Process Synchronization PDF

Process Synchronization  Process Synchronization Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Mutex Locks Semaphores Classic Problems of Synchronization Monitors 5.2  Objectives...

Process Synchronization  Process Synchronization Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Mutex Locks Semaphores Classic Problems of Synchronization Monitors 5.2  Objectives To present the concept of process synchronization. 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 examine several classical process-synchronization problems To explore several tools that are used to solve process synchronization problems 5.3  Background Processes can execute concurrently  May be interrupted at any time, partially completing execution Concurrent access to shared data may result in data inconsistency Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes Illustration of the problem: 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 counter that keeps track of the number of full buffers. Initially, counter 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. 5.4  Producer while (true) { while (counter == BUFFER_SIZE) ; buffer[in] = next_produced; in = (in + 1) % BUFFER_SIZE; counter++; } 5.5  Consumer while (true) { while (counter == 0) ; next_consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; counter--; } 5.6  Race Condition counter++ could be implemented as register1 = counter register1 = register1 + 1 counter = register1 counter-- could be implemented as register2 = counter register2 = register2 - 1 counter = register2 Consider this execution interleaving with “counter = 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 {counter = 6 } S5: consumer execute counter = register2 {counter = 4} 5.7  Critical Section Problem Consider system of n processes {p0, p1, … pn-1} Each process has critical section segment of code  Process may be changing common variables, updating table, writing file, etc  When one process in critical section, no other may be in its critical section Critical section problem is to design protocol to solve this Each process must ask permission to enter critical section in entry section, may follow critical section with exit section, then remainder section 5.8  Critical Section General structure of process Pi 5.9  Algorithm for Process Pi do { while (turn == j); critical section turn = j; remainder section } while (true); 5.10  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 5.11  Critical-Section Handling in OS Two approaches depending on if kernel is preemptive or non- preemptive  Preemptive – allows preemption of process when running in kernel mode  Non-preemptive – runs until exits kernel mode, blocks, or voluntarily yields CPU Essentially free of race conditions in kernel mode 5.12  Peterson’s Solution Good algorithmic description of solving the problem Two process solution Assume that the load and store machine-language instructions are atomic; that is, cannot be interrupted The two processes share two variables:  int turn;  Boolean flag 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! 5.13  Algorithm for Process Pi do { flag[i] = true; turn = j; while (flag[j] && turn = = j); critical section flag[i] = false; remainder section } while (true); 5.14  Peterson’s Solution (Cont.) Provable that the three CS requirement are met: 1. Mutual exclusion is preserved Pi enters CS only if: either flag[j] = false or turn = i 2. Progress requirement is satisfied 3. Bounded-waiting requirement is met 5.15  Synchronization Hardware Many systems provide hardware support for implementing the critical section code. All solutions below based on idea of locking  Protecting critical regions via locks Uniprocessors – could disable interrupts  Currently running code would execute without preemption  Generally too inefficient on multiprocessor systems  Operating systems using this not broadly scalable Modern machines provide special atomic hardware instructions  Atomic = non-interruptible  Either test memory word and set value  Or swap contents of two memory words 5.16  Solution to Critical-section Problem Using Locks do { acquire lock critical section release lock remainder section } while (TRUE); 5.17  test_and_set Instruction Definition: boolean test_and_set (boolean *target) { boolean rv = *target; *target = TRUE; return rv: } 1. Executed atomically 2. Returns the original value of passed parameter 3. Set the new value of passed parameter to “TRUE”. 5.18  Solution using test_and_set() Shared Boolean variable lock, initialized to FALSE Solution: do { while (test_and_set(&lock)) ; lock = false; } while (true); 5.19  compare_and_swap Instruction Definition: int compare _and_swap(int *value, int expected, int new_value) { int temp = *value; if (*value == expected) *value = new_value; return temp; } 1. Executed atomically 2. Returns the original value of passed parameter “value” 3. Set the variable “value” the value of the passed parameter “new_value” but only if “value” ==“expected”. That is, the swap takes place only under this condition. 5.20  Solution using compare_and_swap Shared integer “lock” initialized to 0; Solution: do { while (compare_and_swap(&lock, 0, 1) != 0) ; lock = 0; } while (true); 5.21  Mutex Locks Previous solutions are complicated and generally inaccessible to application programmers OS designers build software tools to solve critical section problem Simplest is mutex lock Protect a critical section by first acquire() a lock then release() the lock Boolean variable indicating if lock is available or not Calls to acquire() and release() must be atomic Usually implemented via hardware atomic instructions But this solution requires busy waiting This lock therefore called a spinlock 5.22  acquire() and release() acquire() { while (!available); available = false;; } release() { available = true; } do { acquire lock critical section release lock remainder section } while (true); 5.23  Semaphore Synchronization tool that provides more sophisticated ways (than Mutex locks) for process to synchronize their activities. Semaphore S – integer variable Can only be accessed via two indivisible (atomic) operations  wait() and signal()  Originally called P() and V() Definition of the wait() operation wait(S) { while (S value--; if (S->value < 0) { add this process to S->list; block(); } } signal(semaphore *S) { S->value++; if (S->value list; wakeup(P); } } 5.28  Deadlock and Starvation Deadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes Let S and Q be two semaphores initialized to 1 P0 P1 wait(S); wait(Q); wait(Q); wait(S);...... signal(S); signal(Q); signal(Q); signal(S); Starvation – indefinite blocking  A process may never be removed from the semaphore queue in which it is suspended Priority Inversion – Scheduling problem when lower-priority process holds a lock needed by higher-priority process  Solved via priority-inheritance protocol 5.29  Classical Problems of Synchronization Classical problems used to test newly-proposed synchronization schemes  Bounded-Buffer Problem  Readers and Writers Problem  Dining-Philosophers Problem  Sleeping Barber Problem Assignment 5.30  Monitors A high-level abstraction that provides a convenient and effective mechanism for process synchronization Abstract data type, internal variables only accessible by code within the procedure Only one process may be active within the monitor at a time But not powerful enough to model some synchronization schemes monitor monitor-name { // shared variable declarations procedure P1 (…) { …. } procedure Pn (…) {……} Initialization code (…) { … } } } 5.31 Schematic view of a Monitor 5.32  Condition Variables condition x, y; Two operations are allowed on a condition variable:  x.wait() – a process that invokes the operation is suspended until x.signal()  x.signal() – resumes one of processes (if any) that invoked x.wait()  If no x.wait() on the variable, then it has no effect on the variable 5.33 Monitor with Condition Variables 5.34  Condition Variables Choices If process P invokes x.signal(), and process Q is suspended in x.wait(), what should happen next?  Both Q and P cannot execute in paralel. If Q is resumed, then P must wait Options include  Signal and wait – P waits until Q either leaves the monitor or it waits for another condition  Signal and continue – Q waits until P either leaves the monitor or it waits for another condition  Both have pros and cons – language implementer can decide  Monitors implemented in Concurrent Pascal compromise  P executing signal immediately leaves the monitor, Q is resumed  Implemented in other languages including Mesa, C#, Java 5.35  Monitor Implementation Using Semaphores Variables semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next_count = 0; Each procedure F will be replaced by wait(mutex); … body of F; … if (next_count > 0) signal(next) else signal(mutex); Mutual exclusion within a monitor is ensured 5.36