Process Concept

1
Process Concept

• An operating system executes a variety of
  programs
• Batch system jobs
• Time-shared systems user programs or tasks
• Process a program in execution
• A process includes
• Text segment
• Data segment
• Stack segment
• Program counter

2
Process State

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

3
Process Control Block (PCB)

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

4
Process Queues

• Job queue set of all PCBs 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 a
  resource, e.g., an I/O device.
• PCBs migrate between the various queues.

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Ready Queue And Various I/O Device Queues
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Context Switch

• When the CPU switches to another process, the
  system must
• Switch to kernel mode
• Save the state of the old process
• Deal with the interrupt
• Load the saved state for the new process.
• Hardware (and possibly OS) saves the state of the
  old process
• The scheduler selects the new process
• The dispatcher loads the new process
• Set the PC
• Switch to user mode
• Context-switch time is overhead the system does
  no useful work while switching
• Time dependent on hardware support.

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CPU Switch From Process to Process
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Process Creation

• Parent process create children processes, which,
  in turn create other processes, forming a tree of
  processes.
• Address space (one of)
• Child duplicate of parent.
• Child has a program loaded into it.
• Execution (one of)
• Parent and child execute concurrently
• Parent waits until child terminates
• Resource sharing (one of)
• Parent and children share all resources.
• Children share subset of parents resources.
• Parent and child share no resources.

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Process Termination

• Process executes last statement and tells the OS
• Output status from child to parent
• Process resources are (eventually) deallocated
  by the operating system
• Parent may terminate execution of children
  processes
• Child has exceeded allocated resources.
• Task assigned to child is no longer required.
• Parent is exiting
• With cascading termination, the operating system
  does not allow child to continue if its parent
  terminates

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UNIX PCBs
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UNIX Shell
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Threads

• A thread (or lightweight process) is a basic unit
  of CPU utilization it consists of
• Program counter
• Register set
• Stack space
• A thread shares with its peer threads its
• Code segment
• Data segment
• Operating-system resources
• A traditional or heavyweight process is equal to
  a task with one thread.

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Single and Multithreaded Processes
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Thread Implementations

• User-level threads library
• POSIX Pthreads
• Mach C-threads
• Solaris threads
• Kernel Threads
• Windows 95/98/NT/2000
• Solaris
• Linux

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Many-to-One

• Many user-level threads mapped to single kernel
  thread.
• Used on systems that do not support kernel
  threads.

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One-to-One

• Each user-level thread maps to kernel thread.
• Examples
• Windows 95/98/NT/2000
• OS/2
• LinuxThreads

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Many-to-Many Model

• Allows many user level threads to be mapped to
  many kernel threads.
• Allows the operating system to create a
  sufficient number of kernel threads.
• Solaris 2
• Windows NT/2000 with the ThreadFiber package

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Pthreads

• A POSIX standard (IEEE 1003.1c) API for thread
  creation and synchronization.
• API specifies behavior of the thread library,
  implementation is up to development of the
  library.
• Common in UNIX operating systems.

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Benefits

• Responsiveness
• In a multiple threaded task, while one thread is
  blocked and waiting, a second thread in the same
  task can run.
• Kernel level threads provide a mechanism that
  allows sequential processes to make blocking
  system calls while also achieving parallelism.
• Resource Sharing
• Economy
• Utilization of MP Architectures