Introduction to Operating Systems

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Introduction to Operating Systems Windows
process and thread management

• In this lecture we will cover
• Threads and processes in Windows
• Thread priority and scheduling
• Thread synchronisation
• Interprocess communication

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Windows structure overview
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Processes and threads

• In Windows a process consists of program code,
  execution context ( the address space of the
  process plus such things as the access token)
  resources allocated to the process i.e. handles,
  one or more threads
• Threads are the units of execution they execute
  program code using the processes context and
  resources

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• A thread has its own context (register values
  including instruction pointer, execution stack,
  and such things as scheduling priority)
• Threads are scheduled onto the processor, not a
  process

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• Processes and threads are managed by 2 components
  in Windows
• the process and thread manager
• and
• the kernel

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kernel

• The kernel is responsible for
• Thread scheduling
• Interrupt and exception handling
• Low level processor synchronisation
• Recovery after power failure
• In normal Operating Systems the kernel refers to
  all the operating systems components that run in
  kernel mode. In Windows as we have seen this
  applies to all the Windows Executive components
  (everything below the line in the diagram).

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• However, perversely, Windows applies the name
  kernel to just one component - a low level layer
  of the OS that manages much of the execution of
  threads within processes
• The kernel uses objects to represent and manage
  low level threads and processes
• However these kernel objects are different from
  those managed by the Object Manager
• They are lower level and provide support for the
  higher level objects used by the Object Manager

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Process and thread manager

• Process manager is part of the Windows Executive
  and implements the representation of processes
  and threads that are available to other parts of
  the Executive and provides high level services to
  manage and control processes and threads
• Process and thread manager provides functions to
• Create and destroy processes
• Control resource allocation to processes
• Keep track of information about processes and
  threads

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• Processes are created by other processes by
  making a CreateProcess system call
• Unlike Unix/Linux the parent and child process
  are independent of each other remember fork in
  Unix/Linux creates the child as a copy of the
  parent and hence has a copy of the parents
  address space
• In Windows child process has completely separate
  address space, etc.
• Hence Windows does not keep track of process
  hierarchies

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Thread scheduling

• Windows maintains a list of threads that are in
  the system
• Threads may be in one of several different states
• Ready thread can execute but waiting for a
  processor
• Running running on a processor
• Standby - selected to run next on a processor

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• Waiting unable to execute until some event
  occurs (typically I/O)
• Terminated
• All processes have at least one thread known as
  the base thread created when the process is
  created

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• The kernel implements the dispatcher code, which
  determines which thread to run next
• The dispatcher implements pre-emptive scheduling
  among threads
• The dispatcher schedules threads without
  reference to the process they belong to hence a
  process that has more threads will if everything
  else is equal have a greater share of the
  processor

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• The scheduling algorithm is based on a multilevel
  priority queue approach with each thread having a
  priority and hence belonging to a given queue
• A ready thread is placed in the queue which
  represents its assigned priority
• There are 32 priority queue levels designated by
  numbers with 31 highest priority and 0 lowest
• Dispatcher starts with highest priority queue and
  schedules threads in order in queue in round
  robin fashion

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• Each thread is given a time quantum in which to
  execute and it either blocks itself waiting on
  some I/O event or synchronisation event or the
  quantum expires
• Once a given queue is empty, the dispatcher then
  proceeds to the next lowest priority queue and so
  on until the queues are all empty or a higher
  level priority thread enters a ready state i.e.
  one of the higher level queues is now no longer
  empty and dispatcher pre-empts lower priority
  running thread

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• Highest priority levels (16-31) is for real-time
  threads (needing immediate responsiveness) the
  real-time priorities are static
• Lower priority levels (0-15) are for dynamic
  priority threads
• A processes base thread is given a base priority
  which is the minimum priority a thread can have

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• Each process has
• a base priority class (a range of priority levels
  which the define the possible range of base
  priorities) and
• a base priority level which specifies the
  relative base priority a threads should have in
  the base priority class

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• Each thread then takes on priority values
  dynamically i.e. it changes over time
• e.g. if the thread is delayed waiting on an I/O
  event, when the I/O event occurs and the thread
  becomes ready again, its priority is increased
  temporarily.
• The size of the increase depends on the length of
  the wait the longer the wait the greater the
  increase in priority

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Traps and exception handling

• Kernel implements a trap handler which deals with
  hardware interrupts and processor exceptions
• The trap handler disables interrupts, determines
  the cause of the interrupt, saves processor
  state, re-enables interrupts and dispatches code
  to deal with type of interrupt/exception found
  (Interrupt service routine for I/O events or
  traps from running code to request some service
  or exception handler to handle problems such as
  attempt to execute invalid instruction)

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Thread synchronisation

• Windows provides a set of dispatcher objects
  which can be used for synchronisation of thread
  behaviour
• These dispatcher objects can be in a signalled
  state (when the event that the thread is waiting
  for occurs) or a unsignaled state (when the event
  has not yet occurred)

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• Event objects represent events such as I/O events
  when the relevant event occurs the object
  manager sets the event object to the signalled
  state
• Mutex objects provide mutual exclusion only one
  thread can have a mutex object it does this by
  setting the object into an unsignaled state. Once
  the thread completes its activity it sets the
  mutex object to the signalled state
• Waitable timer objects these objects remain
  unsignaled until a given time has elapsed

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Interprocess communication

• Threads (and hence also processes) can
  communicate using a variety of methods
• Pipes work very similar to Unix
  unidirectional communication via a shared buffer
• Sockets similar to pipes but usually connect
  processes and threads on different machines
• Remote procedure calls allows a thread in one
  process to invoke the execution of code in
  different processes address space
• File sharing is also implemented