ch6 - Chapter 6: Process Synchronization Chapter Module 6:...

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Chapter 6: Process Synchronization Chapter 6: Process Synchronization
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6.2 Silberschatz, Galvin and Gagne ©2005 th Module 6: Process Synchronization Module 6: Process Synchronization Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Semaphores Classic Problems of Synchronization Monitors Synchronization Examples Atomic Transactions
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6.3 Silberschatz, Galvin and Gagne ©2005 th Background Background Concurrent access to shared data may result in data inconsistency Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes Suppose that we wanted to provide a solution to the consumer-producer problem that fills all the buffers rather than BUFFER_SIZE-1 items. We can do so by having an integer count that keeps track of the number of full buffers. Initially, count is set to 0. It is incremented by the producer after it produces a new buffer and is decremented by the consumer after it consumes a buffer.
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6.4 Silberschatz, Galvin and Gagne ©2005 th Producer Producer while (true) { /* produce an item and put in nextProduced */ while (count == BUFFER_SIZE) ; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; count++; }
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6.5 Silberschatz, Galvin and Gagne ©2005 th Consumer Consumer while (true) { while (count == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--; /* consume the item in nextConsumed }
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6.6 Silberschatz, Galvin and Gagne ©2005 th Race Condition 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 Consider this execution interleaving with “count = 5” initially: S0: producer execute register1 = count {register1 = 5} S1: producer execute register1 = register1 + 1 {register1 = 6} S2: consumer execute register2 = count {register2 = 5} S3: consumer execute register2 = register2 - 1 {register2 = 4} S4: producer execute count = register1 {count = 6 } S5: consumer execute count = register2 {count = 4} To guard against the race condition , we need to ensure that only one process at a time can be manipulating the variable count . We require that processes be synchronized in some way.
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6.7 Silberschatz, Galvin and Gagne ©2005 th Solution to Critical-Section Problem Solution to Critical-Section Problem The section of code implementing request to enter its critical section is entry section . The critical section may be followed by an exit section . The remaining code is the remainder section . Solution must satisfy three requirements: 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,
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ch6 - Chapter 6: Process Synchronization Chapter Module 6:...

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