Chapter 4: Processes

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Chapter 4 Processes

• Process Concept
• Process Scheduling
• Operations on Processes
• Cooperating Processes
• Interprocess Communication
• Communication in Client-Server Systems

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Process Concept

• An operating system executes a variety of
  programs
• Batch system jobs
• Time-shared systems user programs or tasks
• Textbook uses the terms job and process almost
  interchangeably.
• Process a program in execution process
  execution must progress in sequential fashion.
• A process includes
• program counter
• stack
• data section

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Process State

• As a process executes, it changes state
• new The process is being created.
• running Instructions are being executed.
• waiting The process is waiting for some event
  to occur.
• ready The process is waiting to be assigned to
  a process.
• terminated The process has finished execution.

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Diagram of Process State
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Process Control Block (PCB)

• Information associated with each process.
• Process state
• Program counter
• CPU registers
• CPU scheduling information
• Memory-management information
• Accounting information
• I/O status information

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Process Control Block (PCB)
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CPU Switch From Process to Process
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Process Scheduling Queues

• Job queue set of all processes in the system.
• Ready queue set of all processes residing in
  main memory, ready and waiting to execute.
• Device queues set of processes waiting for an
  I/O device.
• Process migration between the various queues.

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Ready Queue And Various I/O Device Queues
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Representation of Process Scheduling
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Schedulers

• Long-term scheduler (or job scheduler) selects
  which processes should be brought into the ready
  queue.
• Short-term scheduler (or CPU scheduler) selects
  which process should be executed next and
  allocates CPU.

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Addition of Medium Term Scheduling
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Schedulers (Cont.)

• Short-term scheduler is invoked very frequently
  (milliseconds) ? (must be fast).
• Long-term scheduler is invoked very infrequently
  (seconds, minutes) ? (may be slow).
• The long-term scheduler controls the degree of
  multiprogramming.
• Processes can be described as either
• I/O-bound process spends more time doing I/O
  than computations, many short CPU bursts.
• CPU-bound process spends more time doing
  computations few very long CPU bursts.

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Context Switch

• When CPU switches to another process, the system
  must save the state of the old process and load
  the saved state for the new process.
• Context-switch time is overhead the system does
  no useful work while switching.
• Time dependent on hardware support.

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Process Creation

• Parent process create children processes, which,
  in turn create other processes, forming a tree of
  processes.
• Resource sharing
• Parent and children share all resources.
• Children share subset of parents resources.
• Parent and child share no resources.
• Execution
• Parent and children execute concurrently.
• Parent waits until children terminate.

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Process Creation (Cont.)

• Address space
• Child duplicate of parent.
• Child has a program loaded into it.
• UNIX examples
• fork system call creates new process
• exec system call used after a fork to replace the
  process memory space with a new program.

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Processes Tree on a UNIX System
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Process Termination

• Process executes last statement and asks the
  operating system to decide it (exit).
• Output data from child to parent (via wait).
• Process resources are deallocated by operating
  system.
• Parent may terminate execution of children
  processes (abort).
• Child has exceeded allocated resources.
• Task assigned to child is no longer required.
• Parent is exiting.
• Operating system does not allow child to continue
  if its parent terminates.
• Cascading termination.

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Cooperating Processes

• Independent process cannot affect or be affected
  by the execution of another process.
• Cooperating process can affect or be affected by
  the execution of another process
• Advantages of process cooperation
• Information sharing
• Computation speed-up
• Modularity
• Convenience

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Producer-Consumer Problem

• Paradigm for cooperating processes, producer
  process produces information that is consumed by
  a consumer process.
• unbounded-buffer places no practical limit on the
  size of the buffer.
• bounded-buffer assumes that there is a fixed
  buffer size.

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Bounded-Buffer Shared-Memory Solution

• Shared data
• define BUFFER_SIZE 10
• Typedef struct
• . . .
• item
• item bufferBUFFER_SIZE
• int in 0
• int out 0
• Solution is correct, but can only use
  BUFFER_SIZE-1 elements

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Bounded-Buffer Producer Process

• item nextProduced
• while (1)
• while (((in 1) BUFFER_SIZE) out)
• / do nothing /
• bufferin nextProduced
• in (in 1) BUFFER_SIZE

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Bounded-Buffer Consumer Process

• item nextConsumed
• while (1)
• while (in out)
• / do nothing /
• nextConsumed bufferout
• out (out 1) BUFFER_SIZE

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Interprocess Communication (IPC)

• Mechanism for processes to communicate and to
  synchronize their actions.
• Message system processes communicate with each
  other without resorting to shared variables.
• IPC facility provides two operations
• send(message) message size fixed or variable
• receive(message)
• If P and Q wish to communicate, they need to
• establish a communication link between them
• exchange messages via send/receive
• Implementation of communication link
• physical (e.g., shared memory, hardware bus)
• logical (e.g., logical properties)

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Implementation Questions

• How are links established?
• Can a link be associated with more than two
  processes?
• How many links can there be between every pair of
  communicating processes?
• What is the capacity of a link?
• Is the size of a message that the link can
  accommodate fixed or variable?
• Is a link unidirectional or bi-directional?

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Direct Communication

• Processes must name each other explicitly
• send (P, message) send a message to process P
• receive(Q, message) receive a message from
  process Q
• Properties of communication link
• Links are established automatically.
• A link is associated with exactly one pair of
  communicating processes.
• Between each pair there exists exactly one link.
• The link may be unidirectional, but is usually
  bi-directional.

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Indirect Communication

• Messages are directed and received from mailboxes
  (also referred to as ports).
• Each mailbox has a unique id.
• Processes can communicate only if they share a
  mailbox.
• Properties of communication link
• Link established only if processes share a common
  mailbox
• A link may be associated with many processes.
• Each pair of processes may share several
  communication links.
• Link may be unidirectional or bi-directional.

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Indirect Communication

• Operations
• create a new mailbox
• send and receive messages through mailbox
• destroy a mailbox
• Primitives are defined as
• send(A, message) send a message to mailbox A
• receive(A, message) receive a message from
  mailbox A

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Indirect Communication

• Mailbox sharing
• P1, P2, and P3 share mailbox A.
• P1, sends P2 and P3 receive.
• Who gets the message?
• Solutions
• Allow a link to be associated with at most two
  processes.
• Allow only one process at a time to execute a
  receive operation.
• Allow the system to select arbitrarily the
  receiver. Sender is notified who the receiver
  was.

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Synchronization

• Message passing may be either blocking or
  non-blocking.
• Blocking is considered synchronous
• Non-blocking is considered asynchronous
• send and receive primitives may be either
  blocking or non-blocking.

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Buffering

• Queue of messages attached to the link
  implemented in one of three ways.
• 1. Zero capacity 0 messagesSender must wait
  for receiver (rendezvous).
• 2. Bounded capacity finite length of n
  messagesSender must wait if link full.
• 3. Unbounded capacity infinite length Sender
  never waits.

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Client-Server Communication

• Sockets
• Remote Procedure Calls
• Remote Method Invocation (Java)

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Sockets

• A socket is defined as an endpoint for
  communication.
• Concatenation of IP address and port
• The socket 161.25.19.81625 refers to port 1625
  on host 161.25.19.8
• Communication consists between a pair of sockets.

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Socket Communication
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Remote Procedure Calls

• Remote procedure call (RPC) abstracts procedure
  calls between processes on networked systems.
• Stubs client-side proxy for the actual
  procedure on the server.
• The client-side stub locates the server and
  marshalls the parameters.
• The server-side stub receives this message,
  unpacks the marshalled parameters, and peforms
  the procedure on the server.

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Execution of RPC
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Remote Method Invocation

• Remote Method Invocation (RMI) is a Java
  mechanism similar to RPCs.
• RMI allows a Java program on one machine to
  invoke a method on a remote object.

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Marshalling Parameters
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Major Requirements of anOperating System

• Interleave the execution of several processes to
  maximize processor utilization while providing
  reasonable response time
• Allocate resources to processes
• Support interprocess communication and user
  creation of processes

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Process

• Also called a task
• Execution of an individual program
• Can be traced
• list the sequence of instructions that execute

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Process Creation

• Submission of a batch job
• User logs on
• Created to provide a service such as printing
• Process creates another process

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Process Termination

• Batch job issues Halt instruction
• User logs off
• Quit an application
• Error and fault conditions

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Reasons for Process Termination

• Normal completion
• Time limit exceeded
• Memory unavailable
• Bounds violation
• Protection error
• example write to read-only file
• Arithmetic error
• Time overrun
• process waited longer than a specified maximum
  for an event

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Reasons for Process Termination

• I/O failure
• Invalid instruction
• happens when try to execute data
• Privileged instruction
• Data misuse
• Operating system intervention
• such as when deadlock occurs
• Parent terminates so child processes terminate
• Parent request

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Using Two Queues
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Suspended Processes

• Processor is faster than I/O so all processes
  could be waiting for I/O
• Swap these processes to disk to free up more
  memory
• Blocked state becomes suspend state when swapped
  to disk
• Two new states
• Blocked, suspend
• Ready, suspend

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One Suspend State
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Two Suspend States
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Reasons for Process Suspension
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Operating System Control Structures

• Information about the current status of each
  process and resource
• Tables are constructed for each entity the
  operating system manages

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Memory Tables

• Allocation of main memory to processes
• Allocation of secondary memory to processes
• Protection attributes for access to shared memory
  regions
• Information needed to manage virtual memory

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I/O Tables

• I/O device is available or assigned
• Status of I/O operation
• Location in main memory being used as the source
  or destination of the I/O transfer

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File Tables

• Existence of files
• Location on secondary memory
• Current Status
• Attributes
• Sometimes this information is maintained by a
  file-management system

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Process Table

• Where process is located
• Attributes necessary for its management
• Process ID
• Process state
• Location in memory

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Process Location

• Process includes set of programs to be executed
• Data locations for local and global variables
• Any defined constants
• Stack
• Process control block
• Collection of attributes
• Process image
• Collection of program, data, stack, and attributes

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Process Control Block

• Process identification
• Identifiers
• Numeric identifiers that may be stored with the
  process control block include
• Identifier of this process
• Identifier of the process that created this
  process (parent process)
• User identifier

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Process Control Block

• Processor State Information
• User-Visible Registers
• A user-visible register is one that may be
  referenced by means of the machine language that
  the processor executes. Typically, there are from
  8 to 32 of these registers, although some RISC
  implementations have over 100.

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Process Control Block

• Processor State Information
• Control and Status Registers
• These are a variety of processor registers that
  are employed to control the operation of the
  processor. These include
• Program counter Contains the address of the
  next instruction to be fetched
• Condition codes Result of the most recent
  arithmetic or logical operation (e.g., sign,
  zero, carry, equal, overflow)
• Status information Includes interrupt
  enabled/disabled flags, execution mode

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Process Control Block

• Processor State Information
• Stack Pointers
• Each process has one or more last-in-first-out
  (LIFO) system stacks associated with it. A stack
  is used to store parameters and calling addresses
  for procedure and system calls. The stack pointer
  points to the top of the stack.

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Process Control Block

• Process Control Information
• Scheduling and State Information
• This is information that is needed by the
  operating system to perform its scheduling
  function. Typical items of information
• Process state defines the readiness of the
  process to be scheduled for execution (e.g.,
  running, ready, waiting, halted).
• Priority One or more fields may be used to
  describe the scheduling priority of the process.
  In some systems, several values are required
  (e.g., default, current, highest-allowable)
• Scheduling-related information This will depend
  on the scheduling algorithm used. Examples are
  the amount of time that the process has been
  waiting and the amount of time that the process
  executed the last time it was running.
• Event Identity of event the process is awaiting
  before it can be resumed

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Process Control Block

• Process Control Information
• Data Structuring
• A process may be linked to other process in a
  queue, ring, or some other structure. For
  example, all processes in a waiting state for a
  particular priority level may be linked in a
  queue. A process may exhibit a parent-child
  (creator-created) relationship with another
  process. The process control block may contain
  pointers to other processes to support these
  structures.

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Process Control Block

• Process Control Information
• Interprocess Communication
• Various flags, signals, and messages may be
  associated with communication between two
  independent processes. Some or all of this
  information may be maintained in the process
  control block.
• Process Privileges
• Processes are granted privileges in terms of the
  memory that may be accessed and the types of
  instructions that may be executed. In addition,
  privileges may apply to the use of system
  utilities and services.

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Process Control Block

• Process Control Information
• Memory Management
• This section may include pointers to segment
  and/or page tables that describe the virtual
  memory assigned to this process.
• Resource Ownership and Utilization
• Resources controlled by the process may be
  indicated, such as opened files. A history of
  utilization of the processor or other resources
  may also be included this information may be
  needed by the scheduler.

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Processor State Information

• Contents of processor registers
• User-visible registers
• Control and status registers
• Stack pointers
• Program status word (PSW)
• contains status information
• Example the EFLAGS register on Pentium machines

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Modes of Execution

• User mode
• Less-privileged mode
• User programs typically execute in this mode
• System mode, control mode, or kernel mode
• More-privileged mode
• Kernel of the operating system

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Process Creation

• Assign a unique process identifier
• Allocate space for the process
• Initialize process control block
• Set up appropriate linkages
• Ex add new process to linked list used for
  scheduling queue
• Create of expand other data structures
• Ex maintain an accounting file

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When to Switch a Process

• Clock interrupt
• process has executed for the maximum allowable
  time slice
• I/O interrupt
• Memory fault
• memory address is in virtual memory so it must be
  brought into main memory

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When to Switch a Process

• Trap
• error occurred
• may cause process to be moved to Exit state
• Supervisor call
• such as file open

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Change of Process State

• Save context of processor including program
  counter and other registers
• Update the process control block of the process
  that is currently running
• Move process control block to appropriate queue -
  ready, blocked
• Select another process for execution

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Change of Process State

• Update the process control block of the process
  selected
• Update memory-management data structures
• Restore context of the selected process

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Execution of the Operating System

• Non-process Kernel
• execute kernel outside of any process
• operating system code is executed as a separate
  entity that operates in privileged mode
• Execution Within User Processes
• operating system software within context of a
  user process
• process executes in privileged mode when
  executing operating system code

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Execution of the Operating System

• Process-Based Operating System
• major kernel functions are separate processes
• Useful in multi-processor or multi-computer
  environment

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UNIX SVR4 Process Management

• Most of the operating system executes within the
  environment of a user process

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UNIX Process States
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