Chapter 6 Process Synchronization Operating System Concepts 8

Chapter 6: Process Synchronization Operating System Concepts – 8 th Edition Silberschatz, Galvin and Gagne © 2009

Process Synchronization n Chapter 6 is concerned with the topic of process synchronization among concurrently executing processes. n Concurrency is generally very hard for students to deal with correctly, and sowe have tried to introduce it and its problems with the classic process coordination problems: mutual exclusion, bounded-buffer, readers/writers, and so on. n An understanding of these problems and their solutions is part of current operating system theory and development. n Additionally, students need to understand the effect that languages have on the way that we try to solve problems, and be familiar with critical regions and monitors. Operating System Concepts – 8 th Edition 6. 2 Silberschatz, Galvin and Gagne © 2009

Module 6: Process Synchronization n n n n Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Semaphores Classic Problems of Synchronization Monitors Synchronization Examples Atomic Transactions Operating System Concepts – 8 th Edition 6. 3 Silberschatz, Galvin and Gagne © 2009

Objectives n To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared data n To present both software and hardware solutions of the critical-section problem n To introduce the concept of an atomic transaction and describe mechanisms to ensure atomicity Operating System Concepts – 8 th Edition 6. 4 Silberschatz, Galvin and Gagne © 2009

Background n Concurrent access to shared data may result in data inconsistency n Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes n 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 – 8 th Edition 6. 5 Silberschatz, Galvin and Gagne © 2009

Producer while (true) { /* produce an item and put in next. Produced */ while (counter == BUFFER_SIZE) ; // do nothing buffer [in] = next. Produced; in = (in + 1) % BUFFER_SIZE; counter++; } Operating System Concepts – 8 th Edition 6. 6 Silberschatz, Galvin and Gagne © 2009

Consumer while (true) { while (counter == 0) ; // do nothing next. Consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; counter--; /* consume the item in next. Consumed */ } Operating System Concepts – 8 th Edition 6. 7 Silberschatz, Galvin and Gagne © 2009

Race Condition n counter++ could be implemented as register 1 = counter register 1 = register 1 + 1 counter = register 1 n counter-- could be implemented as register 2 = counter register 2 = register 2 - 1 count = register 2 n Consider this execution interleaving with “count = 5” initially: S 0: producer execute register 1 = counter {register 1 = 5} S 1: producer execute register 1 = register 1 + 1 {register 1 = 6} S 2: consumer execute register 2 = counter {register 2 = 5} S 3: consumer execute register 2 = register 2 - 1 {register 2 = 4} S 4: producer execute counter = register 1 {count = 6 } S 5: consumer execute counter = register 2 {count = 4} Operating System Concepts – 8 th Edition 6. 8 Silberschatz, Galvin and Gagne © 2009

Critical Section Problem n Consider system of n processes {p 0, p 1, … pn-1} n Each process has critical section segment of code l Process may be changing common variables, updating table, writing file, etc l When one process in critical section, no other may be in its critical section n Critical section problem is to design protocol to solve this n Each process must ask permission to enter critical section in entry section, may follow critical section with exit section, then remainder section n Especially challenging with preemptive kernels Operating System Concepts – 8 th Edition 6. 9 Silberschatz, Galvin and Gagne © 2009

Critical Section n General structure of process pi is Operating System Concepts – 8 th Edition 6. 10 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 – 8 th Edition 6. 11 Silberschatz, Galvin and Gagne © 2009

Peterson’s Solution n Two process solution n Assume that the LOAD and STORE instructions are atomic; that is, cannot be interrupted n The two processes share two variables: l int turn; l Boolean flag[2] n The variable turn indicates whose turn it is to enter the critical section n 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 – 8 th Edition 6. 12 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); n Provable that 1. Mutual exclusion is preserved 2. Progress requirement is satisfied 3. Bounded-waiting requirement is met Operating System Concepts – 8 th Edition 6. 13 Silberschatz, Galvin and Gagne © 2009

Synchronization Hardware n Many systems provide hardware support for critical section code n Uniprocessors – could disable interrupts l Currently running code would execute without preemption l Generally too inefficient on multiprocessor systems 4 Operating systems using this not broadly scalable n Modern machines provide special atomic hardware instructions 4 Atomic = non-interruptable l l Either test memory word and set value Or swap contents of two memory words Operating System Concepts – 8 th Edition 6. 14 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 – 8 th Edition 6. 15 Silberschatz, Galvin and Gagne © 2009

Test. And. Set Instruction n Definition: boolean Test. And. Set (boolean *target) { boolean rv = *target; *target = TRUE; return rv: } Operating System Concepts – 8 th Edition 6. 16 Silberschatz, Galvin and Gagne © 2009

Solution using Test. And. Set n Shared boolean variable lock, initialized to FALSE n Solution: do { while ( Test. And. Set (&lock )) ; // do nothing // critical section lock = FALSE; // remainder section } while (TRUE); Operating System Concepts – 8 th Edition 6. 17 Silberschatz, Galvin and Gagne © 2009

Swap Instruction n Definition: void Swap (boolean *a, boolean *b) { boolean temp = *a; *a = *b; *b = temp: } Operating System Concepts – 8 th Edition 6. 18 Silberschatz, Galvin and Gagne © 2009

Solution using Swap n Shared Boolean variable lock initialized to FALSE; Each process has a local Boolean variable key n Solution: do { key = TRUE; while ( key == TRUE) Swap (&lock, &key ); // critical section lock = FALSE; // remainder section } while (TRUE); Operating System Concepts – 8 th Edition 6. 19 Silberschatz, Galvin and Gagne © 2009

Bounded-waiting Mutual Exclusion with Testand. Set() do { waiting[i] = TRUE; key = TRUE; while (waiting[i] && key) key = Test. And. Set(&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 – 8 th Edition 6. 20 Silberschatz, Galvin and Gagne © 2009

Semaphore Synchronization tool that does not require busy waiting n Semaphore S – integer variable n n Two standard operations modify S: wait() and signal() l Originally called P() and V() Less complicated n Can only be accessed via two indivisible (atomic) operations n l wait (S) { while S <= 0 ; // no-op S--; } l signal (S) { S++; } Operating System Concepts – 8 th Edition 6. 21 Silberschatz, Galvin and Gagne © 2009

Semaphore as General Synchronization Tool n Counting semaphore – integer value can range over an unrestricted domain n Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement l Also known as mutex locks n Can implement a counting semaphore S as a binary semaphore n Provides mutual exclusion Semaphore mutex; // initialized to 1 do { wait (mutex); // Critical Section signal (mutex); // remainder section } while (TRUE); Operating System Concepts – 8 th Edition 6. 22 Silberschatz, Galvin and Gagne © 2009

Semaphore Implementation n Must guarantee that no two processes can execute wait () and signal () on the same semaphore at the same time n Thus, implementation becomes the critical section problem where the wait and signal code are placed in the crtical section l n Could now have busy waiting in critical section implementation 4 But implementation code is short 4 Little busy waiting if critical section rarely occupied Note that applications may spend lots of time in critical sections and therefore this is not a good solution Operating System Concepts – 8 th Edition 6. 23 Silberschatz, Galvin and Gagne © 2009

Semaphore Implementation with no Busy waiting n With each semaphore there is an associated waiting queue n Each entry in a waiting queue has two data items: n l value (of type integer) l pointer to next record in the list Two operations: l block – place the process invoking the operation on the appropriate waiting queue l wakeup – remove one of processes in the waiting queue and place it in the ready queue Operating System Concepts – 8 th Edition 6. 24 Silberschatz, Galvin and Gagne © 2009

Semaphore Implementation with no Busy waiting (Cont. ) Implementation of wait: wait(semaphore *S) { S->value--; if (S->value < 0) { add this process to S->list; block(); } } n Implementation of signal: n signal(semaphore *S) { S->value++; if (S->value <= 0) { remove a process P from S->list; wakeup(P); } } Operating System Concepts – 8 th Edition 6. 25 Silberschatz, Galvin and Gagne © 2009

Deadlock and Starvation n Deadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes n Let S and Q be two semaphores initialized to 1 P 0 P 1 wait (S); wait (Q); . . . signal (S); signal (Q); n n wait (Q); wait (S); . . . signal (Q); signal (S); Starvation – indefinite blocking l 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 higherpriority process l Solved via priority-inheritance protocol Operating System Concepts – 8 th Edition 6. 26 Silberschatz, Galvin and Gagne © 2009

Classical Problems of Synchronization n Classical problems used to test newly-proposed synchronization schemes l Bounded-Buffer Problem l Readers and Writers Problem l Dining-Philosophers Problem Operating System Concepts – 8 th Edition 6. 27 Silberschatz, Galvin and Gagne © 2009

Bounded-Buffer Problem n N buffers, each can hold one item n Semaphore mutex initialized to the value 1 n Semaphore full initialized to the value 0 n Semaphore empty initialized to the value N Operating System Concepts – 8 th Edition 6. 28 Silberschatz, Galvin and Gagne © 2009

Bounded Buffer Problem (Cont. ) n The structure of the producer process do { // produce an item in nextp wait (empty); wait (mutex); // add the item to the buffer signal (mutex); signal (full); } while (TRUE); Operating System Concepts – 8 th Edition 6. 29 Silberschatz, Galvin and Gagne © 2009

Bounded Buffer Problem (Cont. ) n The structure of the consumer process do { wait (full); wait (mutex); // remove an item from buffer to nextc signal (mutex); signal (empty); // consume the item in nextc } while (TRUE); Operating System Concepts – 8 th Edition 6. 30 Silberschatz, Galvin and Gagne © 2009

Readers-Writers Problem n n A data set is shared among a number of concurrent processes l Readers – only read the data set; they do not perform any updates l Writers – can both read and write Problem – allow multiple readers to read at the same time l Only one single writer can access the shared data at the same time n Several variations of how readers and writers are treated – all involve priorities n Shared Data l Data set l Semaphore mutex initialized to 1 l Semaphore wrt initialized to 1 l Integer readcount initialized to 0 Operating System Concepts – 8 th Edition 6. 31 Silberschatz, Galvin and Gagne © 2009

Readers-Writers Problem (Cont. ) n The structure of a writer process do { wait (wrt) ; // writing is performed signal (wrt) ; } while (TRUE); Operating System Concepts – 8 th Edition 6. 32 Silberschatz, Galvin and Gagne © 2009

Readers-Writers Problem (Cont. ) n The structure of a reader process do { wait (mutex) ; readcount ++ ; if (readcount == 1) wait (wrt) ; signal (mutex) // reading is performed wait (mutex) ; readcount - - ; if (readcount == 0) signal (wrt) ; signal (mutex) ; } while (TRUE); Operating System Concepts – 8 th Edition 6. 33 Silberschatz, Galvin and Gagne © 2009

Readers-Writers Problem Variations n First variation – no reader kept waiting unless writer has permission to use shared object n Second variation – once writer is ready, it performs write asap n Both may have starvation leading to even more variations n Problem is solved on some systems by kernel providing reader-writer locks Operating System Concepts – 8 th Edition 6. 34 Silberschatz, Galvin and Gagne © 2009

Dining-Philosophers Problem n Philosophers spend their lives thinking and eating n Don’t interact with their neighbors, occasionally try to pick up 2 chopsticks (one at a time) to eat from bowl l n Need both to eat, then release both when done In the case of 5 philosophers l Shared data 4 Bowl of rice (data set) 4 Semaphore chopstick [5] initialized to 1 Operating System Concepts – 8 th Edition 6. 35 Silberschatz, Galvin and Gagne © 2009

Dining-Philosophers Problem Algorithm n The structure of Philosopher i: do { wait ( chopstick[i] ); wait ( chop. Stick[ (i + 1) % 5] ); // eat signal ( chopstick[i] ); signal (chopstick[ (i + 1) % 5] ); // think } while (TRUE); n What is the problem with this algorithm? Operating System Concepts – 8 th Edition 6. 36 Silberschatz, Galvin and Gagne © 2009

Problems with Semaphores n n Incorrect use of semaphore operations: l signal (mutex) …. wait (mutex) l wait (mutex) … wait (mutex) l Omitting of wait (mutex) or signal (mutex) (or both) Deadlock and starvation Operating System Concepts – 8 th Edition 6. 37 Silberschatz, Galvin and Gagne © 2009

Monitors A high-level abstraction that provides a convenient and effective mechanism for process synchronization n Abstract data type, internal variables only accessible by code within the procedure n Only one process may be active within the monitor at a time n But not powerful enough to model some synchronization schemes n monitor-name { // shared variable declarations procedure P 1 (…) { …. } procedure Pn (…) {……} Initialization code (…) { … } } } Operating System Concepts – 8 th Edition 6. 38 Silberschatz, Galvin and Gagne © 2009

Schematic view of a Monitor Operating System Concepts – 8 th Edition 6. 39 Silberschatz, Galvin and Gagne © 2009

Condition Variables n condition x, y; n Two operations on a condition variable: l x. wait () – a process that invokes the operation is suspended until x. signal () – resumes one of processes (if any) that invoked x. wait () 4 If no x. wait () on the variable, then it has no effect on the variable Operating System Concepts – 8 th Edition 6. 40 Silberschatz, Galvin and Gagne © 2009

Monitor with Condition Variables Operating System Concepts – 8 th Edition 6. 41 Silberschatz, Galvin and Gagne © 2009

Condition Variables Choices n If process P invokes x. signal (), with Q in x. wait () state, what should happen next? l n If Q is resumed, then P must wait Options include l Signal and wait – P waits until Q leaves monitor or waits for another condition l Signal and continue – Q waits until P leaves the monitor or waits for another condition l Both have pros and cons – language implementer can decide l Monitors implemented in Concurrent Pascal compromise 4 l P executing signal immediately leaves the monitor, Q is resumed Implemented in other languages including Mesa, C#, Java Operating System Concepts – 8 th Edition 6. 42 Silberschatz, Galvin and Gagne © 2009

Solution to Dining Philosophers monitor Dining. Philosophers { enum { THINKING; HUNGRY, EATING) state [5] ; condition self [5]; void pickup (int i) { state[i] = HUNGRY; test(i); if (state[i] != EATING) self [i]. wait; } void putdown (int i) { state[i] = THINKING; // test left and right neighbors test((i + 4) % 5); test((i + 1) % 5); } Operating System Concepts – 8 th Edition 6. 43 Silberschatz, Galvin and Gagne © 2009

Solution to Dining Philosophers (Cont. ) void test (int i) { if ( (state[(i + 4) % 5] != EATING) && (state[i] == HUNGRY) && (state[(i + 1) % 5] != EATING) ) { state[i] = EATING ; self[i]. signal () ; } } initialization_code() { for (int i = 0; i < 5; i++) state[i] = THINKING; } } Operating System Concepts – 8 th Edition 6. 44 Silberschatz, Galvin and Gagne © 2009

Solution to Dining Philosophers (Cont. ) n Each philosopher i invokes the operations pickup() and putdown() in the following sequence: Dining. Philosophers. pickup (i); EAT Dining. Philosophers. putdown (i); n No deadlock, but starvation is possible Operating System Concepts – 8 th Edition 6. 45 Silberschatz, Galvin and Gagne © 2009

Monitor Implementation Using Semaphores n Variables semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next_count = 0; n Each procedure F will be replaced by wait(mutex); … body of F; … if (next_count > 0) signal(next) else signal(mutex); n Mutual exclusion within a monitor is ensured Operating System Concepts – 8 th Edition 6. 46 Silberschatz, Galvin and Gagne © 2009

Monitor Implementation – Condition Variables n For each condition variable x, we have: semaphore x_sem; // (initially = 0) int x_count = 0; n The operation x. wait can be implemented as: x-count++; if (next_count > 0) signal(next); else signal(mutex); wait(x_sem); x-count--; Operating System Concepts – 8 th Edition 6. 47 Silberschatz, Galvin and Gagne © 2009

Monitor Implementation (Cont. ) n The operation x. signal can be implemented as: if (x-count > 0) { next_count++; signal(x_sem); wait(next); next_count--; } Operating System Concepts – 8 th Edition 6. 48 Silberschatz, Galvin and Gagne © 2009

Resuming Processes within a Monitor n If several processes queued on condition x, and x. signal() executed, which should be resumed? n FCFS frequently not adequate n conditional-wait construct of the form x. wait(c) l Where c is priority number l Process with lowest number (highest priority) is scheduled next Operating System Concepts – 8 th Edition 6. 49 Silberschatz, Galvin and Gagne © 2009

A Monitor to Allocate Single Resource monitor Resource. Allocator { boolean busy; condition x; void acquire(int time) { if (busy) x. wait(time); busy = TRUE; } void release() { busy = FALSE; x. signal(); } initialization code() { busy = FALSE; } } Operating System Concepts – 8 th Edition 6. 50 Silberschatz, Galvin and Gagne © 2009

Synchronization Examples n Solaris n Windows XP n Linux n Pthreads Operating System Concepts – 8 th Edition 6. 51 Silberschatz, Galvin and Gagne © 2009

Solaris Synchronization n Implements a variety of locks to support multitasking, multithreading (including real-time threads), and multiprocessing n Uses adaptive mutexes for efficiency when protecting data from short code segments l Starts as a standard semaphore spin-lock l If lock held, and by a thread running on another CPU, spins l If lock held by non-run-state thread, block and sleep waiting for signal of lock being released n Uses condition variables n Uses readers-writers locks when longer sections of code need access to data n Uses turnstiles to order the list of threads waiting to acquire either an adaptive mutex or reader-writer lock l n Turnstiles are per-lock-holding-thread, not per-object Priority-inheritance per-turnstile gives the running thread the highest of the priorities of the threads in its turnstile Operating System Concepts – 8 th Edition 6. 52 Silberschatz, Galvin and Gagne © 2009

Windows XP Synchronization n Uses interrupt masks to protect access to global resources on uniprocessor systems n Uses spinlocks on multiprocessor systems l n Spinlocking-thread will never be preempted Also provides dispatcher objects user-land which may act mutexes, semaphores, events, and timers l Events 4 An event acts much like a condition variable l Timers notify one or more thread when time expired l Dispatcher objects either signaled-state (object available) or non-signaled state (thread will block) Operating System Concepts – 8 th Edition 6. 53 Silberschatz, Galvin and Gagne © 2009

Linux Synchronization n Linux: l Prior to kernel Version 2. 6, disables interrupts to implement short critical sections l Version 2. 6 and later, fully preemptive Linux provides: l semaphores l spinlocks l reader-writer versions of both On single-cpu system, spinlocks replaced by enabling and disabling kernel preemption Operating System Concepts – 8 th Edition 6. 54 Silberschatz, Galvin and Gagne © 2009

Pthreads Synchronization n Pthreads API is OS-independent n It provides: n l mutex locks l condition variables Non-portable extensions include: l read-write locks l spinlocks Operating System Concepts – 8 th Edition 6. 55 Silberschatz, Galvin and Gagne © 2009

Atomic Transactions n System Model n Log-based Recovery n Checkpoints n Concurrent Atomic Transactions Operating System Concepts – 8 th Edition 6. 56 Silberschatz, Galvin and Gagne © 2009

System Model n Assures that operations happen as a single logical unit of work, in its entirety, or not at all n Related to field of database systems n Challenge is assuring atomicity despite computer system failures n Transaction - collection of instructions or operations that performs single logical function l Here we are concerned with changes to stable storage – disk l Transaction is series of read and write operations l Terminated by commit (transaction successful) or abort (transaction failed) operation l Aborted transaction must be rolled back to undo any changes it performed Operating System Concepts – 8 th Edition 6. 57 Silberschatz, Galvin and Gagne © 2009

Types of Storage Media n Volatile storage – information stored here does not survive system crashes l n Nonvolatile storage – Information usually survives crashes l n Example: main memory, cache Example: disk and tape Stable storage – Information never lost l Not actually possible, so approximated via replication or RAID to devices with independent failure modes Goal is to assure transaction atomicity where failures cause loss of information on volatile storage Operating System Concepts – 8 th Edition 6. 58 Silberschatz, Galvin and Gagne © 2009

Log-Based Recovery n Record to stable storage information about all modifications by a transaction n Most common is write-ahead logging l Log on stable storage, each log record describes single transaction write operation, including 4 Transaction name 4 Data item name 4 Old value 4 New value l <Ti starts> written to log when transaction Ti starts l <Ti commits> written when Ti commits n Log entry must reach stable storage before operation on data occurs Operating System Concepts – 8 th Edition 6. 59 Silberschatz, Galvin and Gagne © 2009

Log-Based Recovery Algorithm n n Using the log, system can handle any volatile memory errors l Undo(Ti) restores value of all data updated by Ti l Redo(Ti) sets values of all data in transaction Ti to new values Undo(Ti) and redo(Ti) must be idempotent l n Multiple executions must have the same result as one execution If system fails, restore state of all updated data via log l If log contains <Ti starts> without <Ti commits>, undo(Ti) l If log contains <Ti starts> and <Ti commits>, redo(Ti) Operating System Concepts – 8 th Edition 6. 60 Silberschatz, Galvin and Gagne © 2009

Checkpoints n Log could become long, and recovery could take long n Checkpoints shorten log and recovery time. n Checkpoint scheme: n 1. Output all log records currently in volatile storage to stable storage 2. Output all modified data from volatile to stable storage 3. Output a log record <checkpoint> to the log on stable storage Now recovery only includes Ti, such that Ti started executing before the most recent checkpoint, and all transactions after Ti All other transactions already on stable storage Operating System Concepts – 8 th Edition 6. 61 Silberschatz, Galvin and Gagne © 2009

Concurrent Transactions n Must be equivalent to serial execution – serializability n Could perform all transactions in critical section l n Inefficient, too restrictive Concurrency-control algorithms provide serializability Operating System Concepts – 8 th Edition 6. 62 Silberschatz, Galvin and Gagne © 2009

Serializability n Consider two data items A and B n Consider Transactions T 0 and T 1 n Execute T 0, T 1 atomically n Execution sequence called schedule n Atomically executed transaction order called serial schedule n For N transactions, there are N! valid serial schedules Operating System Concepts – 8 th Edition 6. 63 Silberschatz, Galvin and Gagne © 2009

Schedule 1: T 0 then T 1 Operating System Concepts – 8 th Edition 6. 64 Silberschatz, Galvin and Gagne © 2009

Nonserial Schedule n Nonserial schedule allows overlapped execute l n Consider schedule S, operations Oi, Oj l n Conflict if access same data item, with at least one write If Oi, Oj consecutive and operations of different transactions & Oi and Oj don’t conflict l n Resulting execution not necessarily incorrect Then S’ with swapped order Oj Oi equivalent to S If S can become S’ via swapping nonconflicting operations l S is conflict serializable Operating System Concepts – 8 th Edition 6. 65 Silberschatz, Galvin and Gagne © 2009

Schedule 2: Concurrent Serializable Schedule Operating System Concepts – 8 th Edition 6. 66 Silberschatz, Galvin and Gagne © 2009

Locking Protocol n Ensure serializability by associating lock with each data item l n Follow locking protocol for access control Locks l Shared – Ti has shared-mode lock (S) on item Q, Ti can read Q but not write Q l Exclusive – Ti has exclusive-mode lock (X) on Q, Ti can read and write Q n Require every transaction on item Q acquire appropriate lock n If lock already held, new request may have to wait l Similar to readers-writers algorithm Operating System Concepts – 8 th Edition 6. 67 Silberschatz, Galvin and Gagne © 2009

Two-phase Locking Protocol n Generally ensures conflict serializability n Each transaction issues lock and unlock requests in two phases n l Growing – obtaining locks l Shrinking – releasing locks Does not prevent deadlock Operating System Concepts – 8 th Edition 6. 68 Silberschatz, Galvin and Gagne © 2009

Timestamp-based Protocols n Select order among transactions in advance – timestamp-ordering n Transaction Ti associated with timestamp TS(Ti) before Ti starts n l TS(Ti) < TS(Tj) if Ti entered system before Tj l TS can be generated from system clock or as logical counter incremented at each entry of transaction Timestamps determine serializability order l If TS(Ti) < TS(Tj), system must ensure produced schedule equivalent to serial schedule where Ti appears before Tj Operating System Concepts – 8 th Edition 6. 69 Silberschatz, Galvin and Gagne © 2009

Timestamp-based Protocol Implementation n Data item Q gets two timestamps W-timestamp(Q) – largest timestamp of any transaction that executed write(Q) successfully l R-timestamp(Q) – largest timestamp of successful read(Q) l Updated whenever read(Q) or write(Q) executed Timestamp-ordering protocol assures any conflicting read and write executed in timestamp order Suppose Ti executes read(Q) l If TS(Ti) < W-timestamp(Q), Ti needs to read value of Q that was already overwritten 4 read operation rejected and Ti rolled back l If TS(Ti) ≥ W-timestamp(Q) l n n 4 read executed, R-timestamp(Q) set to max(R-timestamp(Q), TS(Ti)) Operating System Concepts – 8 th Edition 6. 70 Silberschatz, Galvin and Gagne © 2009

Timestamp-ordering Protocol n Suppose Ti executes write(Q) l If TS(Ti) < R-timestamp(Q), value Q produced by Ti was needed previously and Ti assumed it would never be produced 4 l If TS(Ti) < W-timestamp(Q), Ti attempting to write obsolete value of Q 4 l Write operation rejected, Ti rolled back Write operation rejected and Ti rolled back Otherwise, write executed n Any rolled back transaction Ti is assigned new timestamp and restarted n Algorithm ensures conflict serializability and freedom from deadlock Operating System Concepts – 8 th Edition 6. 71 Silberschatz, Galvin and Gagne © 2009

Schedule Possible Under Timestamp Protocol Operating System Concepts – 8 th Edition 6. 72 Silberschatz, Galvin and Gagne © 2009

End of Chapter 6 Operating System Concepts – 8 th Edition Silberschatz, Galvin and Gagne © 2009