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Outline • Monitors • Monitors in Java • Barrier synchronization • The sleeping barber problem • Readers and Writers Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors - higher-level synchronization(Hoare, Hansen, 1974-5) • Semaphores and event-counters are low-level and error-prone • Monitors are a programming-language construct • Mutual exclusion constructs generated by the compiler. Internal data structures are invisible. • Monitors combine 2 synchronization mechanisms: • Only one process is activeat any given time inside the monitor. • Condition variables for signaling (wait / signal). Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors - Definition A monitor is a program module for concurrent programming Encapsulate shared storage and operations. A monitor has entry procedures Entry procedures are called by processes (threads); the monitor is passive. The monitor guarantees mutual exclusion for calls of entry procedures: Condition variables are defined in the monitor Condition Variables are used within entry procedures for condition synchronization. Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 3

MonitorsOnly one monitor procedure active at any given time monitorexample integeri; conditionc; entry procedurep1( ); . . . end; entry procedurep2( ); . . . end; end monitor; Slide taken from a presentation by Gadi Taubenfeld from IDC Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors: Condition variables • Monitors guarantee “automatic” mutual exclusion • Conditional variables enable signaling. • Condition variables support two operations: wait and signal • Signaling has no effect if there are no waiting threads • The monitor provides queuing for waiting procedures • When one operation waits and another signals there are two ways to proceed: • The signaled operation will execute first: signaling operation immediately followed by block() or exit_monitor (Hoare semantics) • The signaling operation is allowed to proceed (Java semantics) Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors: Wait/Signal wait (c) The executing process leaves the monitor and waits in a set associated to c, until it is released by a subsequent call signal(c); then the process accesses the monitor again and continues. signal (c): The executing process releases one arbitrary process that waits for c. Which of the two processes immediately continues its execution in the monitor depends on the variant of the signal semantics: signal-and-exit semantics (Hoare’s semantics): The process that executes signal terminates the entry procedure call and leaves the monitor. The released process enters the monitor immediately -without a state change in between and without the mutex being released ever (hand to hand transfer of the mutex from signaling process to waiting process). signal-and-wait semantics: The process that executes signal leaves the monitor and waits to re-enter the monitor. The released process enters the monitor immediately - without a state change in between (hand to hand transfer of mutex). signal-and-continue semantics (Java semantics): The process that executes signal continues execution in the monitor. The released process has to wait until the monitor is free. The state that held at the signal call may be changed meanwhile; the waiting condition has to be checked again. (no hand to hand transfer of mutex.) Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 6

Entry queue Shared data Queues associated with x, y conditions x y … operations Initialization code typemonitor-name =monitor variable declarations procedure entryP1 (…); begin … end; procedure entryP2 (…); begin … end; . . . procedure entryPn (…); begin … end; begin initialization code end Figure 6.20 Monitor with Condition Variable Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Bounded Buffer Producer/Consumer with Monitors This code only works if a signaled thread is the next to enter the monitor (Hoare) Slide taken from a presentation by Gadi Taubenfeld from IDC Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Issues if non-Hoare semantics The buffer is full, k producers (for some k>1) are waiting on the full condition variable. Now, N consumers enter the monitor one after the other, but only the first sends a signal (since count==N-1) holds for it. Therefore only a single producer is released and all others are not. The corresponding problem can occur on the empty semaphore. Slide taken from a presentation by Gadi Taubenfeld from IDC Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Issues if non-Hoare semantics (cont'd) The buffer is full, a single producer p1 sleeps on the full condition variable. A consumer executes and makes p1 ready but then another producer, p2, enters the monitor and fills the buffer. Now p1 continues its execution and adds another item to an already full buffer. Slide taken from a presentation by Gadi Taubenfeld from IDC Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors - some comments • Condition variables do not accumulate signals for later use • wait() must come before signal() in order to be signaled • No race conditions, because monitors have mutual exclusion • Complex to implement – but “difficult work” done by compiler • Implementation issues: • How to interpret nested monitors? • How to define wait, priority scheduling, timeouts, aborts? • How to handle exception conditions? • How to interact with process creation and destruction? Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Implementing Monitors with Semaphores – take 1 semaphore mutex=1; /*control access to monitor*/ semaphore c /*represents condition variable c */ void enter_monitor() { down(mutex); /*only one-at-a-time*/ } void leave() { up(mutex); /*allow other processes in*/ } void leave_with_signal(semaphore c) { /* leave with signaling c*/ up(c); /*release the condition variable, mutex not released */ } void wait(semaphore c) { /* block on a condition c */ up(mutex); /*allow other processes*/ down (c); /*block on the condition variable*/ } Any problem with this code? May deadlock. Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Implementing Monitors with Semaphores – take 1 Signal as last call in a procedure – unblocks a process that did wait and leave the monitorwithout freeing mutex.When wait resumesfrom down(c) it does notneed to acquire mutexbecause mutex was notfreed by signal(c) semaphore mutex=1; /*control access to monitor*/ semaphore c /*represents condition variable c */ void enter_monitor() { down(mutex); /*only one-at-a-time*/ } void leave() { up(mutex); /*allow other processes in*/ } void leave_with_signal(semaphore c) { /* leave with signaling c*/ up(c); /*release the condition variable, mutex not released */ } void wait(semaphore c) { /* block on a condition c */ up(mutex); /*allow other processes*/ down (c); /*block on the condition variable*/ } Hand to hand passing of mutex – but if “missed signal” (signal when no one waiting) – mutex is no freed  deadlock. Solution is that signal must distinguish between signal->wait transitions and signal->exit transitions. Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 13

Implementing Monitors with Semaphores - Correct Semaphore mutex = 1; /* control access to monitor */ Cond c; /* c = {count;semaphore} */ void enter_monitor() { down(mutex); /* only one-at-a-time */ } void leave() { up(mutex); /* allow other processes in */ } void leave_with_signal(cond c) { /* cond c is a struct */ if (c.count == 0) up(mutex); /* no waiting, just leave.. */ else { c.count--; up(c.s); } } void wait(cond c) { /* block on a condition */ c.count++; /* count waiting processes */ up(mutex); /* allow other processes */ down(c.s); /* block on the condition */ } Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Outline • Monitors • Monitors in Java • Barrier synchronization • The sleeping barber problem • Readers and writers Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors in Java (before Java 5) • Java directly adapted the Monitor model with some changes. • Procedures designated as synchronized are like entry procedures. • All Java Object instances include an implicit mutex lock. • No explicit condition variables • Only a single implicit condition variable on “this” • Signaling operations: • Wait() • Notify() • NotifyAll() Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors in Java Util Concurrent Explicit lock interface: Use when synchronized blocks don’t apply Time−outs, back−offs Assuring interruptibility Hand−over−hand locking Need multiple condition variables. Multiple condition variables per lock Condition interface with lock.newCondition() Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 17

Producer-consumer in Java class BoundedBuffer { final Lock lock = new ReentrantLock(); final Condition notFull = lock.newCondition(); final Condition notEmpty = lock.newCondition();final Object[] items = new Object[100]; int putptr, takeptr, count; public void put(Object x) throws InterruptedException { lock.lock(); try { while (count == items.length) notFull.await();items[putptr] = x; if (++putptr == items.length) putptr = 0; ++count; notEmpty.signal();} finally { lock.unlock(); } } public Object take() throws InterruptedException { lock.lock(); try { while (count == 0) notEmpty.await(); Object x = items[takeptr]; if (++takeptr == items.length) takeptr = 0; --count; notFull.signal(); return x; } finally { lock.unlock(); } } } Java7 supports multiple condition variables on a single lock using the Lock interfaceand the newCondition() factory. The java.util.concurrent.ArrayBlockingQueue class provides this functionality (with generic types) so there is no reason to implement this class. condition.await() and signal()require that the underlying lock be acquired – else throw IllegalMonitorState. Observe the pattern:lock.lock();try {…} finally { lock.unlock();}to enforce mutex using an explicit lock(instead of the implicit synchronized). Explicit locks support testing for locking and timeout. From http://www.docjar.com/docs/api/java/util/concurrent/locks/Condition.html Written by Doug Lea with assistance from members of JCP JSR-166 Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Producer-consumer in Java class Producer implements Runnable { private BoundedBuffer bb; public Producer(BoundedBuffer b) {bb = b;} void run() { while(true) { Integer item = produceItem(); bb.insert(item); } } protected Integer produceItem() { …}} class Consumer implements Runnable { private BoundedBuffer bb; public Consumer(BoundedBuffer b) {bb = b;} void run() { while(true) { int item = bb.extract(); consume(item); } } protected void consume(Integer item) { …}} class ProducerConsumer { private Producer p; private Consumer c; private BoundedBuffer b; public ProducerConsumer() { b = new BoundedBuffer(); p = new Producer(b); c = new Producer(b); } public static int main(String args[]){ (new Thread(p)).start(); (new Thread(c)).start(); }} Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Monitors in Java: comments • notify() does not have to be the last statement • wait() adds the calling Thread to the queue of waiting threads • a Thread performing notify() is not blocked - just moves one waiting Thread to state ready • once the monitor is open, all queued ready Threads (including former waiting ones) are contesting for entry (NO hand to hand passing of mutex). • To ensure correctness, wait() operations must be part of a condition-checking loop (because of the “hole” in the mutex between notify() and resume of wait()). Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Barriers • Useful for computations that proceed in phases • Use of a barrier: (a) processes approaching a barrier (b) all processes but one blocked at barrier (c) last process arrives, all are let through Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Using CyclicBarrier in Java importjava.util.concurrent.BrokenBarrierException;importjava.util.concurrent.CyclicBarrier;importjava.util.logging.Level;importjava.util.logging.Logger;publicclassCyclicBarrierExample {//Runnable task for each threadprivatestaticclass Task implementsRunnable {privateCyclicBarrier barrier;public Task(CyclicBarrier barrier) {this.barrier = barrier; }publicvoid run() {try {System.out.println(Thread.currentThread().getName() + " is waiting on barrier");barrier.await();System.out.println(Thread.currentThread().getName() + " has crossed the barrier"); } catch (InterruptedException ex) {Logger.getLogger(CyclicBarrierExample.class.getName()).log(Level.SEVERE, null, ex); } catch (BrokenBarrierException ex) {Logger.getLogger(CyclicBarrierExample.class.getName()).log(Level.SEVERE, null, ex); } } } Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Using CyclicBarrier in Java publicstaticvoid main(String args[]) {//create a CyclicBarrier with 3 parties, i.e., 3 Threads needs to call await()finalCyclicBarrier cb = newCyclicBarrier(3, newRunnable() {publicvoid run(){//This task will be executed once all threads reach barrierSystem.out.println("All parties are arrived at barrier, lets play"); } });newThread(new Task(cb), "Thread 1").start();newThread(new Task(cb), "Thread 2") .start();newThread(new Task(cb), "Thread 3") .start(); }} Output:Thread 1 is waiting on barrierThread 3 is waiting on barrierThread 2 is waiting on barrierAll parties are arrived at barrier, lets playThread 3 has crossed the barrierThread 1 has crossed the barrierThread 2 has crossed the barrier Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Typical Use of CyclicBarrier Fork and Join // Dispatch work to N workers // Then merge the results. class Solver { final int N; final float[][] data; final CyclicBarrier barrier; class Worker implements Runnable { int myRow; Worker(int row) { myRow = row; } public void run() { while (!done()) { processRow(myRow); try { barrier.await(); } catch (InterruptedException ex) { return; } catch (BrokenBarrierException ex) { return; } } } } public Solver(float[][] matrix) { data = matrix; N = matrix.length; barrier = new CyclicBarrier(N, new Runnable() { public void run() { mergeRows(...); } }); for (int i = 0; i < N; ++i) new Thread(new Worker(i)).start(); waitUntilDone(); } } Read more on the Fork-Join Framework http://gee.cs.oswego.edu/dl/cpjslides/fj.pdf Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Fetch-and-increment(w) do atomically prev:=w w:=w+1 return prev Implementing CyclicBarrier The fetch-and-increment instruction Called InterlockedIncrement in Windows Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Why can’t we just do simple? shared integer counter=0 Barrier() Fetch-and-increment(counter) await (counter == n) We want to reset the barrier and re-use it (recycle) and We want to execute a follow-up action only once. Who should reset? Beware the race condition! Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

A simple barrier using fetch-and-inc shared integer counter=0, bit go local local-go, local-counter Barrier() local-go := go local-counter := Fetch-and-increment(counter) if (local-counter == n-1) counter := 0 go := 1-go else await (local-go ≠ go) This is a busy-wait solution – look at Java solution for lock-basedhigher latency barrier: http://www.docjar.com/html/api/java/util/concurrent/CyclicBarrier.java.html Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

public class CyclicBarrier { /** * Each use of the barrier is represented as a generation instance. * The generation changes whenever the barrier is tripped, or * is reset. There can be many generations associated with threads * using the barrier - due to the non-deterministic way the lock * may be allocated to waiting threads - but only one of these * can be active at a time (the one to which <tt>count</tt> applies) * and all the rest are either broken or tripped. * There need not be an active generation if there has been a break * but no subsequent reset. */ private static class Generation { boolean broken = false; } /** The lock for guarding barrier entry */ private final ReentrantLock lock = new ReentrantLock(); /** Condition to wait on until tripped */ private final Condition trip = lock.newCondition(); /** The number of parties */ private final int parties; /* The command to run when tripped */ private final Runnable barrierCommand; /** The current generation */ private Generation generation = new Generation(); /** * Number of parties still waiting. Counts down from parties to 0 * on each generation. It is reset to parties on each new * generation or when broken. */ private int count; CyclicBarrier Java Implementation (1/4) Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 30

/** * Updates state on barrier trip and wakes up everyone. * Called only while holding lock. */ private void nextGeneration() { // signal completion of last generation trip.signalAll(); // set up next generation count = parties; generation = new Generation(); } /** * Sets current barrier generation as broken and wakes up everyone. * Called only while holding lock. */ private void breakBarrier() { generation.broken = true; count = parties; trip.signalAll(); } public CyclicBarrier(int parties, Runnable barrierAction) { if (parties <= 0) throw new IllegalArgumentException(); this.parties = parties; this.count = parties; this.barrierCommand = barrierAction; } public int await() throws InterruptedException, BrokenBarrierException { try { return dowait(false, 0L); } catch (TimeoutException toe) { throw new Error(toe); // cannot happen; } } CyclicBarrier Java Implementation (2/4) Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 31

/** * Main barrier code, covering the various policies. */ private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException { final ReentrantLock lock = this.lock; lock.lock(); try { final Generation g = generation; if (g.broken) throw new BrokenBarrierException(); if (Thread.interrupted()) { breakBarrier(); throw new InterruptedException(); } int index = --count; if (index == 0) { // tripped boolean ranAction = false; try { final Runnable command = barrierCommand; if (command != null) command.run(); ranAction = true; nextGeneration(); return 0; } finally { if (!ranAction) breakBarrier(); } } CyclicBarrier Java Implementation (3/4) Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 32

// loop until tripped, broken, interrupted, or timed out for (;;) { try { if (!timed) trip.await(); else if (nanos > 0L) nanos = trip.awaitNanos(nanos); } catch (InterruptedException ie) { if (g == generation && ! g.broken) { breakBarrier(); throw ie; } else { // We're about to finish waiting even if we had not // been interrupted, so this interrupt is deemed to // "belong" to subsequent execution. Thread.currentThread().interrupt(); } } if (g.broken) throw new BrokenBarrierException(); if (g != generation) return index; if (timed && nanos <= 0L) { breakBarrier(); throw new TimeoutException(); } } } finally { lock.unlock(); } } CyclicBarrier Java Implementation (4/4) Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 33

What makes the code complex? Interruptibility: what happens if any thread that is waiting on barrier is interrupted? Exceptions: what happens if the thread that executes the followup action throws an exception? Timeouts: how can we wait for the barrier for a limited amount of time and give up after timeout? That is – Barrier can exit for 4 different reasons: (1) tripped (good), (2) broken, (3) interrupted or (4) timed out. Introduce “broken barrier” case and “Generation” object: - Can break barrier from the outside to “free” the blocked threads. - Barrier is broken in case of interruption - Threads of different generations can co-exist CyclicBarrier Java Implementation: Notes Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 34

The Sleeping Barber Problem Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

The sleeping barber problem (cont’d) • Barber shop - one service provider; many customers • A finite waiting queue • One customer is served at a time • Service provider, barber, sleeps when no customers are waiting • Customer leaves if shop is full • Customer sleeps while waiting in queue Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

The sleeping barber: implementation #define CHAIRS 5 Semaphore customers = 0; // number of waiting customers Semaphore barbers = 0; // number of available barbers: either 0 or 1 int waiting = 0; // copy of customers for reading Semaphore mutex = 1; // mutex for accessing ‘waiting’ void barber() { while (TRUE) { down(customers); // block if no customers down(mutex); // access to ‘waiting’ waiting = waiting - 1; up(barbers); // barber is in. up(mutex); // release ‘waiting’ // Synchronization complete: // One barber faces one customer. cut_hair(); } } Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

The sleeping barber: implementation (cont’d) void customer() { down(mutex); // access to `waiting’ if (waiting < CHAIRS) { waiting = waiting + 1; up(customers); // wake up barber up(mutex); // release ‘waiting’ down(barbers); // go to sleep if barbers=0 // Synchronization complete: // one customer, one barber get_haircut(); } else { up(mutex); // shop full - leave} } Any problem with this code? Two customers on chair at once Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

The sleeping barber: correct synchronization #define CHAIRS 5 Semaphore customers = 0; // number of waiting customers Semaphore barbers = 0; // number of available barbers: 0 or 1 Semaphore mutex = 1; // mutex for accessing ‘waiting’ Semaphore synch = 0; // synchronize the service operation int waiting = 0; // copy of customers for reading void barber() { while (TRUE) { down(customers); // block if no customers down(mutex); // access to ‘waiting’ waiting = waiting - 1; up(barbers); // barber is in.. up(mutex); // release ‘waiting’ cut_hair(); down(synch) //wait for customer to leave } } Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

The sleeping barber: correct synchronization (cont’d) void customer() { down(mutex); // access to `waiting’ if (waiting < CHAIRS) { waiting = waiting + 1; up(customers); // wake up barber up(mutex); // release ‘waiting’ down(barbers); // go to sleep if barbers=0 get_haircut(); up(sync); // synchronize service // customer leaves chair} else { up(mutex); // shop full .. leave} } Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

The sleeping barberinJava - Barber public class Barber implements Runnable { private Queue<Customer> customers; private Shop shop; public Barber(Shop shop) { this.shop = shop; customers = new LinkedBlockingQueue<Customer>(3);} public void addCustomerToQueue(Customer customer) { customers.add(customer);} public void run() { while (! customers.isEmpty()) { Customer customer = customers.remove(); performHaircut(customer); shop.releaseChair(); } } private void performHaircut(Customer customer) { System.out.println("Customer " + customer.getId() + " has got a haircut"); } } http://bluepi.in/blogs/2013/03/18/actors-in-action-sleeping-barber-problem-with-akka/ Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 41

The sleeping barberinJava - Shop public class Shop { private Barber barber; private Semaphore semaphore; private AtomicInteger count = new AtomicInteger(0); // customers served public Shop() { semaphore = new Semaphore(3); // How many chairs in shop } public void init() { barber = new Barber(this); Executors.newSingleThreadScheduledExecutor() .scheduleAtFixedRate(barber, 100, 1000, TimeUnit.MILLISECONDS); }public void acquireChair(Customer customer) throws InterruptedException { boolean acquired = semaphore.tryAcquire(1, 100, TimeUnit.MILLISECONDS); if (acquired) { count.getAndIncrement(); barber.addCustomerToQueue(customer); } else { System.out.println("Turning customer " + customer.getId() + " away"); } } public void releaseChair() { semaphore.release(1); } public int getSuccessfulHaircutCount() { return count.intValue(); } } Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 42

The sleeping barberinJava - Notes Barber is an active object (Runnable) which works in the context of a Shop. Shop activates Barber in an executor. Shop at the time of initialization passes itself as a callback to the Barber so that all the activities performed by Barber can be recorded by the shop. Shop in the init method starts the barber in a periodic executor to periodically invoke run method. This is instead of the “while (true)” loop of the naïve code (reduces contention on the blockingQueue when many customers arrive at once). When a customer arrives, it tries to acquire a chair which is represented by a Semaphore(N), N is the number of spots in the waiting room of the shop. No more than N customers can wait at any given point. Note how we use the “tryAcquire” method of the semaphore with timeout instead of testing explicitly for a counter as in the naïve code. Note usage of AtomicInteger to safely count served customers. Questions: Complete code that simulates arrival of Customers. Why is init() needed in Shop? (Review “Escape reference” problem from SPL) Is the Barber queue used in a safe manner? (Any possible conflict between thread inserting new Customers in addCustomerToQueue() and Barber taking them in run()?) Write a simpler version exploiting the BlockingQueue synchronization capabilities – compare performance. How does the size of the waiting room impact overall performance? Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels 43

The readers and writers problem • Motivation: database access • Two groups of processes: readers, writers • Multiple readers may access database simultaneously (no conflicts on read-only) • A writing process needs exclusive database access Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Readers and Writers: 1st algorithm Int rc = 0 // # of reading processes semaphore mutex = 1; // controls access to rc semaphore db = 1; // controls database access void reader() { while (TRUE) { down(mutex); rc = rc + 1; if (rc == 1) down(db); up(mutex); read_data_base(); down(mutex); rc = rc - 1; if (rc == 0) up(db); up(mutex); } } void writer() { while(TRUE) { down(db); write_data_base() up(db) } Who is more likely to run: readers or writers? Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Comments on 1st algorithm • No reader is kept waiting, unless a writer has already obtained the db semaphore • Writerprocesses may starve - if readers keep coming in and hold the semaphore db • An alternative version of the readers-writers problem requires that no writer is kept waiting once it is “ready” - when a writer is waiting, no new reader can start reading Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Readers and Writers: Writers priority Int rc, wc = 0 // # of reading/writing processes semaphore Rmutex, Wmutex = 1; // controls readers/writers access to rc/wc semaphore Rdb, Wdb = 1; // controls readers/writers database access void reader() { while(TRUE) { down(Rdb); down(Rmutex); rc = rc + 1; if(rc == 1) down(Wdb); up(Rmutex); up(Rdb) read_data_base(); down(Rmutex); rc = rc - 1; if (rc == 0) up(Wdb); up(Rmutex); } } void writer() { while(TRUE) { down(Wmutex); wc = wc + 1; if (wc == 1) down (Rdb); up(Wmutex); down(Wdb); write_data_base(); up(Wdb); down(Wmutex); wc = wc-1; if (wc == 0) up(Rdb); up(Wmutex); }} Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Comments on 2nd algorithm • When Readers are holding Wdb, the first Writer to arrive decreases Rdb • All Readers arriving later are blocked on Rdb • All Writers arriving later are blocked on Wdb • Only the last Writer to leave Wdb releases Rdb – Readers can wait… • If a writer and a few readers are waiting on Rdb, the writer may still have to wait for these readers. If Rdb is unfair, the writer may again starve. Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

Readers and Writers: improved writers priority Int rc, wc = 0 // # of reading/writing processes semaphore Rmutex, Wmutex, Mutex2 = 1; semaphore Rdb, Wdb = 1; void reader() { while(TRUE) { down(Mutex2) down(Rdb); down(Rmutex); rc = rc + 1; if (rc == 1) down(Wdb); up(Rmutex); up(Rdb) up(Mutex2) read_data_base(); down(Rmutex); rc = rc - 1; if(rc == 0) up(Wdb); up(Rmutex); } } void writer() { while(TRUE) { down(Wmutex); wc = wc + 1 if (wc == 1) down (Rdb); up(Wmutex); down(Wdb); write_data_base(); up(Wdb); down(Wmutex); wc = wc-1; if (wc == 0) up(Rdb); up(Wmutex); } } Ben-Gurion University Operating Systems, 2013, Meni Adler, Michael Elhadad, Amnon Meisels

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