ENGR 3950U / CSCI 3020U UOIT, Fall 2012 Quiz on Process Synchronization SOLUTIONS

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1 Name: Student Number: SOLUTIONS ENGR 3950U / CSCI 3020U (Operating Systems) Quiz on Process Synchronization November 13, 2012, Duration: 40 Minutes (10 questions and 8 pages, 45 Marks) Instructor: Dr. Kamran Sartipi NOTE: for each or question, if the statement is, circle around the part that is wrong. NO explanation is required. Question 1 [4 Marks]. Answer or to each definitions below. 1.a) In concurrent processes Critical Section refers to a part of the Operating System Kernel that accesses to a shared variable or uses a shared I/O resource. 1.b) Mutual Execution is a requirements for concurrent processes such that if one of these processes is executing in its critical section to access a shared resource the other processes must not execute their critical sections to access that resource. 1.c) Race Condition is a situation in which multiple threads read or write a shared data item and the final value of the data depends on the last process that reads the shared data. 1.d) Starvation is a situation in which a process that is in the ready queue is ignored by the scheduler to be scheduled due to a sequence of higher priority processes in the readyqueue that cause a low-priority process to remain in the ready queue for a long time. Question 2 [2 Marks]. Answer or to the following statements about using synchronization hardware (TestAndSet instruction). 2.a) TestAndSet instruction causes busy waiting. 2.b) TestAndSet is applicable to any number of processes not just two processes. Page 1 of 8

2 Question 3 [4 Marks]. Answer or to the following definitions. If, draw a circle around the wrong part. 3.a) Semaphore is a structure that includes an integer variable, a queue for blocked processes, and three operations: initialization, increment and decrement. 3.b) Monitor is a programming language construct that encapsulates variables, access procedures and initialization code within the monitor. 3.c) Condition Variable is a data type that is used to block a process or thread in message passing using Unix Sockets. 3.d) Mailbox is a facility for two processes to exchange information in a synchronous manner. Question 4 [4 Marks]. Figure below shows the solution to two process mutual exclusion problem. Answer or to the following questions: 4.a) This algorithm satisfies the mutual exclusion and progress conditions but not the bounded waiting condition. 4.b) This algorithm has a flaw as the variable turn can be modified by both processes at the same time. 4.c) This algorithm may cause deadlock if both processes set their flags to at the same time. 4.d) Since variable turn can take only two values then it imposes strict alternations between two processes. Page 2 of 8

3 Question 5 [8 Marks]. Figure below shows the Bakery Algorithm. Answer to the following questions: 5.a) ( or ) This algorithm can be used for 2-process mutual-exclusion problem. 5.b) ( or ) The lines choosing[i] = true; and choosing[i]=false; provide mutual exclusion access to line number[i]=max(number(0).)+1;. 5.c) Circle lines around the entry guard and exit guard of the critical section part in the algorithm. 5.d) In the array of choosing shown at the bottom of the figure, add a proper ticket number for the PID= e) ( or ) The above ticket numbers are not correct since two processes received ticket numbers 6. 5.f) In the space below specify the order that different processes can enter to their critical sections. Indicate each process with its PID. Page 3 of 8

4 First: PID= 5 Second: PID= 3 Third: PID= 11 Forth: PID= 13 Fifth: PID= 1 Sixth: PID= 9 Question 6 [4 Marks]. In figure below, Process D is a producer that produces items to be consumed by processes A, B and C. Semaphore s controls the consumption of the produced items. Answer or to the following questions. 6.a) In (1), S=1 means Producer D has produced one item for a consumer to use. 6.b) In (3), Process B is blocked because its time to use the CPU (quantum time) has been expired and context switching has happened. 6.c) In (6), S = - 3 means that three processes are blocked and waiting for the producer to produce data. 6.d) In (7), Producer D, after producing an item, will be put in the Blocked queue behind B and A. Page 4 of 8

5 Question 7 [4 Marks]. Figure below shows the Producer and Consumer s solution using Semaphore. Answer to the following questions. 7.a) Consider the ring-buffer shown in the figure, what are the initial values of the following Semaphores when the ring-buffer is empty? - mutex: 1 - empty: 12 - full: 0 7.b) ( or ) There is an error in the program Producer() since the statement wait(empty) must be paired with a signal(empty). 7.c) ( or ) The required condition Progress is not satisfied by this solution, as the consumer must wait for the producer when the buffer is empty. 7.d) ( or ) If the ring-buffer has 10 slots to store items, it means that in the beginning the speed of the producer can be 10 times faster than the speed of the consumer. Page 5 of 8

6 Question 8 [6 Marks]. Figure below shows the Bounded-Buffer solution for Producer and Consumer problem using the Monitor. Answer to the following questions. 8.a) ( or ) The Monitor encapsulates the critical sections of the producer and consumer processes to provide controlled access to their critical sections. 8.b) ( or ) If producer is faster than consumer, the Monitor will synchronize their speeds. 8.c) Suppose at time t, the variable count = 1 and the producer P and consumer C call producer() and consumer() operations according to the following sequence: C P C C P C P Using the line numbers shown in the above figure, indicate the sequence of lines of code that will be executed to perform the above sequence of producer and consumer operations. Complete the following list by replacing letters with line numbers. Page 6 of 8

7 C: (count = 0) P: (count = 1) C: (count = 0) C: 7.. (consumer blocked) P: (count = 1) C: (count = 0) C: 7.. (consumer blocked) P: (count = 1) C: (count = 0) Question 9 [5 Marks]. Figure below shows the solution to the Readers & Writers problem using semaphore. Writer W1 arrives first and enters in its critical section writing is performed. Answer to the following questions. 9.a) While the Writer process W1 is in its critical section the following Reader and Writer processes arrive in sequence:... R1 R2... W2. Determine to which semaphore ( mutex, wrt ) each process will be blocked (if they will be blocked at all). R1: wrt R2: mutex W2: wrt Page 7 of 8

8 9.b) After W1 exits its critical section, what is the order of processes that enter and exit their critical sections? R1.. R2.. W2 Question 10 [4 Marks]. The instructions counter++ and counter- - are performed by two concurrent processes P and C. The machine language implementations of these two instructions are shown below and these two instructions are not atomic. Implementation of counter++ : 1 register1 = counter 2 register1 = register counter = register1 Implementation of counter-- : 4 register2 = counter 5 register2 = register2 1 6 counter = register2 Assume counter is initially 12. Using the numbers on the left-hand side, show a sequence of instruction that will cause the counter s value to become END, Good Luck! Page 8 of 8

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