Chapter 6 Process Synchronization Module 6 Process Synchronization
- Slides: 66
Chapter 6: Process Synchronization
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 6. 2 Silberschatz, Galvin and Gagne © 2005
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 6. 3 Silberschatz, Galvin and Gagne © 2005
Producer while (true) { /* produce an item and put in next. Produced */ while (count == BUFFER_SIZE) ; // do nothing buffer [in] = next. Produced; in = (in + 1) % BUFFER_SIZE; count++; } Operating System Concepts 6. 4 Silberschatz, Galvin and Gagne © 2005
Consumer while (true) { while (count == 0) ; // do nothing next. Consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--; /* consume the item in next. Consumed */ } Operating System Concepts 6. 5 Silberschatz, Galvin and Gagne © 2005
Race Condition n count++ could be implemented as register 1 = count register 1 = register 1 + 1 count = register 1 n count-- could be implemented as register 2 = count register 2 = register 2 - 1 count = register 2 n Consider this execution interleaving with “count = 5” initially: S 0: producer execute register 1 = count {register 1 = 5} S 1: producer execute register 1 = register 1 + 1 {register 1 = 6} S 2: consumer execute register 2 = count {register 2 = 5} S 3: consumer execute register 2 = register 2 - 1 {register 2 = 4} S 4: producer execute count = register 1 {count = 6 } S 5: consumer execute count = register 2 {count = 4} Operating System Concepts 6. 6 Silberschatz, Galvin and Gagne © 2005
The Critical-Section Problem n Consider n processes in a system, each with a critical section n Critical Section: a section of code in which the process may be changing common variables, updating a table, etc. n When one process is executing in its critical section, no other process should be allowed in its critical section! n The critical-section problem: design a protocol that the processes can use to cooperate safely. Operating System Concepts 6. 7 Silberschatz, Galvin and Gagne © 2005
General Structure of a typical process Pi while (true) { entry section CRITICAL SECTION exit section REMAINDER SECTION } Operating System Concepts 6. 8 Silberschatz, Galvin and Gagne © 2005
Solution to Critical-Section Problem A solution must satisfy the following 3 requirements: 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 some processes wish to enter their critical sections, then only those processes that are not executing in their remainder sections can participate in the decision on which will enter its critical section next, and this selection 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 6. 9 Silberschatz, Galvin and Gagne © 2005
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 6. 10 Silberschatz, Galvin and Gagne © 2005
Algorithm for Process Pi while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j); CRITICAL SECTION flag[i] = FALSE; REMAINDER SECTION } Operating System Concepts 6. 11 Silberschatz, Galvin and Gagne © 2005
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 Either test memory word and set value l Or swap contents of two memory words l Operating System Concepts 6. 12 Silberschatz, Galvin and Gagne © 2005
Test. Andnd. Set Instruction n Definition: boolean Test. And. Set (boolean *target) { boolean rv = *target; *target = TRUE; return rv: } Operating System Concepts 6. 13 Silberschatz, Galvin and Gagne © 2005
Solution using Test. And. Set n Shared Boolean variable lock, initialized to FALSE. n Solution: while (true) { while ( Test. And. Set (&lock )) ; // do nothing // critical section lock = FALSE; // remainder section } Operating System Concepts 6. 14 Silberschatz, Galvin and Gagne © 2005
Swap Instruction n Definition: void Swap (boolean *a, boolean *b) { boolean temp = *a; *a = *b; *b = temp; } Operating System Concepts 6. 15 Silberschatz, Galvin and Gagne © 2005
Solution using Swap n Shared Boolean variable lock initialized to FALSE; n Each process has a local Boolean variable key. n Solution: while (true) { key = TRUE; while ( key == TRUE) Swap (&lock, &key ); // critical section lock = FALSE; // remainder section } n This solution satisfies the mutual-exclusion requirement, but not the bounded-waiting requirement. Figure 6. 8 provides a solution n Implementation of atomic Test. And. Set & Swap not trivial for hardware designers, especially on multi-processor systems! Operating System Concepts 6. 16 Silberschatz, Galvin and Gagne © 2005
Semaphore n Synchronization tool that does not require busy waiting n Semaphore S – integer variable n Two standard operations modify S: wait() and signal() l Originally called P() and V() n Less complicated n Can only be accessed via two indivisible (atomic) operations l wait (S) { while S <= 0 ; // no-op S- -; } l signal (S) { S++; } Operating System Concepts 6. 17 Silberschatz, Galvin and Gagne © 2005
Semaphore as General Synchronization Tool n Counting semaphore – integer value can range over an unrestricted domain n Can be used to control access to a given resource consisting of a finite number of instances: l Semaphore S initialized to number of resources available; l Each process that wishes to use a resource issues: wait(S) l When a process releases a resource, it performs a: signal(S) l (decrements semaphore) (increments semaphore) When S = 0, all resources are being used; processes wishing to use a resource block until count becomes > 0 Operating System Concepts 6. 18 Silberschatz, Galvin and Gagne © 2005
Semaphore as General Synchronization Tool (2) n Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement l Also known as mutex locks l Provides mutual exclusion for critical-section problem: Semaphore S; // initialized to 1 wait (S); Critical Section signal (S); l Can be used to impose an order of execution. Suppose S 1 in process P 1 must execute before S 2 in process P 2. Use a binary sem. synch = 0: In P 1 execute S 1; signal(synch); Operating System Concepts In P 2: wait(synch); S 2; 6. 19 Silberschatz, Galvin and Gagne © 2005
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 critical section. l Could now have busy waiting in critical section implementation 4 But implementation code is short 4 Little busy waiting if critical section rarely occupied n Note that applications may spend lots of time in critical sections and therefore this is not a good solution. n We need an implementation that overcomes the need for busy waiting Operating System Concepts 6. 20 Silberschatz, Galvin and Gagne © 2005
Semaphore Implementation with no Busy waiting n With each semaphore there is an associated waiting queue. Each entry in a waiting queue has two data items: l value (of type integer) l a list of processes waiting on the semaphore n 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. n No guarantee that a process removed from the queue will go to the CPU immediately, as this is up to the scheduling policies! Operating System Concepts 6. 21 Silberschatz, Galvin and Gagne © 2005
Semaphore Implementation with no Busy waiting (Cont. ) n Implementation of wait: wait (S){ value--; if (value < 0) { add this process to waiting queue block(); } } n Implementation of signal: signal (S){ value++; if (value <= 0) { remove a process P from the waiting queue wakeup(P); } } Operating System Concepts 6. 22 Silberschatz, Galvin and Gagne © 2005
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); . . . wait (S); . . . signal (S); signal (Q); signal (S); n Starvation – indefinite blocking. A process may never be removed from the semaphore queue in which it is suspended. A LIFO queue may result in starvation. Operating System Concepts 6. 23 Silberschatz, Galvin and Gagne © 2005
Classical Problems of Synchronization n The following problems represent a large class of concurrency- control problem: l Bounded-Buffer Problem l Readers and Writers Problem l Dining-Philosophers Problem n Every proposed synchronization scheme is tested against those problems Operating System Concepts 6. 24 Silberschatz, Galvin and Gagne © 2005
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 6. 25 Silberschatz, Galvin and Gagne © 2005
Bounded Buffer Problem (Cont. ) n The structure of the producer process while (true) { // produce an item wait (empty); wait (mutex); // add the item to the buffer signal (mutex); signal (full); } Operating System Concepts 6. 26 Silberschatz, Galvin and Gagne © 2005
Bounded Buffer Problem (Cont. ) n The structure of the consumer process while (true) { wait (full); wait (mutex); // remove an item from buffer signal (mutex); signal (empty); // consume the removed item } Operating System Concepts 6. 27 Silberschatz, Galvin and Gagne © 2005
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 – allow multiple readers to read at the same time. Only one single writer can access the shared data at the same time. 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 6. 28 Silberschatz, Galvin and Gagne © 2005
Readers-Writers Problem (Cont. ) n The structure of a writer process while (true) { wait (wrt) ; // writing is performed signal (wrt) ; } n Writers may starve since the first ready will wait(wrt) and thus block any future writer until the last reader executes a signal(wrt). Operating System Concepts 6. 29 Silberschatz, Galvin and Gagne © 2005
Readers-Writers Problem (Cont. ) n The structure of a reader process while (true) { wait (mutex) ; readcount ++ ; if (readcount == 1) wait (wrt) ; signal (mutex) // first reader, no writing // reading is performed wait (mutex) ; readcount - - ; if (readcount == 0) signal (wrt) ; signal (mutex) ; // last reader, ok to write } Operating System Concepts 6. 30 Silberschatz, Galvin and Gagne © 2005
Dining-Philosophers Problem n Shared data l Bowl of rice (data set) l Semaphore chopstick [5] initialized to 1 Operating System Concepts 6. 31 Silberschatz, Galvin and Gagne © 2005
Dining-Philosophers Problem (Cont. ) n The structure of Philosopher i: while (true) { wait ( chopstick[i] ); wait ( chopstick[ (i + 1) % 5] ); // eat signal ( chopstick[i] ); signal (chopstick[ (i + 1) % 5] ); // think } n n May deadlock if all philosophers pick up left (right) chopstick simultaneously Any solution must be deadlock and starvation free. Operating System Concepts 6. 32 Silberschatz, Galvin and Gagne © 2005
Problems with Semaphores n Correct use of semaphore operations may be tricky n Programmer error can cause problems, for example: l interchange order: 4 signal (mutex) …. wait (mutex) 4 Results l replace signal with wait(): 4 wait (mutex) … wait (mutex) 4 Results l in violation of mutual-exclusion in deadlock Omitting of wait (mutex) or signal (mutex) (or both) 4 Results Operating System Concepts in mutual exclusion violation or deadlock 6. 33 Silberschatz, Galvin and Gagne © 2005
Monitors n A high-level abstraction that provides a convenient and effective mechanism for process synchronization n Only one process may be active within the monitor at a time monitor-name { // shared variable declarations procedure P 1 (…) { …. } … procedure Pn (…) {……} initialization code ( …. ) { … } } Operating System Concepts 6. 34 Silberschatz, Galvin and Gagne © 2005
Schematic view of a Monitor Operating System Concepts 6. 35 Silberschatz, Galvin and Gagne © 2005
Condition Variables n Needed for modeling some synchronization schemes, in conjunction with the monitor construct n condition x, y; n Only two operations can be invoked on a condition variable: l x. wait () – a process that invokes the operation is suspended. l x. signal () – resumes exactly one of processes (if any) that invoked x. wait (); has no effect if no process is suspended. (compare with signal() on semaphores) Operating System Concepts 6. 36 Silberschatz, Galvin and Gagne © 2005
Monitor with Condition Variables Operating System Concepts 6. 37 Silberschatz, Galvin and Gagne © 2005
Solution to Dining Philosophers n Restriction: a philosopher may pick up her chopsticks only if both of them are available monitor dp { 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(); // unable to get chopsticks – delay oneself } void putdown (int i) { state[i] = THINKING; // test left and right neighbors test((i + 4) % 5); test((i + 1) % 5); } Operating System Concepts 6. 38 Silberschatz, Galvin and Gagne © 2005
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 6. 39 Silberschatz, Galvin and Gagne © 2005
Solution to Dining Philosophers (cont) n Each philosopher i invokes the operations pickup() and putdown() in the following sequence: dp. pickup (i) EAT dp. putdown (i) Operating System Concepts 6. 40 Silberschatz, Galvin and Gagne © 2005
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 6. 41 Silberschatz, Galvin and Gagne © 2005
Monitor Implementation 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 6. 42 Silberschatz, Galvin and Gagne © 2005
Monitor Implementation 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 6. 43 Silberschatz, Galvin and Gagne © 2005
Synchronization Examples n Solaris n Windows XP n Linux n Pthreads Operating System Concepts 6. 44 Silberschatz, Galvin and Gagne © 2005
Solaris Synchronization n Implements a variety of locks to support multitasking, multithreading (including real-time threads), and multiprocessing n Uses adaptive mutexes (standard semaphore implemented as a spinlock) for efficiency when protecting data from short code segments n Uses condition variables and readers-writers locks when longer sections of code need access to data n Uses turnstiles (a queue structure) to order the list of threads waiting to acquire either an adaptive mutex or reader-writer lock Operating System Concepts 6. 45 Silberschatz, Galvin and Gagne © 2005
Windows XP Synchronization n Uses interrupt masks to protect access to global resources on uniprocessor systems n Uses spinlocks on multiprocessor systems n Also provides dispatcher objects which may act as either mutexes or semaphores n Dispatcher objects may also provide events l An event acts much like a condition variable n Dispatcher objects may also provide timers l Notify one (or more) threads that allotted time has expired Operating System Concepts 6. 46 Silberschatz, Galvin and Gagne © 2005
Linux Synchronization n Linux: l disables interrupts to implement short critical sections on single -processor machines l On SMP machines, spinlocks are used for short periods of time n Linux provides: l semaphores l spin locks Operating System Concepts 6. 47 Silberschatz, Galvin and Gagne © 2005
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 spin locks Operating System Concepts 6. 48 Silberschatz, Galvin and Gagne © 2005
Atomic Transactions n System Model n Log-based Recovery n Checkpoints n Concurrent Atomic Transactions Operating System Concepts 6. 49 Silberschatz, Galvin and Gagne © 2005
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 6. 50 Silberschatz, Galvin and Gagne © 2005
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 6. 51 Silberschatz, Galvin and Gagne © 2005
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 4 Data 4 Old name item name 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 6. 52 Silberschatz, Galvin and Gagne © 2005
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 6. 53 Silberschatz, Galvin and Gagne © 2005
Checkpoints n Log could become long, and recovery could take long n Checkpoints shorten log and recovery time. n Checkpoint scheme: 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 n Now recovery only includes Ti, such that Ti started executing before the most recent checkpoint, and all transactions after Ti n All other transactions already on stable storage Operating System Concepts 6. 54 Silberschatz, Galvin and Gagne © 2005
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 6. 55 Silberschatz, Galvin and Gagne © 2005
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 6. 56 Silberschatz, Galvin and Gagne © 2005
Schedule 1: T 0 then T 1 Operating System Concepts 6. 57 Silberschatz, Galvin and Gagne © 2005
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 being a write operation n If Oi, Oj consecutive are 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 6. 58 Silberschatz, Galvin and Gagne © 2005
Schedule 2: Concurrent Serializable Schedule Operating System Concepts 6. 59 Silberschatz, Galvin and Gagne © 2005
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 6. 60 Silberschatz, Galvin and Gagne © 2005
Two-phase Locking Protocol n Generally ensures conflict serializability n Each transaction issues lock and unlock requests in two phases l Growing – obtaining locks, but not release any l Shrinking – releasing locks, but not obtain any new ones n Does not prevent deadlock Operating System Concepts 6. 61 Silberschatz, Galvin and Gagne © 2005
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 6. 62 Silberschatz, Galvin and Gagne © 2005
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 n Timestamp-ordering protocol assures any conflicting read and write executed in timestamp order n Suppose Ti executes read(Q) l If TS(Ti) < W-timestamp(Q), Ti needs to read value of Q that was already overwritten l 4 read operation rejected and Ti rolled back l If TS(Ti) ≥ W-timestamp(Q) 4 read executed, R-timestamp(Q) set to max(R-timestamp(Q), TS(Ti)) Operating System Concepts 6. 63 Silberschatz, Galvin and Gagne © 2005
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 Write l If TS(Ti) < W-tiimestamp(Q), Ti attempting to write obsolete value of Q 4 Write l operation rejected, Ti rolled back 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 6. 64 Silberschatz, Galvin and Gagne © 2005
Schedule Possible Under Timestamp Protocol Operating System Concepts 6. 65 Silberschatz, Galvin and Gagne © 2005
End of Chapter 6
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