Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process Synchronization (Or The “Joys” of Concurrent.

6.2 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Overview: Process Synchronization  Background  The Critical-Section Problem  Peterson’s Solution  Semaphores  Classic Problems of Synchronization

6.3 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Background  Fact of Life 1: Concurrent access to shared data may result in data inconsistency  Fact of Life 2: Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes  Example?

6.5 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Race Condition  count++ could be implemented as register1 = count register1 = register1 + 1 count = register1  count-- could be implemented as register2 = count register2 = register2 - 1 count = register2  Possible execution (with “count = 5” initially): S0: producer executes register1 = count {register1 = 5} S1: producer executes register1 = register1 + 1 {register1 = 6} S2: consumer executes register2 = count {register2 = 5} S3: consumer executes register2 = register2 - 1 {register2 = 4} S4: producer executes count = register1 {count = 6 } S5: consumer executes count = register2 {count = 4}

6.6 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Solution to Critical-Section Problem 1.Mutual Exclusion - If process P i is executing in its critical section, then no other processes can be executing in their critical sections 2.Progress - If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely 3.Bounded Waiting - A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted Assume that each process executes at a nonzero speed No assumption concerning relative speed of the N processes

6.7 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Critical-Section Problem 1. Race Condition - When there is concurrent access to shared data and the final outcome depends upon order of execution. 2. Critical Section - Section of code where shared data is accessed. 3. Entry Section - Code that requests permission to enter its critical section. 4. Exit Section - Code that is run after exiting the critical section

6.9 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Peterson’s Solution  Two process solution  Assume that the LOAD and STORE instructions are atomic; that is, cannot be interrupted.  The two processes share two variables: int turn; Boolean flag[2]  The variable turn indicates whose turn it is to enter the critical section.  The flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process P i is ready!

6.10 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Algorithm for Process P i Does this work? Why? Can it be made simpler? Legenda: j is the index of the other process.

6.12 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Aside: How Many Shared Variables? Peterson’s mutex algorithm for two processes uses two boolean variables and one integer variable. How many variables does one need in order to achieve deadlock- free mutex? Theorem (James Burns and Nancy Lynch, 1980) N binary variables are necessary and sufficient to achieve deadlock-free mutual exclusion amongst N processes. Question: Is this good news? But....one shared register is enough under timing assumptions! See Michael Fischer’s classic algorithm.

6.13 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Aside: Fischer’s Algorithm Delay is chosen to be larger than the longest time it takes to execute an instruction. (Simulate the Uppaal demo of Fischer’s algorithm!) ! End of aside!

6.15 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Semaphore (Dijkstra)  Synchronization tool that does not require busy waiting  Semaphore S – integer variable  Two standard operations modify S: acquire() and release() Originally called P() (Proberen) and V() (Verhogen)  Can only be accessed via two indivisible (atomic) operations

6.16 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Semaphore as General Synchronization Tool  Counting semaphore – integer value can range over an unrestricted domain  Binary semaphore – integer value can range only between 0 and 1 Also known as mutex locks

6.17 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Semaphore Implementation with no Busy waiting  With each semaphore there is an associated waiting queue and a value (of type integer).  Two operations: block – place the process invoking the operation on the appropriate waiting queue. wakeup – remove one of processes in the waiting queue and place it in the ready queue.

6.18 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Semaphore Implementation with no Busy waiting (Cont.)  Implementation of acquire():  Implementation of release(): So, is the world of concurrency nice and easy?

6.19 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Deadlock and Starvation   Deadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes  Let S and Q be two binary semaphores. P 0 P 1 S.acquire(); Q.acquire(); Q.acquire(); S.acquire();. S.release(); Q.release(); Q.release(); S.release();  Starvation – indefinite blocking. A process may never be removed from the semaphore queue in which it is suspended.

6.20 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Classical Problems of Synchronization  Bounded-Buffer Problem  Readers and Writers Problem  Dining-Philosophers Problem

6.21 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Bounded-Buffer Problem  N buffers, each can hold one item  Semaphore mutex initialized to the value 1  Semaphore full initialized to the value 0  Semaphore empty initialized to the value N.

6.27 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Readers-Writers Problem  A data set is shared among a number of concurrent processes Readers – only read the data set; they do not perform any updates Writers – can both read and write.  Problem – allow multiple readers to read at the same time. Only one writer can access the shared data at the same time.  Shared Data Data set Semaphore mutex initialized to 1. (Ensures mutex when readerCount is updated.) Semaphore db initialized to 1. (Mutex for writers, and prevents writers from entering if db is being read.) Integer readerCount initialized to 0.

6.28 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Readers-Writers Problem Interface for read-write locks How would you implement acquireReadLock and releaseReadLock?

6.32 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Dining-Philosophers Problem (Dijkstra)  Shared data Bowl of rice (data set) Semaphore chopStick [5] initialized to 1

6.34 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Problems with Semaphores  Correct use of semaphore operations: mutex.acquire() …. mutex.release() mutex.wait() … mutex.wait() Omitting of mutex.wait () or mutex.release() (or both)

6.35 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Monitors (Brinch-Hansen, Hoare)  A high-level abstraction that provides a convenient and effective mechanism for process synchronization  Key property: Only one process may be active within the monitor at a time

6.38 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Condition Variables  Condition x, y;  Two operations on a condition variable: x.wait () – a process that invokes the operation is suspended. x.signal () – resumes one of the processes (if any) that invoked x.wait ()

6.41 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Solution to Dining Philosophers (cont)  Each philosopher invokes the operations takeForks(i) and returnForks(i) in the following sequence: dp.takeForks (i) EAT dp.returnForks (i)

Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process Synchronization (Or The “Joys” of Concurrent.

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Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process Synchronization (Or The “Joys” of Concurrent.

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