Semaphores Classical OS Problems

1
SemaphoresClassical O/S Problems

• Ch 2.3 and 2.4
• Thursday, February 1, 2007

2
Todays Schedule

• Sec 2.3 Methods to Protect Critical Regions
• Ch 2.4 Design Issues for O/S
• Mutual exclusion
• Dining Philosophers Problem
• Tuesday, February 6 - Test 1
• Ch 1
• Ch 2 (2.1 2.4)

3
Objectives

• Recognize need for Mutual Exclusion in the design
  of operating systems
• Identify software and hardware techniques for
  protecting critical regions, and compare between
  the methods
• Experience the complexity in guaranteeing mutual
  exclusion among processes competing for the same
  resources

4
Review Critical Regions

• Four conditions to provide mutual exclusion
• No two processes simultaneously in critical
  region
• No assumptions made about speeds or numbers of
  CPUs
• No process running outside its critical region
  may block another process
• No process must wait forever to enter its
  critical region

5
Basic Mutual Exclusion

• Disable Interrupts (BAD!)
• Only works for one Processor
• User must remember to re-enable interrupts
• Lock Variable (GETTING CLOSER)
• How do you lock the lock?
• Lockstep Alternation (ALMOST)
• Violates 3rd rule of Critical Regions

6
Mutual Exclusion with Busy Waiting

• Proposed solution with spin lock and Strict
  alternation
• (a) Process 0. (b) Process 1.

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Mutual Exclusion with Interested Flags Busy
Waiting

• Peterson's solution for achieving mutual exclusion

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Basic Mutual Exclusion (cont.)

• Interested Flags (Petersons Solution)
• Test Set uses hardware
• Atomic Action
• Read Value to see locked
• Automatically set to locked, just in case
• Sleep Wakeup
• System calls
• Block instead of busy waiting

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Semaphores

• Tracks number of Sleeps Wakeups
• Down (decrements) Up (increments)
• Historically P V Dutch (Thanks, Dijkstra!)
• 0 no wakeups saved, 2 2 wakeups pending
• Special Cases
• Down (sleep) from a 0
• Process or Thread Sleeps until a new Wakeup
• Up (wakeup) from 0
• Random Sleeper for this Wakeup is Triggered

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Binary Semaphores

• Mutex
• Like a Standard Lock
• Atomicity Provides Self-Locking
• Idle Wait
• If Locked, Thread Yields
• Simpler Semaphores
• Fundamental Mutual Exclusion Tool

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Monitors Barriers

• Monitors Higher level
• Compiler Based Mutual Exclusion
• Condition Variables (full)and operations on them
• Wait
• Signal
• Barriers
• Forced Resynchronization of Groups of Processes
• Ex. By phases

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Barriers
13
IPC (Dining Philosophers)

• Multiple processes attempting to obtain multiple
  resources
• Not enough resources to satisfy all requests at
  all times (Wait necessary)
• Starvation
• Deadlock
• Fair allocation

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Philosopher Resolution?

• Patterned locking?
• Take left, then take right?
• Locking the entire find fork area?
• Steal your forks while no one else can
• State Array?
• Track who has forks, who is waiting

15
Readers Writers (Models Database)

• Multiple readers can co-exist
• Two people can read the same page
• Multiple writers cannot
• Two people cannot write the same page, they will
  forever be getting in the way
• Writers and readers cannot co-exist
• Cannot read what is not written

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Readers Writers (cont)

• Reader on Reader
• Allow it in
• Reader on Writer
• Writer on Writer
• Must wait for Writer to finish
• Writer on Reader
• Must wait for Reader to finish

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Tuesday, February 6

• Test 1 on Tuesday, Feb 6
• Chapter 1 and Chapter 2 (2.1 2.4)
• Practice/review on website

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Summary

• Processes Threads allow for pseudo-simultaneous
  execution of tasks
• Coordination between processes how?
• Locking Sleeping
• TSL Command
• Counting Wakeups
• Monitors