國國國國國國 國國國國國國 Chapter 6: Process Synchronization
Feb 22, 2016
國立台灣大學資訊工程學系
Chapter 6: Process Synchronization
資工系網媒所 NEWS 實驗室
Objectives
To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared dataTo present both software and hardware solutions of the critical-section problemTo introduce the concept of an atomic transaction and describe mechanisms to ensure atomicity
/762
資工系網媒所 NEWS 實驗室
Module 6: Process Synchronization
BackgroundThe Critical-Section ProblemPeterson’s SolutionSynchronization HardwareSemaphoresClassic Problems of SynchronizationMonitorsSynchronization Examples Atomic Transactions
/763
資工系網媒所 NEWS 實驗室
BackgroundConcurrent access to shared data may result in data inconsistencyMaintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processesSuppose 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.
/764
資工系網媒所 NEWS 實驗室
Producer
while (true) { /* produce an item and put in
nextProduced */ while (count == BUFFER_SIZE)
; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; count++;}
Consumer
while (true) {
while (count == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--;/* consume the item in
nextConsumed */}
/765
資工系網媒所 NEWS 實驗室
Race Conditioncount++ could be implemented as register1 = count register1 = register1 + 1 count = register1count-- could be implemented as register2 = count register2 = register2 - 1 count = register2Consider this execution interleaving with “count = 5” initially:
S0: producer execute register1 = count {register1 = 5}S1: producer execute register1 = register1 + 1 {register1 = 6} S2: consumer execute register2 = count {register2 = 5} S3: consumer execute register2 = register2 - 1 {register2 = 4} S4: producer execute count = register1 {count = 6 } S5: consumer execute count = register2 {count = 4}
/766
資工系網媒所 NEWS 實驗室
Solution to Critical-Section Problem1. Mutual Exclusion - If process Pi is executing in its critical section, then
no other processes can be executing in their critical sections2. 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 grantedAssume that each process executes at a nonzero speed No assumption concerning relative speed of the N processes
/767
資工系網媒所 NEWS 實驗室
Algorithm (1) for Process Pi Pj
do {
while (turn != i) ;
critical section
turn = j;
remainder section} while (1);
do {
while (turn != j) ;
critical section
turn = i;
remainder section} while (1);
/768
資工系網媒所 NEWS 實驗室
Algorithm (2) for Process Pi Pj
do {
flag[i] = TRUE; while (flag[j]) ;
critical section
flag[i] = false;
remainder section} while (1);
do {
flag[j] = TRUE; while (flag[i]) ;
critical section
flag[j] = false;
remainder section} while (1);
/769
資工系網媒所 NEWS 實驗室
Peterson’s Solution
Two-process solutionAssume that the LOAD and STORE instructions are atomic; that is, cannot be interrupted.The two processes share two variables:
int turn; Boolean flag[2]
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!
/7610
資工系網媒所 NEWS 實驗室
Algorithm (3) for Process Pi Pj
while (true) { flag[i] = TRUE; turn = j; while ( flag[j] && turn == j);
CRITICAL SECTION
flag[i] = FALSE;
REMAINDER SECTION }
while (true) { flag[j] = TRUE; turn = i; while ( flag[i] && turn == i);
CRITICAL SECTION
flag[j] = FALSE;
REMAINDER SECTION }
/7611
資工系網媒所 NEWS 實驗室
Synchronization HardwareMany systems provide hardware support for critical section codeUniprocessors – could disable interrupts
Currently running code would execute without preemptionGenerally too inefficient on multiprocessor systems
Operating systems using this not broadly scalable
Modern machines provide special atomic hardware instructions
Atomic = non-interruptableEither test memory word and set valueOr swap contents of two memory words
/7612
資工系網媒所 NEWS 實驗室
Solution to Critical-section Problem Using Locks
do { acquire lock
critical section release lock
remainder section } while (TRUE);
/7613
資工系網媒所 NEWS 實驗室
TestAndndSet Instruction
Definition:
boolean TestAndSet (boolean *target) { boolean rv = *target; *target = TRUE; return rv: }
/7614
資工系網媒所 NEWS 實驗室
Solution using TestAndSetShared boolean variable lock., initialized to false.Solution:
while (true) { while ( TestAndSet (&lock )) ; /* do nothing // critical section lock = FALSE; // remainder section }
/7615
資工系網媒所 NEWS 實驗室
Swap Instruction
Definition:
void Swap (boolean *a, boolean *b) { boolean temp = *a; *a = *b; *b = temp: }
/7616
資工系網媒所 NEWS 實驗室
Solution using SwapShared Boolean variable lock initialized to FALSE; Each process has a local Boolean variable key.Solution:
while (true) { key = TRUE; while ( key == TRUE) Swap (&lock, &key ); // critical section lock = FALSE; // remainder section }
/7617
資工系網媒所 NEWS 實驗室
Bounded-waiting Mutual Exclusion with TestandSet()
do { waiting[i] = TRUE; key = TRUE; while (waiting[i] && key)
key = TestAndSet(&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);
/7618
資工系網媒所 NEWS 實驗室
SemaphoreSynchronization tool that does not require busy waiting Semaphore S – integer variableTwo standard operations modify S: wait() and signal()
Originally called P() and V()Less complicatedCan only be accessed via two indivisible (atomic) operations
wait (S) { while S <= 0
; // no-op S--; }
signal (S) { S++; }
/7619
資工系網媒所 NEWS 實驗室
Semaphore as General Synchronization ToolCounting semaphore – integer value can range over an unrestricted domainBinary semaphore – integer value can range only between 0 and 1; can be simpler to implement
Also known as mutex locksCan implement a counting semaphore S as a binary semaphoreProvides mutual exclusionSemaphore mutex; // initialized to 1do {
wait (mutex); // critical Section signal (mutex);
// remainder section} while (TRUE);
/7620
資工系網媒所 NEWS 實驗室
Semaphore ImplementationMust guarantee that no two processes can execute wait () and signal () on the same semaphore at the same timeThus, implementation becomes the critical section problem where the wait and signal code are placed in the critical section.
Could now have busy waiting in critical section implementationBut implementation code is shortLittle 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.
/7621
資工系網媒所 NEWS 實驗室
With each semaphore there is an associated waiting queue. Each entry in a waiting queue has two data items:
value (of type integer) pointer to next record in the list
Two operations:block – place the process invoking the operation on the appropriate waiting queue.wakeup – remove one of processes in the waiting queue and place it in the ready queue.
Semaphore Implementation with no Busy waiting (1/2)
/7622
資工系網媒所 NEWS 實驗室
Semaphore Implementation with no Busy waiting (2/2)
Implementation of wait: wait(semaphore *S) {
S->value--; if (S->value < 0) {
add this process to S->list; block();
} }
Implementation of signal:signal(semaphore *S) {
S->value++; if (S->value <= 0) {
remove a process P from S->list; wakeup(P);
}}
/7623
資工系網媒所 NEWS 實驗室
Deadlock and StarvationDeadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processesLet 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
/7624
資工系網媒所 NEWS 實驗室
Priority Inversion
Task 1 (H)
Task 2 (M)
Task 3 (L)
Priority Inversion
Task 3 Get Semaphore
Task 1 Preempts Task 3
Task 1 Tries to get Semaphore
Task 2 Preempts Task 3
Task 3 Resumes
Task 3 Releases the Semaphore
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
/7625
資工系網媒所 NEWS 實驗室
Priority Inheritance
Task 1 (H)
Task 2 (M)
Task 3 (L)
Priority Inversion
Task 3 Get Semaphore
Task 1 Preempts Task 3
Task 1 Tries to get Semaphore(Priority of Task 3 is raised to Task 1's)
Task 3 Releases the Semaphore(Task 1 Resumes)
Task 1 Completes
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
/7626
資工系網媒所 NEWS 實驗室
Classical Problems of Synchronization
Bounded-Buffer ProblemReaders and Writers ProblemDining-Philosophers Problem
/7627
資工系網媒所 NEWS 實驗室
Bounded-Buffer Problem (1/2)N buffers, each can hold one itemSemaphore mutex initialized to the value 1Semaphore full initialized to the value 0Semaphore empty initialized to the value N.
/7628
資工系網媒所 NEWS 實驗室
Bounded Buffer Problem (2/2)
producer process
do { // produce an item wait (empty); wait (mutex);
// add the item to the buffer
signal (mutex); signal (full);} while (TRUE);
consumer process
do { wait (full); wait (mutex);
// remove an item from buffer signal (mutex); signal (empty); // consume the item
} while (TRUE);
/7629
資工系網媒所 NEWS 實驗室
Readers-Writers Problem (1/3)A data set is shared among a number of concurrent processes
Readers – only read the data set; they do not perform any updatesWriters – can both read and write
Problem – allow multiple readers to read at the same time. Only one single writer can access the shared data at the same timeShared Data
Data setSemaphore mutex initialized to 1Semaphore wrt initialized to 1Integer readcount initialized to 0
/7630
資工系網媒所 NEWS 實驗室
Readers-Writers Problem (2/3)The structure of a writer process
do { wait (wrt) ; // writing is performed
signal (wrt) ; } while (TRUE);
/7631
資工系網媒所 NEWS 實驗室
Readers-Writers Problem (3/3)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);
/7632
資工系網媒所 NEWS 實驗室
Dining-Philosophers Problem (1/2)
Shared data Bowl of rice (data set)Semaphore chopstick [5] initialized to 1
/7633
資工系網媒所 NEWS 實驗室
Dining-Philosophers Problem (2/2)
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
}
/7634
資工系網媒所 NEWS 實驗室
Problems with Semaphores Correct use of semaphore operations:
signal (mutex) …. wait (mutex)
wait (mutex) … wait (mutex)
Omitting of wait (mutex) or signal (mutex) (or both)
/7635
資工系網媒所 NEWS 實驗室
二元號誌 (1/2)- binary semaphore
二元號誌的值只限定為 0 或 1 。利用硬體對二元數值的運算支援,二元號誌的實作要比計數號誌簡單快速得多。 可以利用二元號誌來實作計數號誌。
/7636
資工系網媒所 NEWS 實驗室
二元號誌 (2/2)
計數號誌可以利用兩個二元號誌以及一個整數實作。 void wait(S) { wait(S1); C--; if (C < 0) { signal(S1); wait(S2); } signal(S1);}
void signal(S) { wait( S1); C++; if (C <= 0) signal(S2); else signal(S1);}
/7637
資工系網媒所 NEWS 實驗室
臨界區域 (1/4)- critical region 臨界區域的使用非常方便。 以下宣告一個具有共享變數 v 的臨界區域,在 B 條件式成立下,如果沒有其他行程在此臨界區域中執行,就會執行 S 敘述:
利用臨界區域來實作,程式設計師不用煩惱同步的問題,只要正確地把問題描述在臨界區域內。 有限緩衝區問題可以用臨界區域來簡單地解決同步的問題。
region v when B do S;
/7638
資工系網媒所 NEWS 實驗室
臨界區域 (2/4)
生產者與消耗者程式可以分別以臨界區域實作如下。region buffer when (count < n) { pool[in] = nextp; in = (in + 1) % n; count++;}
region buffer when (count > 0) { nextc = pool[out]; out = (out + 1) % n; count--;}
生 產 者 消 耗 者
/7639
資工系網媒所 NEWS 實驗室
臨界區域 (3/4)臨界區域 region v when B do S 可利用 mutex 、first_delay 及 second_delay 三個號誌實作。
mutex 號誌是用來確保臨界區的互斥條件成立。 如果行程因為 B 為 FALSE 而無法進入臨界區,該行程將會在號誌 first_delay 等待。 在號誌 first_delay 等待的行程重新檢查 B 值之前,會離開號誌 first_delay ,而在號誌 second_delay 等待。 分成 first_delay 與 second_delay 兩段式等待的原因,是為了要避免行程持續忙碌地檢查 B 值。 當一個行程離開了臨界區之後,可能因為執行了敘述 S 而改變了 B 的值,所以需要重新檢查。
/7640
資工系網媒所 NEWS 實驗室
臨界區域 (4/4)wait(mutex);while (!B) { first_count++; if (second_count > 0) signal(second_delay); else signal(mutex); wait(first_delay); first_count--; second_count++; if (first_count > 0) signal(first_delay); else signal(second_delay); wait(second_delay); second_count--;}S;if (first_count > 0) signal(first_delay);else if (second_count > 0) signal(second_delay);else signal(mutex);
wait(mutex);while (!B) { first_count++; if (first_count > 0) signal(first_delay); else signal(mutex); wait(first_delay); first_count--; first_count++; if (first_count > 0) signal(first_delay); else signal(first_delay); wait(first_delay); first_count--;}S;if (first_count > 0) signal(first_delay);else if (first_count > 0) signal(first_delay);else signal(mutex);
wait(mutex);while (!B) { first_count++; if (first_count > 0) signal(first_delay); else signal(mutex); wait(first_delay); first_count--;}S;if (first_count > 0) signal(first_delay);else signal(mutex);
wait(mutex);while (!B) { first_count++; signal(mutex); wait(first_delay); first_count--;}S; if (first_count > 0) signal(first_delay);else signal(mutex);
/7641
資工系網媒所 NEWS 實驗室
MonitorsA high-level abstraction that provides a convenient and effective mechanism for process synchronizationOnly one process may be active within the monitor at a time
monitor monitor-name{
// shared variable declarationsprocedure P1 (…) { …. }
…
procedure Pn (…) {……}
Initialization code ( ….) { … }…
}}
/7642
資工系網媒所 NEWS 實驗室
Schematic view of a Monitor
/7643
資工系網媒所 NEWS 實驗室
Condition Variables
condition x, y;
Two operations on a condition variable:x.wait () – a process that invokes the operation is
suspended.x.signal () – resumes one of processes (if any) that
invoked x.wait ()
/7644
資工系網媒所 NEWS 實驗室
Solution to Dining Philosophers (1/3)
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;}
void putdown (int i) { state[i] = THINKING;
// test left and right neighbors test((i + 4) % 5); test((i + 1) % 5);
}
/7645
資工系網媒所 NEWS 實驗室
Solution to Dining Philosophers (2/3)
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;}
}
/7646
資工系網媒所 NEWS 實驗室
Solution to Dining Philosophers (3/3)
Each philosopher I invokes the operations pickup() and putdown() in the following sequence:
dp.pickup (i)
EAT
dp.putdown (i)
/7647
資工系網媒所 NEWS 實驗室
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.
/7648
資工系網媒所 NEWS 實驗室
Monitor ImplementationFor each condition variable x, we have:
semaphore x_sem; // (initially = 0)int x-count = 0;
The operation x.wait can be implemented as:
x-count++;if (next_count > 0)
signal(next);else
signal(mutex);wait(x_sem);x-count--;
/7649
資工系網媒所 NEWS 實驗室
Monitor Implementation
The operation x.signal can be implemented as:
if (x-count > 0) {next_count++;signal(x_sem);wait(next);next_count--;
}
/7650
資工系網媒所 NEWS 實驗室
semaphore mutex; // (initially = 1)semaphore next; // (initially = 0)int next-count = 0;semaphore x-sem; // (initially = 0)int x-count = 0;
wait(mutex); …
body of F; …
if (next-count > 0)signal(next)
else signal(mutex);
if (x-count > 0) {next-count++;signal(x-sem);wait(next);next-count--;
}
x-count++;if (next-count > 0)
signal(next);else
signal(mutex);wait(x-sem);x-count--;
x.wait()
x.signal()
/7651
資工系網媒所 NEWS 實驗室
A Monitor to Allocate Single Resourcemonitor ResourceAllocator {
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; }
}/7652
資工系網媒所 NEWS 實驗室
Synchronization Examples
SolarisWindows XPLinuxPthreads
/7653
資工系網媒所 NEWS 實驗室
Solaris Synchronization
Implements a variety of locks to support multitasking, multithreading (including real-time threads), and multiprocessingUses adaptive mutexes for efficiency when protecting data from short code segmentsUses condition variables and readers-writers locks when longer sections of code need access to dataUses turnstiles to order the list of threads waiting to acquire either an adaptive mutex or reader-writer lock
/7654
資工系網媒所 NEWS 實驗室
Windows XP Synchronization
Uses interrupt masks to protect access to global resources on uniprocessor systemsUses spinlocks on multiprocessor systemsAlso provides dispatcher objects which may act as either mutexes and semaphoresDispatcher objects may also provide events
An event acts much like a condition variable
/7655
資工系網媒所 NEWS 實驗室
Linux Synchronization
Linux:Prior to kernel Version 2.6, disables interrupts to implement short critical sectionsVersion 2.6 and later, fully preemptive
Linux provides:semaphoresspin locks
Single processor Multiple Processor
Disable Kernel preemption
Acquire spin lock
Enable Kernel preemption
Release spin lock
/7656
資工系網媒所 NEWS 實驗室
Pthreads Synchronization
Pthreads API is OS-independentIt provides:
mutex lockscondition variables
Non-portable extensions include:read-write locksspin locks
/7657
資工系網媒所 NEWS 實驗室
Atomic Transactions
• System Model• Log-based Recovery• Checkpoints• Concurrent Atomic Transactions
/7658
資工系網媒所 NEWS 實驗室
System ModelAssures that operations happen as a single logical unit of work, in its entirety, or not at allRelated to field of database systemsChallenge is assuring atomicity despite computer system failuresTransaction - collection of instructions or operations that performs single logical function
Here we are concerned with changes to stable storage – diskTransaction is series of read and write operationsTerminated by commit (transaction successful) or abort (transaction failed) operationAborted transaction must be rolled back to undo any changes it performed
/7659
資工系網媒所 NEWS 實驗室
Transactional Memory (TM)TM provides an alternative strategy for developing a thread-save concurrent applications.A memory transaction is a sequence of memory read-write operations that are atomic.As the number of threads increases, traditional locking does not scale well.STM – Software TM
Inserting instrumentation code inside transaction blocks by a compiler
HTM – Hardware TMNeed to modify cache hierarchy and cache coherency protocol
/7660
資工系網媒所 NEWS 實驗室
Types of Storage Media
Volatile storage – information stored here does not survive system crashes
Example: main memory, cache
Nonvolatile storage – Information usually survives crashesExample: disk and tape
Stable storage – Information never lostNot actually possible, so approximated via replication or RAID to devices with independent failure modes
The goal is to assure transaction atomicity where failures cause loss of information on
volatile storage
/7661
資工系網媒所 NEWS 實驗室
Log-Based RecoveryRecord to stable storage information about all modifications by a transactionMost common is write-ahead logging
Log on stable storage, each log record describes single transaction write operation, including
Transaction nameData item nameOld valueNew value
<Ti starts> written to log when transaction Ti starts<Ti commits> written when Ti commits
Log entry must reach stable storage before operation on data occurs
/7662
資工系網媒所 NEWS 實驗室
Log-Based Recovery AlgorithmUsing the log, system can handle any volatile memory errors
Undo(Ti) restores value of all data updated by Ti
Redo(Ti) sets values of all data in transaction Ti to new values
Undo(Ti) and redo(Ti) must be idempotentMultiple executions must have the same result as one execution
If system fails, restore state of all updated data via logIf log contains <Ti starts> without <Ti commits>, undo(Ti)If log contains <Ti starts> and <Ti commits>, redo(Ti)
/7663
資工系網媒所 NEWS 實驗室
CheckpointsLog could become long, and recovery could take longCheckpoints shorten log and recovery time.Checkpoint scheme:1. Output all log records currently in volatile storage to stable
storage2. Output all modified data from volatile to stable storage3. 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
/7664
資工系網媒所 NEWS 實驗室
Concurrent Transactions
Must be equivalent to serial execution – serializabilityCould perform all transactions in critical section
Inefficient, too restrictive
Concurrency-control algorithms provide serializability
/7665
資工系網媒所 NEWS 實驗室
SerializabilityConsider two data items A and BConsider Transactions T0 and T1
Execute T0, T1 atomicallyExecution sequence called scheduleAtomically executed transaction order called serial scheduleFor N transactions, there are N! valid serial schedules
/7666
資工系網媒所 NEWS 實驗室
Serial Schedule 1: T0 then T1
/7667
資工系網媒所 NEWS 實驗室
Nonserial Schedule
Nonserial schedule allows overlapped executeResulting execution not necessarily incorrect
Consider schedule S, consecutive operations Oi, Oj
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
Then S’ with swapped order Oj Oi equivalent to S
If S can become a serial S’ via swapping nonconflicting operations
S is conflict serializable
/7668
資工系網媒所 NEWS 實驗室
Schedule 2: Concurrent Serializable Schedule
/7669
資工系網媒所 NEWS 實驗室
Locking Protocol
Ensure serializability by associating lock with each data item
Follow locking protocol for access control
LocksShared – Ti has shared-mode lock (S) on item Q, Ti can read Q but not write QExclusive – Ti has exclusive-mode lock (X) on Q, Ti can read and write Q
Require every transaction on item Q acquire appropriate lockIf lock already held, new request may have to wait
Similar to readers-writers algorithm
/7670
資工系網媒所 NEWS 實驗室
Two-phase Locking Protocol
Generally ensures conflict serializabilityEach transaction issues lock and unlock requests in two phases
Growing – obtaining locksShrinking – releasing locks
Does not prevent deadlock
/7671
資工系網媒所 NEWS 實驗室
Timestamp-based Protocols
Select order among transactions in advance – timestamp-orderingTransaction Ti associated with timestamp TS(Ti) before Ti starts
TS(Ti) < TS(Tj) if Ti entered system before Tj
TS can be generated from system clock or as logical counter incremented at each entry of transaction
Timestamps determine serializability orderIf TS(Ti) < TS(Tj), system must ensure produced schedule equivalent to serial schedule where Ti appears before Tj
/7672
資工系網媒所 NEWS 實驗室
Timestamp-based Protocol Implementation
Data item Q gets two timestampsW-timestamp(Q) – largest timestamp of any transaction that executed write(Q) successfullyR-timestamp(Q) – largest timestamp of successful read(Q)Updated whenever read(Q) or write(Q) executed
Timestamp-ordering protocol assures any conflicting read and write executed in timestamp orderSuppose Ti executes read(Q)
If TS(Ti) < W-timestamp(Q), Ti needs to read value of Q that was already overwritten
read operation rejected and Ti rolled backIf TS(Ti) ≥ W-timestamp(Q)
read executed, R-timestamp(Q) set to max(R-timestamp(Q), TS(Ti))
/7673
資工系網媒所 NEWS 實驗室
Timestamp-ordering Protocol
Suppose Ti executes write(Q)If TS(Ti) < R-timestamp(Q), value Q produced by Ti was needed previously and Ti assumed it would never be produced
Write operation rejected, Ti rolled back
If TS(Ti) < W-tiimestamp(Q), Ti attempting to write obsolete value of Q
Write operation rejected and Ti rolled back
Otherwise, write executed
Any rolled back transaction Ti is assigned new timestamp and restartedAlgorithm ensures conflict serializability and freedom from deadlock
/7674
資工系網媒所 NEWS 實驗室
Schedule Possible Under Timestamp Protocol
/7675
國立台灣大學資訊工程學系
End of Chapter 6