1. ITEC 502 컴퓨터 시스템 및 실습 Chapter 3-2: Process Synchronization Mi-Jung Choi mjchoi@postech.ac.kr DPNM Lab. Dept. of CSE, POSTECH

2. ITEC502 컴퓨터 시스템 및 실습 2 Contents 1.Monitors 2.Message Passing 3.Barriers

3. ITEC502 컴퓨터 시스템 및 실습 3 Problems with Semaphores  Be careful not to misuse the semaphore  Examples of misuse: –mutex is a semaphore which is initialized to 1 –signal (mutex) …. wait (mutex) results in a violation of the mutual exclusion requirement –wait (mutex) … wait (mutex) results in deadlock –Omitting of wait (mutex) or signal (mutex) (or both) Either mutual exclusion is violated or a deadlock will occur  To deal with such errors, the monitor may be used

4. ITEC502 컴퓨터 시스템 및 실습 4 Monitors - Concept  A high-level abstraction that provides a convenient and effective mechanism for process synchronization  Monitor is defined as an abstract data type like class in object-oriented language (C++, Java)  A monitor instance is shared among multiple processes  Only one process may be active within the monitor at a time monitor monitor-name { shared variable declarations procedure P 1 (…) { …. } … procedure P n (…) {……} Initialization code ( ….) { … } } Private Data Public Methods Initialization Method

5. ITEC502 컴퓨터 시스템 및 실습 5 Schematic view of a Monitor  Shared data  Operations  Initialization code  The shared data can be accessed by operations  Only one process is active within a monitor at a time  Programmer don’t need to code this synchronization

6. ITEC502 컴퓨터 시스템 및 실습 6 Condition Variables  Basic monitor is not sufficiently powerful for modeling some synchronization schemes  To solve, condition variables are introduced –condition x, y; –A programmer can define one or more variables of type condition  Two operations on a condition variable: –x.wait () – a process that invokes the operation is suspended –x.signal () – resumes one of processes (if any) that invoked x.wait () –x.signal() resumes exactly one suspended process –If no process is suspended, then nothing happens

7. ITEC502 컴퓨터 시스템 및 실습 7 Monitor with Condition Variables

8. ITEC502 컴퓨터 시스템 및 실습 8 Condition Variables – issues  Suppose that, when the x.signal() is invoked by a process P, there is a suspended process Q with condition x  If the suspended process Q is allowed to resume its execution, the signaling process P must wait –Otherwise both Q and P would be active simultaneously  Two possibilities: –Signal and Wait: P either waits until Q leaves the monitor or waits for another condition –Signal and Continue: Q either waits until P leaves the monitor or waits for another condition

9. ITEC502 컴퓨터 시스템 및 실습 9 Monitors (1) Example of a monitor

10. ITEC502 컴퓨터 시스템 및 실습 10 Monitors (2)  Outline of producer-consumer problem with monitors –only one monitor procedure active at one time –buffer has N slots

11. ITEC502 컴퓨터 시스템 및 실습 11 Monitors (3) Solution to producer-consumer problem in Java (part 1)

12. ITEC502 컴퓨터 시스템 및 실습 12 Monitors (4) Solution to producer-consumer problem in Java (part 2)

13. ITEC502 컴퓨터 시스템 및 실습 13 Message Passing The producer-consumer problem with N messages

14. ITEC502 컴퓨터 시스템 및 실습 14 Barriers  Use of a barrier –processes approaching a barrier –all processes but one blocked at barrier –last process arrives, all are let through

15. ITEC502 컴퓨터 시스템 및 실습 15 Summary  To deal with errors of semaphores, the monitor may be used  Monitors provides the synchronization mechanism for sharing abstract data types –Condition variables provides a method by which a monitor procedure can block its execution until it is signaled to continue