Operating Systems CSE 411

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Operating SystemsCSE 411

• CPU Management
• Oct. 13 2006 - Lecture 14
• Instructor Bhuvan Urgaonkar

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Some announcements

• Will discuss Exam 1 on Monday
• Hopefully also hand-out the graded exam
• The question on Quiz 4 comparing RR and PS has
  puzzled many
• Has been removed
• Will describe what I had intended on Monday
• Project
• Arjun will hold extra TA hours on Saturday 4-6pm

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Comments on Petersons Approach

• This is a purely software approach, and it does
  not require any support from hardware except
  atomic loads and stores
• However, it may cause "busy waiting
• Wastes CPU cycles

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Synchronization Hardware

• Many systems provide hardware support for
  critical section code
• Modern machines provide special atomic hardware
  instructions
• Atomic non-interruptible
• Either test memory word and set value
• Or swap contents of two memory words

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TestAndndSet Instruction

• Definition
• boolean TestAndSet (boolean target)
• boolean rv target
• target TRUE
• return rv

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Solution using TestAndSet

• Shared boolean variable lock, initialized to
  false
• Solution
• while (true)
• while ( TestAndSet (lock ))
• / do
  nothing
• // critical
  section
• lock FALSE
• // remainder
  section

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Swap Instruction

• Definition
• void Swap (boolean a, boolean b)
• boolean temp a
• a b
• b temp

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Solution using Swap

• Shared Boolean variable lock initialized to
  FALSE Each process
• has a local Boolean variable key.
• Solution
• while (true)
• key TRUE
• while ( key TRUE)
• Swap (lock, key )
• // critical
  section
• lock FALSE
• // remainder
  section

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• Notes
• Previous solutions do not satisfy bounded-waiting
  requirement
• Figure 6.8 describes how to do this
• Implementing these instructions on
  multiprocessors can be quite difficult
• Also, complicated for a programmer to use
• Is there an easier and more efficient way?
• Semaphores

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Semaphore

• Semaphore noun
• 1. an apparatus for conveying information by
  means of visual signals, as a light whose
  position may be changed.
• 2. any of various devices for signaling by
  changing the position of a light, flag, etc.
• 3. a system of signaling, esp. a system by which
  a special flag is held in each hand and various
  positions of the arms indicate specific letters,
  numbers, etc.

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Semaphore

• Synchronization tool that reduces busy waiting
• Invented by Edsger Wybe Dijkstra
• First used in THE operating system
• Dijkstra is also noted for owning only one
  computer (late in life) and rarely actually using
  them, in keeping with his conviction that
  computer science was more abstract than mere
  programming, expressed in a number of famous
  sayings such as "Computer Science is no more
  about computers than astronomy is about
  telescopes."

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Semaphore

• Synchronization tool that does not require busy
  waiting
• Semaphore S integer variable
• Two standard operations modify S wait() and
  signal()
• Originally called P() and V()
• From Dutch words/phrases probeer te verlagen
  (try-and-decrease) and verhoog ("increase)
• Less complicated
• Can only be accessed via two indivisible (atomic)
  operations
• wait (S)
• while S lt 0 // no-op
• S--
• signal (S)
• S

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Semaphore as General Synchronization Tool

• Counting semaphore integer value can range over
  an unrestricted domain
• Synchronize access to a resource with multiple
  copies/instances
• Binary semaphore integer value can range only
  between 0 and 1 can be simpler to implement
• Also known as mutex locks (MUTUAL EXCLUSION)
• Can implement a counting semaphore S as a binary
  semaphore
• Provides mutual exclusion
• Semaphore S // initialized to 1
• wait (S)
• Critical Section
• signal (S)

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Semaphore Implementation with no Busy waiting

• With each semaphore there is an associated
  waiting queue. A waiting queue has two data
  items
• value (of type integer)
• pointer to a list of PCBs
• Introduce a pointer in the PCB structure
• FIFO ordering gt bounded-waiting
• Two operations
• block place the process invoking the operation
  on the appropriate waiting queue.
• wakeup remove one of processes in the waiting
  queue and place it in the ready queue.
• Is busy waiting
  completely gone?

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Semaphore Implementation with (almost) no Busy
waiting

• Implementation of wait
• wait (S)
• value--
• if (value lt 0)
• add this process to waiting
  queue
• block()
• Implementation of signal
• Signal (S)
• value
• if (value lt 0)
• remove a process P from the
  waiting queue
• wakeup(P)

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Deadlock and Starvation

• Deadlock two or more processes are waiting
  indefinitely for an event that can be caused by
  only one of the waiting processes
• Let S and Q be two semaphores initialized to 1
• P0 P1
• wait (S)
  wait (Q)
• wait (Q)
  wait (S)
• . .
• . .
• . .
• signal (S)
  signal (Q)
• signal (Q)
  signal (S)
• Starvation indefinite blocking. A process may
  never be removed from the semaphore queue in
  which it is suspended.

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Classical Problems of Synchronization

• Bounded-Buffer Problem
• Readers and Writers Problem
• Dining-Philosophers Problem

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Bounded-Buffer Problem

• N buffers, each can hold one item
• Semaphore mutex initialized to the value 1
• Semaphore full initialized to the value 0
• Semaphore empty initialized to the value N

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Bounded Buffer Problem (Cont.)

• The structure of the producer process
• while (true)
• // produce an item
• wait (empty)
• wait (mutex)
• // add the item to the
  buffer
• signal (mutex)
• signal (full)

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Bounded Buffer Problem (Cont.)

• The structure of the consumer process
• while (true)
• wait (full)
• wait (mutex)
• // remove an item
  from buffer
• signal (mutex)
• signal (empty)
• // consume the
  removed item

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Readers-Writers Problem

• A data set is shared among a number of concurrent
  processes
• Readers only read the data set they do not
  perform any updates
• Writers can both read and write.
• Problem allow multiple readers to read at the
  same time. Only one writer can access the
  shared data at the same time.
• Shared Data
• Data set
• Semaphore mutex initialized to 1.
• Semaphore wrt initialized to 1.
• Integer readcount initialized to 0.

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Readers-Writers Problem (Cont.)

• The structure of a writer process
• while (true)
• wait (wrt)
• // writing is
  performed
• signal (wrt)

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Readers-Writers Problem (Cont.)

• The structure of a reader process
• while (true)
• wait (mutex)
• readcount
• if (readcount 1) wait
  (wrt)
• signal (mutex)
• // reading is
  performed
• wait (mutex)
• readcount - -
• if (readcount 0)
  signal (wrt)
• signal (mutex)

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Dining-Philosophers Problem

• Shared data
• Bowl of rice (data set)
• Semaphore chopstick 5 initialized to 1

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Dining-Philosophers Problem (Cont.)

• The structure of Philosopher i
• while (true)
• wait ( chopsticki )
• wait ( chopStick (i 1) 5 )
• // eat
• signal ( chopsticki )
• signal (chopstick (i 1) 5 )
• // think

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Problems with Semaphores

• Correct use of semaphore operations
• signal (mutex) . wait (mutex)
• wait (mutex) wait (mutex)
• Omitting of wait (mutex) or signal (mutex) (or
  both)

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Synchronization on Multi-processors
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Monitors
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System Bootstrap
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Multi-processor Scheduling
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Some History
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UNIX
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The POSIX Standard
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Course Outline

• Resource Management (and some services an OS
  provides to programmers)
• CPU management
• Memory management
• I/O management (emphasis Disk)
• Cross-cutting design considerations and
  techniques
• Quality-of-service/fairness, monitoring,
  accounting, caching, software design methodology,
  security and isolation
• Advanced topics
• Distributed systems
• Data centers, multi-media systems, real-time
  systems,
• virtual machines