1. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 1 Instructor: Dr. Khalil Synchronization Tools for Distributed Operating System Survey Paper Team Members: Mazen Hammad Chuck Mann Vrushali Nidgundi Hong Zhang Course: CSE 8343 Advanced Operating Systems Professor: Dr. Mohamed Khalil (Group 2)

2. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 2 Instructor: Dr. Khalil Outline Mutual Exclusion Atomicity Concurrency Semaphores Message Passing Deadlock Handling

3. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 3 Instructor: Dr. Khalil Mutual Exclusion Mutual-exclusion guarantees that certain sections of code (critical sections) will not be executed by more than one process simultaneously. These sections of code usually access shared variables in a common store or access shared hardware. The standard solution to kernel-level mutual- exclusion in uniprocessor systems is to momentarily disable interrupts to guarantee that the process accessing the sensitive data will not be preempted before the access has been completed. This solution is not available for multiprocessor systems, since processes on these are truly concurrent.

4. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 4 Instructor: Dr. Khalil Mutual Exclusion (Continued) A critical section of code is framed by an entry section at the beginning and an exit section at the end; these sections act to grab and release the “lock” on that section. One safety property of mutual exclusion is, no more than one process should have its program counter (PC) in the critical code at the same time.

5. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 5 Instructor: Dr. Khalil Mutual Exclusion (Continued) Different algorithms for implementing mutual exclusion Centralized Approach: One of the processes in the system is chosen to coordinate the entry to the critical section. Fully Distributed Approach: This algorithm is based on the event ordering scheme. Token Passing Approach: Another method of providing mutual exclusion is to circulate a token among the processes in the system.

6. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 6 Instructor: Dr. Khalil Atomicity Atomic transaction is a program that must be executed atomically. That is, either all the operations associated with it are executed to completion, or none are performed. The two phase commit protocol is used to ensure atomicity.

7. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 7 Instructor: Dr. Khalil Concurrency Different concurrency control schemes are modified so that they can be used in a distributed environment: Locking Protocols Timestamping

8. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 8 Instructor: Dr. Khalil Distributed Semaphores Semaphores provide a basic synchronization mechanism in uni and multi processor systems Supporting semaphores in distributed systems has not received much attention Implementation of semaphores very difficult in a distributed system

9. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 9 Instructor: Dr. Khalil Distributed Semaphores (Continued) Distributed Semaphore is a semaphore-like mechanism It does not require shared memory Implemented using conditional synchronous message-passing mechanism

10. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 10 Instructor: Dr. Khalil Distributed Semaphores (Continued) Synchronization is achieved using LEMMA (MESSAGE QUEUE STABILITY) approach and is used to ensure consistency in distributed database systems Once the Lemma equation is satisfied then the following things are also satisfied: –A proxy message will not be queued indefinitely –A request message will not be queued indefinitely –Every P request message eventually reaches the semaphore holder –Two or more nodes will not form a cycle while waiting for a semaphore –A node’s request for P and V will not form a cycle

11. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 11 Instructor: Dr. Khalil Message Passing Minimum set of primitives needed for processes to conduct message passing are: –Send (destination, message) –Receive (source, message) Process A sends message to process B with send primitive designating B as destination Process B receives message with receive primitive designating A as the source

12. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 12 Instructor: Dr. Khalil Message Passing Shared Data Send Message Process A Recv Message Process B

13. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 13 Instructor: Dr. Khalil Common Synchronization Combinations Blocking send and blocking receive –Rendezvous – Both sender and receiver are blocked until the message is delivered –Example – Remote Procedure Calls (RPCs) Nonblocking send and blocking receive –Sender can send messages to several different recipient processes –Receiver that must obtain data from message before it can do useful work waits for the data Nonblocking send and nonblocking receive –Neither process waits but recipient should poll

14. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 14 Instructor: Dr. Khalil Synchronization with Receive Messages Time NonblockingBlockingTimeout pvm_trecv()pvm_recv()pvm_nrecv() Function Called Time Expired Message Arrives Waiting Running

15. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 15 Instructor: Dr. Khalil Synchronization Point Group barrier synchronizes a group of processes at a point in time Indirect message passing via daemons Each early member process in a group performs a virtual blocking receive Last member process performs a virtual nonblocking send to all the other processes

16. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 16 Instructor: Dr. Khalil Synchronization with a Group Barrier Time Process 1Process 2Process 3 barrier call Synchronization Point barrier call Waiting Running Note: Syntax of barrier call is pvm_barrier(“g2”,3) where the group name is g2 and the number of processes to rendezvous is 3.

17. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 17 Instructor: Dr. Khalil Deadlock Handling Processes compete for resources

18. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 18 Instructor: Dr. Khalil Deadlock Characterization & Handling Approaches Deadlock will happen if four conditions hold simultaneously –Mutual exclusion –Hold and wait –No preemption –Circular wait Deadlock Handling Approaches –Prevention –Avoidance –Detection –Recovery

19. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 19 Instructor: Dr. Khalil Deadlock Prevention Ensure at least one of these conditions cannot hold Mutual Exclusion - Not required for sharable resources, must hold for non-sharable resources Hold and Wait - Whenever a process requests a resource, it does not hold any other resources. No Preemption - Preempt resources held by a process, which is requesting another resource that cannot be immediately allocated to it. Circular Wait - Impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.

20. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 20 Instructor: Dr. Khalil Deadlock Avoidance Wound-Wait Scheme (Preemptive) Process P 0 requests a resource held by process P 1, P 0 will be allowed to wait only if it has a larger timestamp than P 1, i.e. P 0 is younger than P 1. Otherwise, P 1 is rolled back (P 1 is wound by P 0 ). Wait-Die Scheme (Non-preemptive) Process P 0 requests a resource held by process P 1, P 0 will be allowed to wait only if it has a smaller timestamp than P 1, i.e. P 0 is older than P 1. Otherwise, P 0 is rolled back (dies).

21. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 21 Instructor: Dr. Khalil Deadlock Detection Centralized Approach Fully Distributed Approach

22. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 22 Instructor: Dr. Khalil Recovery from Deadlock Process Termination –Abort all deadlocked processes –Abort one process at a time until the deadlock cycle is eliminated –In which order to abort Resource Preemption –Selecting a victim - minimize cost –Rollback - return to some safe state, restart process from that state –Avoid starvation - same process may always be picked as victim, include number of rollbacks in cost factor

23. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 23 Instructor: Dr. Khalil References [1]Comer, D. (2000), Internetworking with TCP/IP: Principles, Protocols, and Architectures, 4th Ed., Prentice-Hall, Upper Saddle River, NJ. [2]Coulouris, G.; Dollimore, J.; Kindberg, T. (2001), Distributed Systems: Concepts and Design, 3rd Ed., Addison-Wesley, Reading, Mass. [3]El-Rewini, H. (2003), Classroom Lectures, CSE 8380 - Parallel and Distributed Processing, Southern Methodist University, Spring 2003. [4] El-Rewini, H. and Lewis, T. (1998), Distributed and Parallel Computing, Manning & Prentice Hall, Greenwich, CT. [5] Fiorini, P. "Distributed Deadlock", University of Southern Maine, Portland, ME. [6] Holliday, J. and Abbadi, A. “Distributed Deadlock Detection”, Encyclopedia of Distributed Computing, Kluwer Academic Publishers. [7]Silberschatz, A. and Galvin, P. (1998), "Operating System Concepts", 5th Ed., Addison-Wesley, Read-ing Mass. [8]Stallings, W. (2001), "Operating Systems: Internals and Design Principles", 4th Ed., Prentice-Hall, Up-per Saddle River, NJ. [9]Tanenbaum, A. and van Steen, M. (2002), "Distributed Systems: Principles and Paradigms", Prentice-Hall, Upper Saddle River, NJ.

24. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 24 Instructor: Dr. Khalil Questions & Discussion

25. November 22, 2003 SMU School of Engineering Group 2: Hammad, Mann, Nidgundi, & Zhang CSE 8343 Advanced Operating Systems Slide 25 Instructor: Dr. Khalil Thank You!