Reader/writer problem

Reader/writer problem

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  Reader/Writer problem Name :Rinku Monani Subject : Operating Systems
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  INTRODUCTION TO PROBLEM •Multiple reader/writer activities can run simultaneously. • At any time, a reader activity may wish to read data. • Or a writer activity may wish to write or modify the data at any instant. • Simultaneous reading and writing to the data can cause inconsistency. • Therefore, to avoid this situation, a set of rules is followed, • Any number of readers can access the data simultaneously. • But during the time a writer is writing, no other reader or writer may access the shared data.
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  SHARING OF DATA •Readers:only read the data set they do not perform any updates •Writers: can both read and write
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  Active Readers
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  Active Writers
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  I. First readers/writersproblem (reader priority):  No reader will wait (for other readers to finish) even if a writer is waiting  Writer starvation possible Semaphore solution II. Second reader/writers problem (writer priority):  No new readers allowed once a writer has asked for access  Solution can be implemented using monitors
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  SEMAPHORE Definition: Semaphore is non-negativeinteger variable (It’s denoted by ‘S’). Types of semaphore: 1) Binary Semaphore(value between 0 & 1) 2) Counting Semaphore(value are 1,2,3…..) Functions are used to modify the value of semaphore variable. 1) wait(S) = S – 1 2) Signal(S) = S + 1
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  Here we haveused three semaphore variables for solving this problem: 1)Mutex initialize to 1 (Binary Semaphore) 2)Wrt initialize to 1 (Binary Semaphore) 3)Readcount initialize to 0 (Counting Semaphore SOLUTION USING SEMAPHORE Mutex: Used to lock critical section for both readers and writers. Wrt: Used to block the writers from entering the critical section. Read count : Helps counting the number of readers in the critical section and permits the writer to enter when it become zero.
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  STRUCTURE OF READERPROCESS wait (mutex); readcount ++; if (readcount == 1) wait (wrt); signal(mutex); . .READ THE OBJECT . . wait(mutex); readcount --; if readcount == 0) signal (wrt); s signal(mutex); • The mutex semaphore ensures mutual exclusion and wrt handles the writing mechanism and is common to the reader and writer process code. • As soon as readcount becomes 1, wait operation is used on wrt. • This means that a writer cannot access the object anymore. After the read operation is done, readcount is decremented. When readcount becomes 0, signal operation is used on wrt. So a writer can access the object now.
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  STRUCTURE OF WRITERPROCESS • If a writer wants to access the object, wait operation is performed on wrt. • After that no other writer can access the object. • When a writer is done writing into the object, signal operation is performed on wrt. do { // writer requests for critical section wait(wrt); // performs the write // leaves the critical section signal(wrt); }while(true);
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  THANK YOU