Process Synchronization III

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Process Synchronization III
CS423UG Operating Systems

• Indranil Gupta
• Lecture 10
• Sep 16, 2005

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Todays Agenda

• Classical synchronization problems
• Producer-Consumer problem
• Bounded Buffer Problem
• Reader Writer Problem
• Dining Philosophers Problem
• Sleeping Barber Problem
• IPC

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III. Reader-Writer Problem

• A reader read data
• A writer write data
• Rules
• Multiple readers may read the data simultaneously
• Only one writer can write the data at any time
• A reader and a writer cannot access data
  simultaneously
• Locking table whether any two can be in the
  critical section simultaneously

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Reader-Writer Solution

• Does it work? Why?
• Ask Systematic questions, and look at What if ?
  scenarios
• Play the devils advocate!
• Semaphore mutex, wrt // shared and initialized
  to 1
• int readcount // shared and initialized
  to 0
• // Writer
  // Reader
• Down(mutex)
• readcountreadcoun
  t1
• Down(wrt) if
  readcount 1 then Down(wrt)
• ......
  Up(mutex)
• writing performed ....
• ..... reading
  performed
• 
  Down(mutex)
• Up(wrt)
  readcountreadcount-1
• if
  readcount 0 then Up(wrt)

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IV. Dining Philosophers an intellectual game
0

• N philosophers and N forks
• Philosophers eat/think
• Eating needs 2 forks
• Pick one fork at a time

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0
4
1
2
4
3
3
2
N5
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Does this solve the Dining Philosophers Problem?

• A non-solution to the dining philosophers problem
• Deadlock everyone picks up their left fork
  first, then waits gt Starvation!

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Dining Philosophers Solution
(sn inited to 0s)
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Dining Philosophers Solution
Can prove that this solution is deadlock-free and
starvation-free
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V. The Sleeping Barber Problem

• N customer chairs (waiting chairs)
• One barber who can cut one customers hair at any
  time
• No waiting customer gt barber sleeps
• Customer enters gt
• If all waiting chairs full, customer leaves
• Otherwise, if barber asleep, wake up barber and
  make him work
• Otherwise, barber is busy wait in a chair

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The Sleeping Barber Solution (1)
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The Sleeping Barber Solution (2)
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The Sleeping Barber Solution (3)
Solution to sleeping barber problem.
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Epilogue IPC (Interprocess Communication)

• Real life analogy email!
• Can two processes on a machine send email to
  each other?
• IPC An indirect way of coordination, where the
  OS (and/or network) helps processes!
• An IPC primitive Message Passing
• Send (destination, message)
• Receive (source, message)
• Message size Fixed or Variable size.

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Message Passing
Example Unix pipes
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IPCIndirect Communication Implicit Coordination
provided through library/system calls

• send(A,message) / send a message to mailbox A
  /
• receive(A,message) / receive a message from
  mailbox A /
• Mailbox is an abstract object into which a
  message can be placed in, or removed from.

Example Unix message queues
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Advantage of Indirect Communication

• Allows greater variety of schemes
• two processes per link
• 1 link per pair of processes
• Uni- or bi-directional
• allow 1 process to receive a message from a link
  
• allow 1 process to receive all messages from a
  link
• What if a process sends when queue is full?
  Receive when queue is empty?
• Pipe and message queue implementations themselves
  use sempahores/mutexes within them!

Unix msgget() and msgsnd() for message queues
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Reminders

• Reading for this weeks lectures were Sections
  2.3-2.4
• Reading for next week Chapter 3 (full)
• MP1 due next Monday