Chapter 6 Process Synchronization Operating System Concepts 8

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

Module 6: Process Synchronization n n Classic Problems of Synchronization Monitors Synchronization Examples Atomic Transactions Operating System Concepts – 8 th Edition 6. 2 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. 3 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. 4 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. 5 Silberschatz, Galvin and Gagne © 2009

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

Readers-Writers Problem 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 n Problem: l 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. 7 Silberschatz, Galvin and Gagne © 2009

Readers-Writers Problem n The structure of a writer process do { wait (wrt) ; // writing is performed signal (wrt) ; 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 n The structure of a reader process 6. 8 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. 9 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 Need both to eat, then release both when done n 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. 10 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. 11 Silberschatz, Galvin and Gagne © 2009

Problems with Semaphores 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) n Deadlock and starvation Operating System Concepts – 8 th Edition 6. 12 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. 13 Silberschatz, Galvin and Gagne © 2009

Schematic view of a Monitor Operating System Concepts – 8 th Edition 6. 14 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. 15 Silberschatz, Galvin and Gagne © 2009

Monitor with Condition Variables Operating System Concepts – 8 th Edition 6. 16 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 If Q is resumed, then P must wait n 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. 17 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. 18 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; } } 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. 19 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. 20 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. 21 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. 22 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. 23 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. 24 Silberschatz, Galvin and Gagne © 2009

Synchronization Examples n Solaris n Windows XP n Linux n Pthreads Operating System Concepts – 8 th Edition 6. 25 Silberschatz, Galvin and Gagne © 2009

Solaris Synchronization n Implements a variety of locks to support multitasking, multithreading (including real-time threads), and multiprocessing n Adaptive mutexes are used to protect access to every critical data item. Solaris uses them for efficiency when protecting data from short code segments. n Uses condition variables n Readers-writers locks are used to protect data that are accessed frequently but usually accessed in read-only manner n Turnstiles are used to order the list of threads waiting to acquire either an adaptive mutex or reader-writer lock l Turnstiles are queue structures containing threads blocked on a lock Operating System Concepts – 8 th Edition 6. 26 Silberschatz, Galvin and Gagne © 2009

Windows XP Synchronization n Uses spinlocks on multiprocessor systems l Spinlocking-thread will never be preempted n Also provides dispatcher objects – treads synchronize using according to different mechanisms, including 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 nonsignaled state (thread will block) Operating System Concepts – 8 th Edition 6. 27 Silberschatz, Galvin and Gagne © 2009

Linux Synchronization n Linux: l Prior to Version 2. 6, Linux was a non-preemptive kernel Version 2. 6 and later, fully preemptive n Linux provides: l semaphores l spinlocks l reader-writer versions of both n On single-cpu system, spinlocks replaced by enabling and disabling kernel preemption Operating System Concepts – 8 th Edition 6. 28 Silberschatz, Galvin and Gagne © 2009

Pthreads Synchronization n Pthreads API is OS-independent n It provides: l mutex locks l condition variables n Non-portable extensions include: l read-write locks l spinlocks Operating System Concepts – 8 th Edition 6. 29 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. 30 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. 31 Silberschatz, Galvin and Gagne © 2009

Types of Storage Media n Volatile storage – information stored here does not survive system crashes l Example: main memory, cache n Nonvolatile storage – Information usually survives crashes l Example: disk and tape n 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. 32 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. 33 Silberschatz, Galvin and Gagne © 2009

Log-Based Recovery Algorithm 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 n Undo(Ti) and redo(Ti) must be idempotent l Multiple executions must have the same result as one execution n 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. 34 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 from 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. 35 Silberschatz, Galvin and Gagne © 2009

Concurrent Transactions n Must be equivalent to serial execution – serializability n Could perform all transactions in critical section l Inefficient, too restrictive n Concurrency-control algorithms provide serializability Operating System Concepts – 8 th Edition 6. 36 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. 37 Silberschatz, Galvin and Gagne © 2009

Schedule 1: T 0 then T 1 Operating System Concepts – 8 th Edition 6. 38 Silberschatz, Galvin and Gagne © 2009

Nonserial Schedule n Nonserial schedule allows overlapped execute l Resulting execution not necessarily incorrect n Consider schedule S, operations Oi, Oj l Conflict if access same data item, with at least one write n If Oi, Oj consecutive and operations of different transactions & Oi and Oj don’t conflict l Then S’ with swapped order Oj Oi equivalent to S n If S can become S’ via swapping nonconflicting operations l S is conflict serializable Operating System Concepts – 8 th Edition 6. 39 Silberschatz, Galvin and Gagne © 2009

Schedule 2: Concurrent Serializable Schedule Operating System Concepts – 8 th Edition 6. 40 Silberschatz, Galvin and Gagne © 2009

Locking Protocol n Ensure serializability by associating lock with each data item l Follow locking protocol for access control n 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. 41 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 l Growing – obtaining locks l Shrinking – releasing locks n Does not prevent deadlock Operating System Concepts – 8 th Edition 6. 42 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 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 n 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. 43 Silberschatz, Galvin and Gagne © 2009

Schedule Possible Under Timestamp Protocol Operating System Concepts – 8 th Edition 6. 44 Silberschatz, Galvin and Gagne © 2009

Synchronization using semaphores n Operating System Concepts – 8 th Edition 6. 45 Silberschatz, Galvin and Gagne © 2009

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