1. Issues in Concurrent Systems

2. 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 LB

3. 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 LB

4. Final Exam Schedule CS1311 Sections E/F Tuesday/Thursday 2:00 P.M. Exam Scheduled for 2:50 Wednesday May 3, 2000 Physics L1 LB

5. 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

6. Mutual Exclusion By providing mutual exclusion, we can protect a resource from being “corrupted.” Last Seat on flight 121 Operator 1 Operator 2

7. 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 Time Slice LB

8. 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 LB

9. 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

10. First Come, First Served Fred - 10 hours Wilma - 5 minutes Dino - 2 seconds Fred’s long job blocks others

11. Shortest Job First Dino - 2 seconds Priority Queue priority: 1 Wilma - 5 minutes priority: 2 Fred - 10 hours priority: 10 Fast jobs first, but what if new fast jobs keep arriving? Will Fred’s job ever get served?

12. Multiple Queues 1 minute jobs2 minute jobs4 minute jobs 8 minute jobs Job 123Job 86 Job 5 Job 123 Job 13 Job 23 Job 12 LB

13. 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. LB

14. Deadlock When sharing resources, jobs can get into never-ending waits: Job A Job B resource C resource D A has resource C, needs D B has resource D, needs C

15. 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.

16. Dining Philosophers Chopsticks LB

17. 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 LB

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27. Dining Philosophers Yak Delicious! LB

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29. Dining Philosophers Deadlock! LB

30. 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! LB

31. 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.

32. Dining Philosophers 4 3 2 1 LB

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35. Dining Philosophers 4 3 2 1 Yummy! LB

36. 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. LB

37. Time Computer 1 Computer 2 Computer 3 Whose system clock is correct in a distributed system?

38. 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. LB

39. Synchronization Process 1 Process 2 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

40. Impact of Processing Overhead 2481632641281 2 4 8 16 32 64 128 1 R = 0 R = 0.001 R = 0.02 R = 0.1 Actual Number of Processors Effective number of Processors R = Context switch Time Task Time

41. 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.

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

43. Questions?