CS 3214 Computer Systems

1. CS 3214Computer Systems Godmar Back Lecture 22

2. Announcements • Project 4 due Nov 10 • Exercise 9 due Nov 11 CS 3214 Fall 2010

3. MULTI-THREADING CS 3214 Fall 2010

4. Coordinating Multiple Threads • Aside from coordinating access to shared items, threads may need to communicate about events • “has event A already happened in another thread?” • aka “precedence constraint”, or “scheduling constraint” • Do B after A • Must do so • Correctly (never miss that event A has occurred when in fact it has) • Efficiently • Don’t waste resources in the process • Don’t unnecessarily delay notification of event A CS 3214 Fall 2010

5. intcoin_flip; static void * thread1(void *_) { coin_flip = rand() % 2; printf("Thread 1: flipped coin %d\n", coin_flip); return NULL; } static void * thread2(void *_) { printf("Thread 2: flipped coin %d\n", coin_flip); return NULL; } int main() { inti, N = 2; pthread_t t[N]; srand(getpid()); pthread_create(&t[1], NULL, thread2, NULL); pthread_create(&t[0], NULL, thread1, NULL); for (i = 0; i < N; i++) pthread_join(t[i], NULL); return 0; } Q.: How can thread2 make sure that ‘coin_flip’ has occurred before printing its outcome? CS 3214 Fall 2010

6. intcoin_flip; volatile boolcoin_flip_done; static void * thread1(void *_) { coin_flip = rand() % 2; coin_flip_done = true; printf("Thread 1: flipped coin %d\n", coin_flip); return NULL; } Thread 2 could “busy-wait” – spin until thread 1 completes the coin flip.Exceptions not withstanding, this is practically never an acceptable solution. The somewhat less wasteful variant of busy-waiting: while (!coin_flip_done)sched_yield(); is not acceptable, either. • wastes CPU cycles • is fragile (volatile needed when using –O) • does not document semantics static void * thread2(void *_) { /* Thread 2 spins, "busy-waits" until the coin flip is done. * This is an unacceptable solution. Bad for the planet, too. */ while (!coin_flip_done) continue; printf("Thread 2: flipped coin %d\n", coin_flip); return NULL; } CS 3214 Fall 2010

7. Source: inter.scoutnet.org Semaphores • Invented by Edsger Dijkstra in 1960s • Counter S, initialized to some value, with two operations: • P(S) or “down” or “wait” – if counter greater than zero, decrement. Else wait until greater than zero, then decrement • V(S) or “up” or “signal” or “post” – increment counter, wake up any threads stuck in P. • Semaphores don’t go negative: • #V + InitialValue - #P >= 0 • Note: direct access to counter value after initialization is not allowed • Counting Semaphores vs Binary Semaphores • Binary: counter can only be 0 or 1 • Simple to implement, yet powerful • Can be used for many synchronization problems CS 3214 Fall 2010

8. intcoin_flip; sem_tcoin_flip_done; // semaphore for thread 1 to signal coin flip static void * thread1(void *_) { coin_flip = rand() % 2; sem_post(&coin_flip_done); // raise semaphore, increment, 'up' printf("Thread 1: flipped coin %d\n", coin_flip); } POSIX Semaphores Notice the 3rd argument of sem_init() – it gives the initial value of the semaphore: ‘0’ means the semaphore is used to express scheduling constraint static void * thread2(void *_) { // wait until semaphore is raised, // then decrement, 'down' sem_wait(&coin_flip_done); printf("Thread 2: flipped coin %d\n", coin_flip); } int main() { … sem_init(&coin_flip_done, 0, 0); pthread_create(&t[1], NULL, thread2, NULL); pthread_create(&t[0], NULL, thread1, NULL); … } CS 3214 Fall 2010

9. Semaphores can be used to build locks Must initialize semaphore with 1 to allow one thread to enter critical section This is not a recommended style, despite of what Bryant & O’Hallaron suggest – you should use a mutex instead [Cantrill & Bonwick 2008] Easily generalized to allow at most N simultaneous threads: multiplex pattern (i.e., a resource can be accessed by at most N threads) Implementing Mutual Exclusion with Semaphores sem_t S; sem_init(&S, 0, 1); lock_acquire() { // try to decrement, wait if 0 sem_wait (S); } lock_release() { // increment (wake up waiters if any) sem_post(S); } CS 3214 Fall 2010

10. Condition Variables - Intro • Besides (and perhaps more so) than semaphores, condition variables are another widely used form to implement ‘signaling’ kinds of coordination/synchronization • In POSIX Threads, Java, C# • Based on the concept of a Monitor • ADT that combines protected access to state and signaling • Confusing terminology alert: • Word ‘signal’ is overloaded 3 times • Semaphore signal (V(), “up”, “post”) • Monitor/Condition variable signal (“signal”, “notify”) • Unix signals • Word ‘wait’ is overloaded • Semaphore wait (P(), “down”) • Monitor/Condition variable wait • Unix wait() for child process CS 3214 Fall 2010

11. Monitors • A monitor combines a set of shared variables & operations to access them • Think of a Java class with no public fields & all public methods carrying the attribute ‘synchronized’ • A monitor provides implicit synchronization (only one thread can access private variables simultaneously) • Single lock is used to ensure all code associated with monitor is within critical section • A monitor provides a general signaling facility • Wait/Signal pattern (similar to, but different from semaphores) • May declare & maintain multiple signaling queues CS 3214 Fall 2010

12. Monitors (cont’d) • Classic monitors are embedded in programming languages • Invented by Hoare & Brinch-Hansen 1972/73 • First used in Mesa/Cedar System @ Xerox PARC 1978 • Adapted version available in Java/C# • (Classic) Monitors are safer than semaphores • can’t forget to lock data – compiler checks this • In contemporary C, monitors are a synchronization pattern that is achieved using locks & condition variables • Helps to understand monitor abstraction to use it correctly CS 3214 Fall 2010

13. Infinite Buffer w/ Monitor monitor buffer { /* implied: struct lock mlock;*/ private: char buffer[]; int head, tail; public: produce(item); item consume(); } buffer::produce(item i) { /* try { lock_acquire(&mlock); */ buffer[head++] = i; /* } finally {lock_release(&mlock);} */ } buffer::consume() { /* try { lock_acquire(&mlock); */ return buffer[tail++]; /* } finally {lock_release(&mlock);} */ } • Monitors provide implicit protection for their internal variables • Still need to add the signaling part CS 3214 Fall 2010

14. Condition Variables • Used by a monitor for signaling a condition • a general (programmer-defined) condition, not just integer increment as with semaphores • Somewhat weird: the condition is actually not stored in the variable – it’s typically some boolean predicate of monitor variables, e.g. “buffer.size > 0” • the condition variable itself is better thought of as a signaling queue • Monitor can have more than one condition variable • Three operations: • Wait(): leave monitor, wait for condition to be signaled, reenter monitor • Signal(): signal one thread waiting on condition • Broadcast(): signal all threads waiting on condition CS 3214 Fall 2010

15. Enter Region of mutual exclusion Wait Signal Wait Signal Exit Condition Variables as Queues • A condition variable’s state is just a queue of waiters: • Wait(): adds current thread to (end of queue) & block • Signal(): pick one thread from queue & unblock it • Broadcast(): unblock all threads Note on style: best practice is to leave monitor only once, and near the procedure’s entry. CS 3214 Fall 2010

16. Bounded Buffer w/ Monitor monitor buffer { condition items_avail; condition slots_avail; private: char buffer[]; int head, tail; public: produce(item); item consume(); } buffer::produce(item i) { while ((tail+1–head)%CAPACITY==0) slots_avail.wait(); buffer[head++] = i; items_avail.signal(); } buffer::consume() { while (head == tail) items_avail.wait(); item i = buffer[tail++]; slots_avail.signal(); return i; } CS 3214 Fall 2010

17. Bounded Buffer w/ Monitor monitor buffer { condition items_avail; condition slots_avail; private: char buffer[]; int head, tail; public: produce(item); item consume(); } buffer::produce(item i) { while ((tail+1–head)%CAPACITY==0) slots_avail.wait(); buffer[head++] = i; items_avail.signal(); } buffer::consume() { while (head == tail) items_avail.wait(); item i = buffer[tail++]; slots_avail.signal(); return i; } Q1.: How is lost update problem avoided? lock_release(&mlock); block_on(items_avail); lock_acquire(&mlock); Q2.: Why while() and not if()? CS 3214 Fall 2010

18. cond_signal semantics • cond_signal keeps lock, so it leaves signaling thread in monitor • waiter is made READY, but can’t enter until signaler gives up lock • There is no guarantee whether signaled thread will enter monitor next or some other thread will (who may be already waiting!) • so must always use “while()” when checking condition – cannot assume that condition set by signaling thread will still hold when monitor is reentered by signaled thread • This semantics is also referred to as “Mesa-Style” after the system in which it was first used • POSIX Threads, Java, and C# use this semantics CS 3214 Fall 2010

19. Condition Variables Signals are lost if nobody’s on the queue (e.g., nothing happens) Wait() always blocks Semaphores Signals (calls to V() or sem_post()) are remembered even if nobody’s current waiting Wait (e.g., P() or sem_wait()) may or may not block Condition Variables vs. Semaphores CS 3214 Fall 2010

20. Monitors in C • POSIX Threads as well as many custom environments • No compiler support, must do it manually • must declare locks & condition vars • must call pthread_mutex_lock/unlock when entering & leaving the monitor • must use pthread_cond_wait/pthread_cond_signal to wait for/signal condition • Note: pthread_cond_wait(&c, &m) takes monitor lock as parameter • necessary so monitor can be left & reentered without losing signals • pthread_cond_signal() does not • leaving room for programmer error! CS 3214 Fall 2010

21. synchronized block means enter monitor execute block leave monitor wait()/notify() use condition variable associated with receiver Every object in Java can function as a condition variable (just like it can function as a lock) More restrictive than Pthreads/C which allow multiple condition variables (signaling conditions) to be used in connection with a lock protecting state Monitors in Java class buffer { private char buffer[]; private int head, tail; public synchronized produce(item i) { while (buffer_full()) this.wait(); buffer[head++] = i; this.notifyAll(); } public synchronized item consume() { while (buffer_empty()) this.wait(); i = buffer[tail++]; this.notifyAll(); return ; } } CS 3214 Fall 2010

22. Previous slide (bounded buffer) is actually an example of where Java’s built-in monitors suck Needed “notifyAll()” to make sure one at least one of the right kind of threads was woken up Unacceptably inefficient Use java.util.concurrent.- locks.Condition instead in cases where multiple condition queues are needed import java.util.concurrent.locks.*; class buffer { private ReentrantLockmonitorlock = new ReentrantLock(); private Condition items_available = monitorlock.newCondition(); private Condition slots_available = monitorlock.newCondition(); public /* NO SYNCHRONIZED here */ void produce(item i) { monitorlock.lock(); try { while (buffer_full()) slots_available.await(); buffer[head++] = i; items_available.signal(); } finally { monitorlock.unlock(); } } /* consume analogous */ } Monitors in Java, Take 2 CS 3214 Fall 2010

23. A ReadWrite Lock Implementation struct lock mlock; // protects rdrs & wrtrs int readers = 0, writers = 0; structcondvarcanread, canwrite; void read_lock_acquire() { lock_acquire(&mlock); while (writers > 0) cond_wait(&canread, &mlock); readers++; lock_release(&mlock); } void read_lock_release() { lock_acquire(&mlock); if (--readers == 0) cond_signal(&canwrite); lock_release(&mlock); } void write_lock_acquire() { lock_acquire(&mlock); while (readers > 0 || writers > 0) cond_wait(&canwrite, &mlock); writers++; lock_release(&mlock); } void write_lock_release() { lock_acquire(&mlock); writers--; ASSERT(writers == 0); cond_broadcast(&canread); cond_signal(&canwrite); lock_release(&mlock); } Note: this is a naïve implementation that may lead to livelock – no guarantees a reader or writer ever enters the locked section even if every threads eventually leaves it CS 3214 Fall 2010

24. Summary • Semaphores & Condition Variables provide signaling facilities • Condition variables are loosely based on “monitor” concept • Java/C# provide syntactic sugar • Semaphores have “memory” • But require that # of signals matches # of waits • Good for rendezvous, precedence constraints – if problem lends itself to semaphore, use one • Always use idiomatic “while (!cond) *_wait()” pattern when using condition variables (in C) or Object.wait() (in Java) CS 3214 Fall 2010