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 seconds
Priority 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 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
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 Philosophers
Yak
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Dining Philosophers
Yak
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Dining Philosophers
Yak
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Dining Philosophers
Yak
Delicious!
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Dining Philosophers
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Dining Philosophers
Deadlock!
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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
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
2
4
8
16
32
64
128
1
R = 0
R = 0.001
R = 0.02
R = 0.1
Actual Number of Processors
Eff
ect
ive
nu
mb
er
of
Pro
ces
so
rs
R = Context switch Time
Task 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?