Critical Region Classical OS Problems

1
Critical RegionClassical O/S Problems

• Ch 2.3 and 2.4
• Tuesday, January 30, 2007

2
Todays Schedule

• Questions over homework 2???
• Return Quick Check from Thursday, Jan 25
• Sec 2.3 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
Interprocess CommunicationRace Conditions

• Two processes want to access shared memory at
  same time

5
Critical Regions (2)

• Mutual exclusion using critical regions

6
IPC Basics

• Race Conditions
• Critical Region
• One at a Time
• Speed Independent
• Non-Critical Regions Immaterial
• Finite Waiting
• Solution?
• Mutual Exclusion

7
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

8
Basic Mutual Exclusion (cont.)

• Interested Flags (Petersons Solution)
• Test Set
• Atomic Action
• Read Value to see locked
• Automatically set to locked, just in case
• Sleep Wakeup

9
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

10
Mutual Exclusion with Busy Waiting (1)

• Proposed solution to critical region problem
• (a) Process 0. (b) Process 1.

11
Mutual Exclusion with Busy Waiting (2)

• Peterson's solution for achieving mutual exclusion

12
IPC (Dining Philosophers)

• Multiple processes attempting to obtain multiple
  resources
• Not enough resources to satisfy all requests at
  all times (Wait necessary)
• Why is this relevant?

13
Dining Philosophers Demonstration
14
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
Thursday, February 1

• Continue Reading Chapter 2.3 Monitors,
  Semaphores, Readers Writers pg 108-115, pg
  123-129
• Bring questions from Ch 1 and 2
• Test 1 on Tuesday, Feb 6