Chapter05.pdf

Chapter 5: Process Synchronization Chapter 5: Process Synchronization Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Mutex Locks Semaphores Classic Problems of Synchronization Monitors Synchronization Examples Alternative...

Chapter 5: Process Synchronization Chapter 5: Process Synchronization Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Mutex Locks Semaphores Classic Problems of Synchronization Monitors Synchronization Examples Alternative Approaches 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 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. 4 Producer while (true) { while (counter == BUFFER_SIZE); buffer[in] = next_produced; in = (in + 1) % BUFFER_SIZE; counter++; } 5 Consumer while (true) { while (counter == 0) ; next_consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; counter--; } 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 “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 {counter = 6 } S5: consumer execute counter = register2 {counter = 4} 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 8 Critical Section General structure of process Pi 9 Algorithm for Process Pi do { while (turn == j); critical section turn = j; remainder section } while (true); 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 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 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! 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); 14 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag T Turn = 0 15 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag T Turn = 1 16 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag T T Turn = 1 17 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag T T Turn = 0 18 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag T T Turn = 0 19 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag T T Turn = 0 20 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag T T Turn = 0 21 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag F T Turn = 0 22 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag F T Turn = 0 23 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag F T Turn = 0 24 Algorithm for Process Pi Process 1 Process 0 do { do { flag = true; flag = true; turn = 0; turn = 1; while (flag && turn = = 0); while (flag && turn = = 1); critical section critical section flag = false; flag = false; remainder section remainder section } while (true); } while (true); Flag F F Turn = 0 25 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 Only if a process is in the entry section can it take part in selecting a process. 3. Bounded-waiting requirement is met Process i cannot go into the critical section again while j is waiting to go into its critical section. i could only run once 26 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 27 Solution to Critical-section Problem Using Locks do { acquire lock critical section release lock remainder section } while (TRUE); 28 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”. 29 Solution using test_and_set() Shared Boolean variable lock, initialized to FALSE Solution: do { while (test_and_set(&lock)); lock = false; } while (true); 30 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. 31 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); 32 Bounded-waiting Mutual Exclusion with test_and_set do { waiting[i] = true; key = true; while (waiting[i] && key) key = test_and_set(&lock); waiting[i] = false; j = (i + 1) % n; while ((j != i) && !waiting[j]) j = (j + 1) % n; if (j == i) lock = false; else waiting[j] = false; } while (true); 33 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 34 acquire() and release() acquire() { while (!available) ; available = false;; } release() { available = true; } do { acquire lock critical section release lock remainder section } while (true); 35 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); } } 40 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 41 Problems with Semaphores Incorrect use of semaphore operations: signal (mutex) …. wait (mutex) wait (mutex) … wait (mutex) Omitting of wait (mutex) or signal (mutex) (or both) Deadlock and starvation are possible. 53 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 (…) { … } } 54 Schematic view of a Monitor 55 Alternative Approaches Transactional Memory OpenMP Functional Programming Languages 73 Transactional Memory A memory transaction is a sequence of read-write operations to memory that are performed atomically. void update() { } 74 OpenMP OpenMP is a set of compiler directives and API that support parallel progamming. void update(int value) { #pragma omp critical { count += value } } The code contained within the #pragma omp critical directive is treated as a critical section and performed atomically. 75 Functional Programming Languages Functional programming languages offer a different paradigm than procedural languages in that they do not maintain state. Variables are treated as immutable and cannot change state once they have been assigned a value. There is increasing interest in functional languages such as Erlang and Scala for their approach in handling data races. 76 Deadlocks Explain how a system uses resources. Request. Use. Release. Deadlock characterisation Resource Allocation Graphs Handling deadlocks 77 System Model System consists of resources Resource types R1, R2,..., Rm CPU cycles, memory space, I/O devices Each resource type Ri has Wi instances. Each process utilizes a resource as follows: request use release 78 Deadlock Characterization Deadlock can arise if four conditions hold simultaneously. Mutual exclusion: only one process at a time can use a resource Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task Circular wait: there exists a set {P0, P1, …, Pn} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0. 79 Resource-Allocation Graph A set of vertices V and a set of edges E. V is partitioned into two types: P = {P1, P2, …, Pn}, the set consisting of all the processes in the system R = {R1, R2, …, Rm}, the set consisting of all resource types in the system request edge – directed edge Pi → Rj assignment edge – directed edge Rj → Pi 80 Resource-Allocation Graph (Cont.) Process Resource Type with 4 instances Pi requests instance of Rj Pi Rj Pi is holding an instance of Rj Pi Rj 81 Example of a Resource Allocation Graph 82 Resource Allocation Graph With A Deadlock 83 Graph With A Cycle But No Deadlock 84 Basic Facts If graph contains no cycles  no deadlock If graph contains a cycle  if only one instance per resource type, then deadlock if several instances per resource type, possibility of deadlock 85 Methods for Handling Deadlocks Ensure that the system will never enter a deadlock state: Deadlock prevention Deadlock avoidence Allow the system to enter a deadlock state and then recover Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX 86