Bài giảng Hệ điều hành nâng cao - Chapter 6: Process Synchronization

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Bài giảng Hệ điều hành nâng cao - Chapter 6: Process Synchronization trình bày về những vấn đề cơ bản của đồng bộ hóa, giải pháp của Peterson, phần cứng đồng bộ, vấn đề cổ điển của đồng bộ, màn hình, đồng bộ hóa, giao dịch nguyên tử,...

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Nội dung Text: Bài giảng Hệ điều hành nâng cao - Chapter 6: Process Synchronization

Chapter 6: Process Synchronization Operating System Concepts – 8th Edition Operating System Concepts – 8th Edition 6.1 Silberschatz, Galvin and Gagne ©2009
Module 6: Process Synchronization s Background s The Critical-Section Problem s Peterson’s Solution s Synchronization Hardware s Semaphores s Classic Problems of Synchronization s Monitors s Synchronization Examples s Atomic Transactions Operating System Concepts – 8th Edition 6.2 Silberschatz, Galvin and Gagne ©2009
Objectives s To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared data s To present both software and hardware solutions of the critical-section problem s To introduce the concept of an atomic transaction and describe mechanisms to ensure atomicity Operating System Concepts – 8th Edition 6.3 Silberschatz, Galvin and Gagne ©2009
Background s Concurrent access to shared data may result in data inconsistency s Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes s Suppose that we wanted to provide a solution to the consumer-producer problem that fills all the buffers. 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. Operating System Concepts – 8th Edition 6.4 Silberschatz, Galvin and Gagne ©2009
Producer while (true) { /* produce an item and put in nextProduced */ while (counter == BUFFER_SIZE) ; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; counter++; } Operating System Concepts – 8th Edition 6.5 Silberschatz, Galvin and Gagne ©2009
Consumer while (true) { while (counter == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; counter--; /* consume the item in nextConsumed */ } Operating System Concepts – 8th Edition 6.6 Silberschatz, Galvin and Gagne ©2009
Race Condition s counter++ could be implemented as register1 = counter register1 = register1 + 1 counter = register1 s counter-- could be implemented as register2 = counter register2 = register2 - 1 count = register2 s Consider this execution interleaving with “count = 5” initially: S0: producer execute register1 = counter {register1 = 5} S1: producer execute register1 = register1 + 1 {register1 = 6} S2: consumer execute register2 = counter {register2 = 5} S3: consumer execute register2 = register2 - 1 {register2 = 4} S4: producer execute counter = register1 {count = 6 } S5: consumer execute counter = register2 {count = 4} Operating System Concepts – 8th Edition 6.7 Silberschatz, Galvin and Gagne ©2009
Critical Section Problem s Consider system of n processes {p0, p1, … pn-1} s Each process has critical section segment of code q Process may be changing common variables, updating table, writing file, etc q When one process in critical section, no other may be in its critical section s Critical section problem is to design protocol to solve this s Each process must ask permission to enter critical section in entry section, may follow critical section with exit section, then remainder section s Especially challenging with preemptive kernels Operating System Concepts – 8th Edition 6.8 Silberschatz, Galvin and Gagne ©2009
Critical Section s General structure of process pi is Operating System Concepts – 8th Edition 6.9 Silberschatz, Galvin and Gagne ©2009
Solution to Critical-Section Problem 1. Mutual Exclusion - If process Pi 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, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely 3. Bounded Waiting - A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted Assume that each process executes at a nonzero speed No assumption concerning relative speed of the n processes Operating System Concepts – 8th Edition 6.10 Silberschatz, Galvin and Gagne ©2009
Peterson’s Solution s Two process solution s Assume that the LOAD and STORE instructions are atomic; that is, cannot be interrupted s The two processes share two variables: q int turn; q Boolean flag[2] s The variable turn indicates whose turn it is to enter the critical section s The flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process Pi is ready! Operating System Concepts – 8th Edition 6.11 Silberschatz, Galvin and Gagne ©2009
Algorithm for Process Pi do { flag[i] = TRUE; turn = j; while (flag[j] && turn == j); critical section flag[i] = FALSE; remainder section } while (TRUE); s Provable that 1. Mutual exclusion is preserved 2. Progress requirement is satisfied 3. Bounded-waiting requirement is met Operating System Concepts – 8th Edition 6.12 Silberschatz, Galvin and Gagne ©2009
Synchronization Hardware s Many systems provide hardware support for critical section code s Uniprocessors – could disable interrupts q Currently running code would execute without preemption q Generally too inefficient on multiprocessor systems 4 Operating systems using this not broadly scalable s Modern machines provide special atomic hardware instructions 4 Atomic = non-interruptable q Either test memory word and set value q Or swap contents of two memory words Operating System Concepts – 8th Edition 6.13 Silberschatz, Galvin and Gagne ©2009
Solution to Critical-section Problem Using Locks do { acquire lock critical section release lock remainder section } while (TRUE); Operating System Concepts – 8th Edition 6.14 Silberschatz, Galvin and Gagne ©2009
TestAndSet Instruction s Definition: boolean TestAndSet (boolean *target) { boolean rv = *target; *target = TRUE; return rv: } Operating System Concepts – 8th Edition 6.15 Silberschatz, Galvin and Gagne ©2009
Solution using TestAndSet s Shared boolean variable lock, initialized to FALSE s Solution: do { while ( TestAndSet (&lock )) ; // do nothing // critical section lock = FALSE; // remainder section } while (TRUE); Operating System Concepts – 8th Edition 6.16 Silberschatz, Galvin and Gagne ©2009
Swap Instruction s Definition: void Swap (boolean *a, boolean *b) { boolean temp = *a; *a = *b; *b = temp: } Operating System Concepts – 8th Edition 6.17 Silberschatz, Galvin and Gagne ©2009
Solution using Swap s Shared Boolean variable lock initialized to FALSE; Each process has a local Boolean variable key s Solution: do { key = TRUE; while ( key == TRUE) Swap (&lock, &key ); // critical section lock = FALSE; // remainder section } while (TRUE); Operating System Concepts – 8th Edition 6.18 Silberschatz, Galvin and Gagne ©2009
Bounded-waiting Mutual Exclusion with TestandSet() do { waiting[i] = TRUE; key = TRUE; while (waiting[i] && key) key = TestAndSet(&lock); waiting[i] = FALSE; // critical section j = (i + 1) % n; while ((j != i) && !waiting[j]) j = (j + 1) % n; if (j == i) lock = FALSE; else waiting[j] = FALSE; // remainder section } while (TRUE); Operating System Concepts – 8th Edition 6.19 Silberschatz, Galvin and Gagne ©2009
Semaphore s Synchronization tool that does not require busy waiting s Semaphore S – integer variable s Two standard operations modify S: wait() and signal() q Originally called P() and V() s Less complicated s Can only be accessed via two indivisible (atomic) operations q wait (S) { while S