Synchronization Tools for Distributed Operating System Survey Paper

1. Synchronization Tools for Distributed Operating SystemSurvey Paper (Group 2) • Team Members:Mazen HammadChuck MannVrushali Nidgundi Hong Zhang • Course:CSE 8343 Advanced Operating Systems • Professor:Dr. Mohamed Khalil Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

2. Outline • Mutual Exclusion • Atomicity • Concurrency • Semaphores • Message Passing • Deadlock Handling Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

3. 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. Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

4. 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. Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

5. 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. Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

6. 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. Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

7. Concurrency Different concurrency control schemes are modified so that they can be used in a distributed environment: • Locking Protocols • Timestamping Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

8. 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 Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

9. Distributed Semaphores (Continued) • Distributed Semaphore isa semaphore-like mechanism • It does not require shared memory • Implemented using conditional synchronous message-passing mechanism Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

10. 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 Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

11. 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 Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

12. Send Message Recv Message Shared Data Process B Message Passing Process A Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

13. 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 Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

14. Synchronization with Receive Messages Nonblocking Blocking Timeout Function Called pvm_nrecv() pvm_recv() pvm_trecv() Waiting Time Time Expired Waiting Running Message Arrives Running Running Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

15. 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 Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

16. Synchronization with a Group Barrier Process 1 Process 2 Process 3 barrier call Running Time barrier call Waiting Waiting Synchronization Point barrier call Running Running 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. Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

17. Deadlock Handling Processes compete for resources Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

18. 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 Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

19. 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. Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

20. Deadlock Avoidance • Wound-Wait Scheme (Preemptive) Process P0 requests a resource held by process P1, P0 will be allowed to wait only if it has a larger timestamp than P1, i.e. P0 is younger than P1. Otherwise, P1 is rolled back (P1 is wound by P0). • Wait-Die Scheme (Non-preemptive) Process P0 requests a resource held by process P1, P0 will be allowed to wait only if it has a smaller timestamp than P1, i.e. P0 is older than P1. Otherwise, P0 is rolled back (dies). Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

21. Deadlock Detection • Centralized Approach • Fully Distributed Approach Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

22. 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 Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

23. 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. Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

24. Questions & Discussion Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems

25. Thank You! Group 2: Hammad, Mann, Nidgundi, & ZhangCSE 8343 Advanced Operating Systems