Studies
• Conclusions

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Chapter Overview Processes

• Motivation
• Objectives
• Processes and Programs
• Process States
• Operations on Processes
• Operating System Control Structures
• Process Control Block

• Processes and Threads
• Process Scheduling
• Cooperating Processes
• Interprocess Communication
• Important Concepts and Terms
• Chapter Summary

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Motivation

• applications can only run through the use of
  processes
• processes are the fundamental unit of operation
  in an operating system
• most components of the operating systems provide
  services for the execution of processes
• processes require resources for their execution

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Objectives

• understand the use of processes for running
  programs
• know the requirements and mechanisms for the
  execution of processes
• understand the principles of cooperation between
  processes
• acquire the terminology related to processes and
  their execution

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Processes and Programs

• both terms are used to describe the activities
  performed by a computer
• program refers to the instructions as specified
  by the programmer
• process refers to the activities performed by the
  computer as the instructions of a program are
  executed

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Terminology

• program
• application
• job
• task
• process
• thread

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Program

• set of instructions specifying the activities
  necessary to accomplish a task
• frequently used in a broad sense
• static usually doesnt change unless modified by
  the programmer

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Application

• program used for a specific task
• often used for programs available to the user, in
  contrast to programs used internally by the OS

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Job

• used to describe the unit of work in a batch
  system
• frequently used synonymously to process

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Task

• unit of work from the users perspective in a
  time-sharing or multitasking system
• often corresponds to an application program
• may comprise several processes

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Process

• program in execution
• unit of work from the OS perspective, in
  particular with respect to resource ownership
• dynamic changes its state over time (during
  execution)
• may consist of several threads

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Thread

• smallest dispatchable unit in the OS
• several threads are usually grouped into a
  process, and can cooperate on a task
• sometimes also called lightweight processes

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Execution of Programs

• multiprogramming
• multiprocessing
• multitasking
• multithreading

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Example Processes

• consider three processes
• process A executes 100 instructions, reads two
  blocks from hard disk, and executes another 100
  instructions
• process B reads one block from hard disk,
  executes 100 instructions, and writes one block
  to disk
• process C executes 1000 instructions, and writes
  two blocks to disk
• timing
• 10 ns per CPU instruction (100 MHertz clock
  frequency)
• 10 ms average transfer time per block between
  hard disk and memory

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Example Single-Programming
1,000 ns
A
B
C
10,000 ns
1,000 ns
1,000 ns
1,000 ns
I/O
I/O
I/O
I/O
20 ,000 ,000 ns
10 ,000 ,000 ns
10 ,000 ,000 ns
20 ,000 ,000 ns

• overall execution time for single-programming20,0
  02,000 20,001,000 20,010,000 ns 60,013,000
  ns
• CPU is idle for 60,000,000 ns

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Multiprogramming

• several programs are simultaneously under
  execution
• they are between start and finish
• simultaneously refers to a human time-scale
  (seconds)
• at the CPU time scale, only one process is
  handled by the CPU (single processor systems)
• creates logical parallelism
• mainly used in batch systems to increase CPU
  utilization

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Example Multiprogramming
1,000 ns
1,000 ns
A

• logical parallelism all three programs are run
  simultaneously
• problem only one CPU available

B
C
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Example Multiprogramming
1,000 ns
1,000 ns
Attention Dimensions not to scale
10,000 ns
1,000 ns
1,000 ns
1,000 ns
I/O A
20 ,000 ,000 ns
10 ,000 ,000 ns
10 ,000 ,000 ns
20 ,000 ,000 ns

• solution
• multiplexing of the CPU by switching between
  processes
• I/O operations concurrently with CPU operations
• problem
• more complex
• requires process management

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Multiprocessing

• several processes run simultaneously on different
  CPUs
• creates physical parallelism
• advantages
• short overall execution time
• problems
• CPUs might have to sharememory, I/O devices
• low CPU utilization
• communication between processes
• process allocation and load balancing

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Example Multiprocessing
1,000 ns
1,000 ns

• physical parallelism all three programs are run
  simultaneously on three different CPUs

CPU 1
CPU 2
CPU 3
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Multitasking

• conceptually similar to multiprogramming better
  CPU utilization by switching between processes
• more frequent switching so that users can
  interact with the program
• necessary for time-sharing system

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Multithreading

• within one single process, multiple threads of
  execution are used
• independent activities within one program or
  application can be performed in parallel
• either on different CPUs, or via switching
  between threads
• decreases the overhead of switching between
  processes

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Process States

• over their existence, processes can be in
  different states
• newly created
• ready to run on the CPU
• running on the CPU
• blocked because it is waiting for an event or an
  I/O operation
• terminated

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Process State Diagram
new
terminated
admission
release
dispatch
ready
running
time-out
I/O or event wait
I/O or event completion
blocked
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Operations on Processes

• process creation
• new batch job, user login, OS service, child
  process
• process termination
• normal completion, time limit exceeded, resources
  unavailable, protection error, calculation error,
  invalid instruction, OS intervention, parent
  termination, parent request
• context switch
• execution of one process is stopped, and another
  process continues
• change of process state
• implicit via context switch, explicit by the OS

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Operating System Control Structures

• used to maintain information about important
  entities and activities in the computer system
• management of processes and resources
• information about the status of processes and
  devices
• usually stored in tables, possibly with pointers
  to further information
• cross-references must exist between different
  tables
• the OS must know the basic configuration of the
  computer system

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OS Control Structures
Process Image
Process Image
adapted from Stallings 98
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Memory Tables

• used to keep track of memory usage
• allocation of main memory to processes
• OS and user processes
• allocation of secondary memory to processes
• swap space or virtual memory
• protection attributes of memory segments
• access permissions

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I/O Tables

• used for the management of I/O devices
• allocation of devices to processes
• status of devices
• available, allocated to a process
• I/O operation in progress
• memory segment involved in the I/O operation
• in some operating systems, I/O devices are
  integrated into the file system

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File Tables

• used for the management of files and directories
• access information for files
• name, path
• location on secondary memory
• file status information
• open, closed
• processes using the file

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Process Tables

• used for the management of processes
• location of the process
• main memory, secondary storage
• process attributes
• process identification (pid)
• unique number, often used as index into the
  process table
• parent process, affiliated user, children

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Process Image

• user program
• program to be executed
• user data
• program data, user stack, modifiable parts of the
  program
• system stack
• parameters, procedure call and return addresses
• at least one per process
• process control block
• essential process data needed by the OS

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Process Control Block

• process identification
• processor state information
• process control information

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Process Identification

• process id, parent process, user id
• used by the operating system for all activities
  involving processes
• process management, main memory, I/O devices,
  interprocess communication
• cross-reference to between process and other OS
  tables

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Processor State Information

• user registers
• registers available to the user program
• system registers
• used for control and status information
• program counter, condition codes (e.g. division
  by zero, overflow), status register (interrupt
  enabled, system/user mode)
• stack pointers
• points to the top of system stacks
• these stacks contain parameters, procedure call
  and return addresses, and execution-related data

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Process Control Information

• scheduling and state information
• process state, priority, time in ready queue,
  etc.
• process relations
• links to other processes (waiting queues,
  parent/child)interprocess communications
• flags, signals, messages, shared memory
• process privileges
• memory access, instruction execution, resources
• memory management
• pointers to memory segments used by the process
• resources
• ownership and utilization of resources

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Processes and Address Spaces
Process 1
Process 2
Process n
Process Identification
Process Identification
Process Identification
Process Control Block
Process State Information
Process State Information
Process State Information
Process Control Information
Process Control Information
Process Control Information
System Stack
System Stack
System Stack
User Stack
User Stack
User Stack
User Address Space
User Address Space
User Address Space
Shared Address Space
Shared Address Space
adapted from Stallings 98
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Processes in Memory

• several processes need to be accomodated
• OS has its own memory section
• simplified view
• larger number of processes
• processes do not occupy one single section in
  memory, but several smaller ones (non-contiguous
  allocation)
• not the whole process image is always present in
  memory (virtual memory)

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Process Scheduling

• objective efficient allocation of CPU processing
  time to processes
• in uniprocessor systems multiplexing of the CPU
  between processes
• in multiprocessor systems allocation of
  processes to CPUs, load balancing across CPUs,
  multiplexing if there are more processes than
  processors
• very important in multiprogramming and
  multitasking
• maximization of CPU utilization
• interaction between programs and users

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Schedulers

• job scheduler
• long-term scheduling
• medium-term scheduler
• not used in all systems
• CPU scheduler
• short-term scheduling

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Job Scheduler

• manages processes that cant be executed
  immediately
• not enough memory available
• CPU load too high
• processes waiting for I/O are kept separately
• controls the degree of multiprogramming
• number of processes in main memory
• is not invoked too frequently
• mainly when processes enter or leave the system

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Medium-Term Scheduler

• processes are temporarily moved out of main
  memory to secondary storage (swapping)
• memory space restrictions
• to improve the process mix (balance between
  CPU-intensive and I/O-intensive processes)
• determines which processes to swap out and in

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CPU scheduler

• manages processes in the ready queue
• processes have all the resources they need,
  except for CPU time
• is invoked very frequently
• a process requests an I/O operation
• time slice of a process is over
• interrupt or trap
• OS intervention
• must be very fast to reduce overhead

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Scheduling Queues

• first-in, first-out (FIFO) data structures used
  to administer the scheduling of processes
• types of queues
• job queue newly created processes not yet ready
  for execution
• ready queue processes in main memory and ready
  for execution
• device queue processes waiting for a particular
  device
• event queue processes waiting for an event
• queuing diagrams display the interdependencies
• can be derived from the process state diagram

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Two-State Process Model
Queuing Diagram
release
dispatch
admission
CPU
pause
adapted from Stallings 98
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Five-State Process Model
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Queuing Diagram Five-State
admission
release
forward
dispatch
CPU
Ready Queue
time-out
I/O or event wait
I/O or event completion
. . .
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Context Switch

• the execution of one process on the CPU is
  halted, and the CPU continues with another
• information about the old process must be saved
• saved information about the new process must be
  restored
• highly dependent on hardware support
• multiple sets of registers
• special instructions, e.g. to load and store
  registers
• context switching time is pure overhead
• no productive work is done
• should be kept as low as possible

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Threads

• reduce the overhead of context switching
• only essential information about individual
  threads is saved for a thread switch
• program counter, register set, stack
• other information is shared by a group of threads
  within a process or task
• code section, data section, resources
• enable asynchronous and distributed processing
• support modular programs
• also sometimes referred to as lightweight
  processes

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Types of Threads

• user-level threads
• managed by the user process instead of the OS
• uses user-level libraries instead of system calls
• often more efficient since the OS is not involved
• can be difficult to program
• kernel-level threads
• scheduled by the OS
• threads may be distributed over several processors

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Multithreaded Process Model
Process
Process Control Block
adapted from Stallings 98
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Cooperating Processes

• processes existing simultaneously may be
  independent or cooperating processes
• independent not affected by the execution of
  other processes in the system
• processes are not completely independent since
  they have to share resources
• cooperating influences the execution of other
  processes
• information sharing
• computation speedup
• modularity

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Example Producer-Consumer

• a producer process produces data that is consumed
  by a consumer process
• a buffer is used to allow producer and consumer
  processes to run concurrently
• unbounded buffer unlimited size
• bounded buffer limited size
• explicitly coded (shared memory) or provided by
  the OS (interprocess communication)
• shared data are accessed through regular memory
  read and write operations

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Interprocess Communication

• mechanisms for processes to communicate and
  synchronize their activities
• does not rely on shared variables or shared
  memory
• communication links must be established between
  processes
• basic operations
• send(message)
• receive(message)

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Direct Communication

• processes must explicitly name the recipient or
  sender of the messages
• send(receiver, message)
• receive(sender, message)
• processes must know each others identity
• properties of direct links
• links are established automatically by the OS
• one link between a pair of processes
• a link is associated with exactly two processes

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Indirect Communication

• messages are sent to and received from mailboxes
• send(mailbox_1, message)
• receive(mailbox_1 , message)
• processes can communicate only if they share a
  mailbox
• properties of direct links
• links are established only if there is a common
  mailbox
• mailboxes are created by the OS
• there may be several links between a pair of
  processes
• a mailbox may be associated with several processes

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Important Concepts and Terms

• address space
• CPU scheduler
• computer system
• context switch
• graphical user interface
• hardware
• interprocess communication
• job
• kernel-level thread
• lightweight process
• link
• mailbox
• message
• message passing

• multiprocessing
• multiprogramming
• multitasking
• multithreading
• port
• process
• register
• resources, services
• scheduling
• shared memory
• stack
• task
• thread
• user-level thread

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Chapter Summary Processes

• a process is a program in execution
• it is the dynamic entity associated with a static
  program
• a process requires resources
• CPU time, main memory, I/O devices
• a process can be in one of several states
• new, ready, running, blocked, terminated
• process control block contains important
  information
• processes are scheduled for execution
• short, medium, long term scheduling
• processes can run concurrently
• independent or cooperating
• multiplexing in a single-processor system