Threads

1
Threads

• Many software packages are multi-threaded
• Web browser one thread display images, another
  thread retrieves data from the network
• Word processor threads for displaying graphics,
  reading keystrokes from the user, performing
  spelling and grammar checking in the background
• A thread is sometimes called a lightweight
  process
• It is comprised over a thread ID, program
  counter, a register set and a stack
• It shares with other threads belonging to the
  same process its code section, data section and
  other OS resources (e.g., open files)
• A process that has multiples threads can do more
  than one task at a time

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Benefits

• Responsiveness
• One part of a program can continue running even
  if another part is blocked
• Resource Sharing
• Threads of the same process share the same memory
  space and resources
• Economy
• Much less time consuming to create and manage
  threads than processes

3
Single and Multithreaded Processes
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Lifecycle of a Thread (or Process)

• As a thread executes, it changes state
• new The thread is being created
• ready The thread is waiting to run
• running Instructions are being executed
• waiting Thread waiting for some event to occur
• terminated The thread has finished execution
• Active threads are represented by their TCBs
• TCBs organized into queues based on their state

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Java Thread States
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Ready Queue And Various I/O Device Queues

• Thread not running ? TCB is in some scheduler
  queue
• Separate queue for each device/signal/condition
• Each queue can have a different scheduler policy

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Dispatch Loop

• Conceptually, the dispatching loop of the
  operating system looks as follows
• Loop
• RunThread()
• ChooseNextThread()
• SaveStateOfCPU(curTCB)
• LoadStateOfCPU(newTCB)

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Running a thread

• Consider first portion RunThread()
• How do I run a thread?
• Load its state (registers, PC, stack pointer)
  into CPU
• Load environment (virtual memory space, etc)
• Jump to the PC
• How does the dispatcher get control back?
• Internal events thread returns control
  voluntarily
• External events thread gets preempted

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Internal Events

• Blocking on I/O
• The act of requesting I/O implicitly yields the
  CPU
• Waiting on a signal from other thread
• Thread asks to wait and thus yields the CPU
• Thread executes a yield()
• Thread volunteers to give up CPU
• computePI()
• while(TRUE)
• ComputeNextDigit()
• yield()

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Choosing a Thread to Run

• How does Dispatcher decide what to run?
• Zero ready threads dispatcher loops
• Alternative is to create an idle thread
• Can put machine into low-power mode
• Exactly one ready thread easy
• More than one ready thread use scheduling
  priorities
• Possible priorities
• LIFO (last in, first out)
• put ready threads on front of list, remove from
  front
• Pick one at random
• FIFO (first in, first out)
• Put ready threads on back of list, pull them from
  front
• This is fair and is what Nachos does
• Priority queue
• keep ready list sorted by TCB priority field

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Summary

• The state of a thread is contained in the TCB
• Registers, PC, stack pointer
• States New, Ready, Running, Waiting, or
  Terminated
• Multithreading provides simple illusion of
  multiple CPUs
• Switch registers and stack to dispatch new thread
• Provide mechanism to ensure dispatcher regains
  control
• Many scheduling options
• Decision of which thread to run