Issues in Concurrent Systems

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Issues in Concurrent Systems
Lecture 27
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Online Survey
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• The Spring term course/instructor opinion survey
  will be available during the period Monday, April
  17th through Friday, April 28th from 6am to
  1159pm each day
• http//www.coursesurvey.gatech.edu

Fabulous Prizes!
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Final Exam Schedule
LB

• CS1311 Sections L/M/N Tuesday/Thursday 1000 A.M.
• Exam Scheduled for 800 Friday May 5, 2000
• Physics L1

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Final Exam Schedule
LB

• CS1311 Sections E/F Tuesday/Thursday 200 P.M.
• Exam Scheduled for 250 Wednesday May 3, 2000
• Physics L1

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Issues in Concurrent Systems

• Concurrency is great, but there are some problems
  to be addressed
• Mutual exclusion (protection)
• Starvation (fairness)
• Deadlock
• Time
• Synchronization
• Increased Cost of Overhead

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Mutual Exclusion

• By providing mutual exclusion, we can protect a
  resource from being corrupted.

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Banking Example
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• Gertrude makes 100 deposit
• Get current balance of 500
• balance 500 100 600
• Write new balance back to disk thus on disk 600

• Heathcliffe requests 100 withdrawl
• Get current balance of 500
• balance 500 - 100 400
• Write new balance back to disk thus on disk 400

Time Slice
Final Answer?
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Banking Example
LB

• Gertrude makes 100 deposit
• Request and get AcctLock
• Get current balance of 500 from disk
• balance 500 100 600
• Write back balance 600
• Release AcctLock

• Heathcliffe requests 100 withdrawl
• Request AcctLock
• Wait(AcctLock)
• Get AcctLock
• Read balance 600
• balance 600 - 100 500
• Write back 500

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Fairness

• Multiple processors (users) competing for a
  resource.
• Make sure that all jobs get serviced (have access
  to the resource) fairly.
• Policies include
• Shortest job first
• First come, first served
• More sophisticated schemes

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First Come, First Served
Fred - 10 hours
Wilma - 5 minutes
Dino - 2 seconds
Freds long job blocks others
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Shortest Job First
Priority Queue
Dino - 2 seconds
priority 1
Fast jobs first, but what if new fast jobs keep
arriving? Will Freds job ever get served?
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Multiple Queues
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Job 123
Job 86
Job 5
Job 13
Job 23
8 minute jobs
Job 123
Job 12
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Typical Multiple Queue Rules
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• Jobs that take longer than expected get moved
  down.
• Jobs that less time than expected get moved up.
• Jobs that have been waiting a long time get moved
  up.

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Deadlock

• When sharing resources, jobs can get into
  never-ending waits

Job B
Job A
resource D
resource C
A has resource C, needs D B has resource D, needs
C
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Preventing Deadlock

• There are several algorithms to prevent deadlock
• Require resources to always be acquired in a
  particular order.
• If the job cant acquire all the resources it
  needs for a particular task, it frees all the
  resources it has reserved.
• Establish priorities for resource queues.

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Dining Philosophers
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• Algorithm DiningPhilosophers
• Loop
• exitif(no more food)
• Engage in philosophical discussion
• Get left chopstick
• Get right chopstick
• Eat some food with chopsticks
• Put chopsicks down
• Endloop

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Dining Philosophers
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
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Dining Philosophers
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Yak
Delicious!
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Dining Philosophers
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Dining Philosophers
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Deadlock!
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LB

• Algorithm DiningPhilosophers
• Loop
• exitif(no more food)
• Engage in philosophical discussion
• Get left chopstick
• Get right chopstick
• Eat some food with chopsticks
• Put chopsicks down
• Endloop

Stuck here!
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Preventing Philosopher Deadlock

• Add a resource
• Add a chopstick in the middle.
• Alter the order of acquisition
• Number the chopsticks in sequence
• Change algorithm to take odd numbered chopstick
  first.

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Dining Philosophers
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1
4
2
3
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Dining Philosophers
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1
4
2
3
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Dining Philosophers
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1
4
2
3
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Dining Philosophers
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Yummy!
4
2
3
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Preventing Philosopher Deadlock
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• Add a resource
• Add a chopstick in the middle.
• Alter the order of acquisition
• Number the chopsticks in sequence
• Change algorithm to take odd numbered chopstick
  first.
• Have a back-off and try again policy
• If a philosopher waits for a chopstick for more
  than a few minutes, release held chopstick.

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Time

• Whose system clock is correct in a distributed
  system?

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Time
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• Hardware Approach Develop highly sophisticated
  atomic clocks and synchronize off of US standard
  time clock.
• Software Approach Use algorithms that dont rely
  on real time at all!
• Mix of the two.

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Synchronization

• What if one process wants to exchange data with
  another process?
• How do the processes get together to coordinate
  their communication?
• Asynchronous vs Synchronous Operations

Process 2
Process 1
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Impact of Processing Overhead
128
R 0
64
R 0.001
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Effective number of Processors
16
R 0.02
8
R 0.1
4
2
1
2
4
8
16
32
64
128
1
Actual Number of Processors
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CS Specialties

• Operating Systems people worry about the
  effective allocation and coordination of
  resources within a single computer system.
• Networking people worry about effective
  communication between multiple computer systems
  over networks.
• Database people worry about providing fast access
  to specific data (located within large amounts of
  data) while protecting the integrity of data from
  corruption by multiple concurrent users.
• Hardware Architecture people worry about how
  processors are designed so that they can work
  effectively with algorithms, compilers and data
  structures.

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CS Core Specialty Areas
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• Systems
• Theory
• Languages/Compilers
• Architecture
• Database
• Networking/Telecom
• HCI
• Security
• Biocomputing
• ...

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Questions?
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