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 nextProduced */ while (count == BUFFER_SIZE) ; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; count++; }

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

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

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

Race Condition Consider this execution interleaving with “count = 5” initially Producer Consumer register1 = count {register1 = 5} register1 = register1 + 1 {register1 = 6} count = register1 {count = 6 } R register2 = count {register2 = 5} register2 = register2 - 1 {register2 = 4} count = register2 {count = 4} R w w

Race Condition Consider this execution interleaving with “count = 5” initially Producer Consumer register1 = count {register1 = 5} register1 = register1 + 1 {register1 = 6} count = register1 {count = 6 } R register2 = count {register2 = 6} register2 = register2 - 1 {register2 = 5} count = register2 {count = 5} w R w

深入思考 使用Count實現 使用in, out實現 二個process都會對Count進行讀寫 節省記憶體的使用量 只有一個process會對in進行寫入，另一個process對in只會讀取 只有一個process會對out進行寫入，另一個process對out只會讀取 增加平行度

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 Mutual Exclusion “計算結果”的正確性 Progress 有效率的使用資源 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*/ }

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

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

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

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

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

TestAndndSet Instruction Definition: boolean TestAndSet (boolean *target) { boolean rv = *target; *target = TRUE; return rv: }

Solution using TestAndSet Shared boolean variable lock., initialized to false. while (true) { 　while ( TestAndSet (&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 “一個較為抽象化的同步運算子”

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

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

Semaphore The definition of semaphore S: S=0 S>0 S<0 S: 有多少人可以進入到critical section S=0 沒有人 S>0 有S個人可以進去 S<0 有S個人已經進去（需要等他們都離開） 有|S|的人在等待

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() } } Signal (S) { value++; if (value <= 0) { remove a process P from the waiting queue wakeup(P); } } (coding in assembly)