itec 502 컴퓨터 시스템 및 실습 chapter 3-2: process synchronization mi-jung choi...

ITEC 502 컴퓨터 시스템 및 실습

Chapter 3-2:Process Synchronization

Mi-Jung Choimjchoi@postech.ac.kr

DPNM Lab. Dept. of CSE, POSTECH

ITEC502 컴퓨터 시스템 및 실습

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Contents

1. Monitors2. Message Passing3. Barriers

ITEC502 컴퓨터 시스템 및 실습

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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

ITEC502 컴퓨터 시스템 및 실습

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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 P1 (…) { …. }

…

procedure Pn (…) {……}

Initialization code ( ….) { … }

}

Private Data

Public Methods

Initialization Method

ITEC502 컴퓨터 시스템 및 실습

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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

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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 conditio

n 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.w

ait () – x.signal() resumes exactly one suspended process– If no process is suspended, then nothing happens

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Monitor with Condition Variables

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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 simultaneousl

y 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

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Monitors (1)

Example of a monitor

ITEC502 컴퓨터 시스템 및 실습

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Monitors (2)

Outline of producer-consumer problem with monitors– only one monitor procedure active at one time– buffer has N slots

ITEC502 컴퓨터 시스템 및 실습

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Monitors (3)

Solution to producer-consumer problem in Java (part 1)

ITEC502 컴퓨터 시스템 및 실습

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Monitors (4)

Solution to producer-consumer problem in Java (part 2)

ITEC502 컴퓨터 시스템 및 실습

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Message Passing

The producer-consumer problem with N messages

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Barriers

Use of a barrier– processes approaching a barrier– all processes but one blocked at barrier– last process arrives, all are let through

ITEC502 컴퓨터 시스템 및 실습

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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