Issues in Concurrent Systems

Online Survey

The Spring term course/instructor opinion survey will be available during the period Monday, April 17th through Friday, April 28th from 6am to 11:59pm each day:

http://www.coursesurvey.gatech.edu

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Final Exam Schedule

• CS1311 Sections L/M/N Tuesday/Thursday 10:00 A.M.

• Exam Scheduled for 8:00 Friday May 5, 2000• Physics L1

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Final Exam Schedule

• CS1311 Sections E/F Tuesday/Thursday 2:00 P.M.

• Exam Scheduled for 2:50 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

Mutual Exclusion

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

Last Seaton flight 121

Operator 1 Operator 2

Banking Example

• 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

TimeSlice

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Banking Example• 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

First Come, First Served

Fred - 10 hours

Wilma - 5 minutes

Dino - 2 seconds

Fred’s long job blocks others

Shortest Job First

Dino - 2 secondsPriority Queue

priority: 1

Wilma - 5 minutespriority: 2

Fred - 10 hourspriority: 10

Fast jobs first, but what if new fast jobs keep arriving? Will Fred’s job ever get served?

Multiple Queues

1 minute jobs

2 minute jobs

4 minute jobs

8 minute jobs

Job 123 Job 86

Job 5

Job 123

Job 13

Job 23

Job 12

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Typical Multiple Queue Rules

• 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 A Job B

resource C resource D

A has resource C, needs DB has resource D, needs C

Preventing Deadlock

• There are several algorithms to prevent deadlock:– Require resources to always be

acquired in a particular order.– If the job can’t acquire all the resources

it needs for a particular task, it frees all the resources it has reserved.

– Establish priorities for resource queues.

Dining Philosophers

Chopsticks

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Algorithm DiningPhilosophersLoopexitif(no more food)

Engage in philosophical discussion

Get left chopstick

Get right chopstick

Eat some food with chopsticks

Put chopsicks downEndloop

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Dining Philosophers

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Dining PhilosophersYak

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Dining Philosophers

Yak

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Dining Philosophers

Yak

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Dining Philosophers

Yak

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Dining Philosophers

Yak

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Dining Philosophers

Yak

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Dining PhilosophersYak

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Dining Philosophers

Yak

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Dining PhilosophersYak

Delicious!

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Dining Philosophers

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Dining Philosophers

Deadlock!

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Algorithm DiningPhilosophersLoopexitif(no more food)

Engage in philosophical discussion

Get left chopstick

Get right chopstick

Eat some food with chopsticks

Put chopsicks downEndloop

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.

Dining Philosophers

4

3

2

1

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Dining Philosophers

4

3

2

1

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Dining Philosophers

4

3

2

1

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Dining Philosophers

4

3

2

1

Yummy!

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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.• 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

Computer 1Computer 2

Computer 3

• Whose system clock is correct in a distributed system?

Time

• Hardware Approach: Develop highly sophisticated atomic clocks and synchronize off of US standard time clock.

• Software Approach: Use algorithms that don’t rely on real time at all!

• Mix of the two.

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Synchronization

Process1

Process2

• 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

Impact of Processing Overhead

2 4 8 16 32 64 1281

248

163264

128

1

R = 0

R = 0.001

R = 0.02

R = 0.1

Actual Number of Processors

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ctiv

e nu

mbe

r of

Proc

esso

rs

R = Context switch TimeTask Time

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.

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

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Questions?