Chapter 6 Process Synchronization Process Synchronization Background The

Chapter 6: Process Synchronization

Process Synchronization • • • Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Semaphores Classic Problems of Synchronization Monitors Synchronization Examples Atomic Transactions

Objectives • To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared data • To present both software and hardware solutions of the critical-section problem • To introduce the concept of an atomic transaction and describe mechanisms to ensure atomicity

Background • Concurrent access to shared data may result in data inconsistency • Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes「意義上等價於某 個執行順序即可」

Background - Example 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, the 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.

Producer while (true) { 　/* produce an item and put in next. Produced */ 　while (count == BUFFER_SIZE) ; // do nothing 　buffer [in] = next. Produced; 　in = (in + 1) % BUFFER_SIZE; 　count++; }

Consumer while (true) { 　while (count == 0) 　 ; // do nothing 　next. Consumed = buffer[out]; 　out = (out + 1) % BUFFER_SIZE; 　count--; 　/*consume the item in next. Consumed*/ }

Race condition Producer (Thread 1) Consumer (Thread 2) while (true) { 　while (count == 0) 　 ; // do nothing 　next. Consumed = buffer[out]; 　out = (out + 1) % BUFFER_SIZE; 　count--; 　/*consume the item in next. Consumed*/ } 　/* produce an item and put in next. Produced */ 　while (count == BUFFER_SIZE) ; // do nothing 　buffer [in] = next. Produced; 　in = (in + 1) % BUFFER_SIZE; 　count++; }

Race Condition • count++ could be implemented as register 1 = count register 1 = register 1 + 1 count = register 1 • count-- could be implemented as register 2 = count register 2 = register 2 - 1 count = register 2

Race Condition Consider this execution interleaving with “count = 5” initially Producer Consumer register 1 = count R {register 1 = 5} register 1 = register 1 + 1 {register 1 = 6} R register 2 = count {register 2 = 5} register 2 = register 2 - 1 {register 2 = 4} count = register 1 {count = 6 } w count = register 2 {count = 4} w

Race Condition Consider this execution interleaving with “count = 5” initially Producer Consumer register 1 = count R {register 1 = 5} register 1 = register 1 + 1 {register 1 = 6} count = register 1 w {count = 6 } R register 2 = count {register 2 = 6} register 2 = register 2 - 1 {register 2 = 5} w count = register 2 {count = 5}

有很多方法解決race condition，critical section是其中一種方式！！

Solution to Critical-Section Problem 1. Mutual Exclusion - If a process 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 one of the processes will enter the critical section 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

Solution to Critical-Section Problem 1. Mutual Exclusion – “計算結果”的正確性 2. Progress – 有效率的使用資源 3. Bounded Waiting – 在某個程度上資源的分配是公平的

Peterson’s Solution • Two process solution • Assume that the LOAD and STORE instructions are atomic; that is, cannot be interrupted. • The two processes share two variables: – int turn; – Boolean flag[2] • The variable turn indicates whose turn it is to enter the critical section. • 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!

Peterson’s Solution • 純粹軟體？

Peterson’s Solution • 純粹軟體？ • 假設CPU對memory的寫入及讀出動作是「 atomic operation」

Algorithm for Process Pi while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }

Mutual Exclusion while (true) { flag[i] = TRUE; while (true) { flag[j] = TRUE; turn = j; turn = i; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ } turn = i或j； 並且flag[i]及flag[j]已 經給了正確的� while ( flag[i] && turn == i); /*CRITICAL SECTION*/ flag[j] = FALSE; /*REMAINDER SECTION*/ }

progress while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ } while (true) { turn=j；但 flag[j]=false flag[j] = TRUE; turn = i; while ( flag[j] && turn == i); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }

Bounded Waiting while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; 展開loop (loop /*REMAINDER SECTION*/ flag[i] = FALSE; unrolling) /*REMAINDER SECTION*/ flag[i] = TRUE; } turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/

flag[j] = TRUE; Bounded Waiting flag[i] = TRUE; turn = j; turn = i; while ( flag[i] && turn == i); while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); while ( flag[i] && turn == i); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ /*CRITICAL SECTION*/ flag[i] = FALSE;

Algorithm for Process Pi while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }

Algorithm for Process Pi (? ? ? ) while (true) { turn = j; flag[i] = TRUE; while ( flag[j] && turn == j); /*CRITICAL SECTION*/ flag[i] = FALSE; /*REMAINDER SECTION*/ }

Algorithm for Process Pi while (true) { 　turn = j; 　 　 　flag[i] = TRUE; 　while ( flag[j] && turn == j); j 　　/*CRITICAL SECTION*/ 　flag[i] = FALSE; 　　/*REMAINDER SECTION*/ } Algorithm for Process Pj while (true) { 　turn = i; 　flag[j] = TRUE; 　while ( flag[i] && turn == i); 　　/*CRITICAL SECTION*/ 　flag[j] = FALSE; 　　/*REMAINDER SECTION*/ }

Synchronization Hardware • Many systems provide hardware support for critical section code • Uniprocessors – could disable interrupts – Currently running code would execute without preemption – Generally too inefficient on multiprocessor systems • Modern machines provide special atomic hardware instructions – test memory word and set value – swap contents of two memory words

Test. Andnd. Set Instruction • Definition: boolean Test. And. Set (boolean *target) { boolean rv = *target; *target = TRUE; return rv: }

Solution using Test. And. Set • Shared boolean variable lock. , initialized to false. while (true) { 　while ( Test. And. Set (&lock )) 　　; // do nothing 　　// critical section 　　lock = FALSE; 　　// remainder section }

Swap Instruction • Definition: void Swap (boolean *a, boolean *b) { 　boolean temp = *a; 　*a = *b; 　*b = temp: }

Solution using Swap • Shared Boolean variable lock initialized to FALSE; Each process has a local Boolean variable key. while (true) { 　　key = TRUE; 　　while ( key == TRUE) { 　　　Swap (&lock, &key ); } 　　　//critical section 　　　lock = FALSE; 　　　//remainder section }

Semaphore • Synchronization tool that does not require busy waiting • Semaphore S – integer variable • Two standard operations modify S: wait() and signal()

Semaphore • The definition of wait(): wait (S) { while (S <= 0) ; // no-op S--; } • The definition of signal(): signal (S) { S++; }

Semaphore as General Synchronization Tool • Counting semaphore – integer value can range over an unrestricted domain • Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement – Also known as mutex locks • Provides mutual exclusion Semaphore S; // initialized to 1 wait (S); 　　//Critical Section signal (S);

Semaphore Implementation wait (S) { while (S <= 0) ; // no-op S--; } signal (S) { S++; } Busy waiting wastes CPU time! This type of semaphore also called “spinlock” (coding in assembly)

Semaphore Implementation with no Busy waiting • With each semaphore there is an associated waiting queue. Each entry in a waiting queue has two data items: – value (of type integer) – pointer to next record in the list • Two operations: – block – place the process invoking the operation on the appropriate waiting queue. – wakeup – remove one of processes in the waiting queue and place it in the ready queue.

Semaphore Implementation with no Busy waiting wait (S){ value--; 　if (value < 0) { add this process to waiting queue block(); //krl_fn_sleep() 　} } (coding in assembly) Signal (S) { 　value++; 　if (value <= 0) { 　　remove a process P from the waiting queue 　　wakeup(P); 　} }