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15-213 Lectures 22 and 23: Synchronization 15-213 Lectures 22 and 23: Synchronization Carnegie Mellon Carnegie Mellon Introduction Last Time: Threads Introduction to Computer Systems Concepts 15-213/18-243, fall 2009 Synchronization with Semaphores 23 rd Lecture, Nov 19 Today: Synchronization in Greater Depth Producer Consumer Buffer Pools and Condition Variables Instructors: Message Queues Roger B. Dannenberg and Greg Ganger Message Passing 2 Carnegie Mellon Carnegie Mellon Notifying With Semaphores Producer-Consumer on a Buffer That Holds One Item producer shared consumer /* buf1.c - producer-consumer int main() { thread buffer thread on 1-element buffer */ pthread_t tid_producer; #include “csapp.h” pthread_t tid_consumer; #define NITERS 5 /* initialize the semaphores */ Common synchronization pattern: Sem_init(&shared.empty, 0, 1); Producer waits for slot, inserts item in buffer, and notifies consumer void *producer(void *arg); Sem_init(&shared.full, 0, 0); Consumer waits for item, removes it from buffer, and notifies producer void *consumer(void *arg); /* create threads and wait */ struct { Pthread_create(&tid_producer, NULL, int buf; /* shared var */ producer, NULL); Examples sem_t full; /* sems */ Pthread_create(&tid_consumer, NULL, Multimedia processing: sem_t empty; consumer, NULL); } shared; Pthread_join(tid_producer, NULL); Producer creates MPEG video frames, consumer renders them Pthread_join(tid_consumer, NULL); Event-driven graphical user interfaces Producer detects mouse clicks, mouse movements, and keyboard hits exit(0); and inserts corresponding events in buffer } Consumer retrieves events from buffer and paints the display 3 4 Carnegie Mellon Carnegie Mellon Producer-Consumer (cont) Counting with Semaphores Initially: empty = 1, full = 0 Remember, it’s a non-negative integer /* producer thread */ /* consumer thread */ So, values greater than 1 are legal void *producer(void *arg) { void *consumer(void *arg) { Lets repeat thing_5() 5 times for every 3 of thing_3() int i, item; int i, item; /* thing_5 and thing_3 */ int main() { for (i=0; i<NITERS; i++) { for (i=0; i<NITERS; i++) { #include “csapp.h” pthread_t tid_five, tid_three; /* produce item */ /* read item from buf */ item = i; P(&shared.full); sem_t five; /* initialize the semaphores */ printf("produced %d\n", item = shared.buf; sem_t three; Sem_init(&five, 0, 5); item); V(&shared.empty); Sem_init(&three, 0, 3); void *five_times(void *arg); /* write item to buf */ /* consume item */ void *three_times(void *arg); /* create threads and wait */ P(&shared.empty); printf("consumed %d\n“, item); Pthread_create(&tid_five, NULL, shared.buf = item; } five_times, NULL); V(&shared.full); return NULL; Pthread_create(&tid_three, NULL, } } three_times, NULL); return NULL; . } . } 5 6 15-213 Lectures 22 and 23: Synchronization Carnegie Mellon Carnegie Mellon Counting with semaphores (cont) Producer-Consumer, Circular Buffer Initially: five = 5, three = 3 /* thing_5() thread */ /* thing_3() thread */ /* bufn.c - producer-consumer int main() { void *five_times(void *arg) { void *three_times(void *arg) { on n-element buffer */ pthread_t tid_producer; int i; int i; #include “csapp.h” pthread_t tid_consumer; while (1) { while (1) { #define NITERS 100 /* initialize the semaphores */ for (i=0; i<5; i++) { for (i=0; i<3; i++) { #define N 20 /* buffer size */ Sem_init(&shared.empty, 0, N); /* wait & thing_5() */ /* wait & thing_3() */ Sem_init(&shared.full, 0, 0); P(&five); P(&three); void *producer(void *arg); thing_5(); thing_3(); void *consumer(void *arg); /* create threads and wait */ } } Pthread_create(&tid_producer, NULL, V(&three); V(&five); struct { producer, NULL); V(&three); V(&five); int buf [N] ; /* shared var */ Pthread_create(&tid_consumer, NULL, V(&three); V(&five); sem_t full; /* sems */ consumer, NULL); } V(&five); sem_t empty; Pthread_join(tid_producer, NULL); return NULL; V(&five); } shared; Pthread_join(tid_consumer, NULL); } } return NULL; exit(0); } } 7 8 Carnegie Mellon Carnegie Mellon Producer-Consumer, Circular Buffer (cont) Do You Need Semaphores? Initially, shared.full = 0, shared.empty = N /* data structure */ /* producer */ /* producer thread */ /* consumer thread */ int shared; while (shared != 0) ; void *producer(void *arg) { void *consumer(void *arg) { shared = any_non_zero_data; int i, item; int i, item; /* initially */ shared = 0; for (i=0; i<NITERS; i++) { for (i=0; i<NITERS; i++) { /* consumer */ /* produce item */ /* read item from buf */ while (shared == 0) ; item = i; P(&shared.full); data_to_consume = shared; printf("produced %d\n", item = shared.buf [i%N]; shared = 0; item); V(&shared.empty); /* write item to buf */ /* consume item */ P(&shared.empty); printf("consumed %d\n“, item); Notes: shared.buf [i%N] = item; } V(&shared.full); return NULL; Producer notifies Consumer by writing non-zero } } Consumer notifies Producer by writing zero return NULL; } Don’t try this at home! (loops, hot-spot) This is just a mental warm-up 9 10 Carnegie Mellon Carnegie Mellon Do You Need Semaphores? (cont) Sharing With Dependencies /* data structure */ /* producer */ Threads interact through a resource manager #define N 20 int i = (shared.tail + 1) % N; struct { if (i == shared.head) return FAIL; (Standard terminology: monitor ) int head; shared.buf[i] = produced_data; One thread may need to wait for something another int tail; tail = i; /* atomic update */ int buf[N]; thread provides } shared; /* consumer */ if (shared.tail == shared.head) Example 1: previous circular buffer /* initiallize */ return FAIL; But that had a simple semaphore solution shared.head = 0; data_to_consume = shared.buf[shared.head]; Shared.tail = 0; /* atomic update: */ Example 2: memory buffer pool shared.head = (shared.head + 1) % N; Multiple threads checking out and returning buffers Notes: Might have to wait for buffer of the right size to show up Producer notifies Consumer by updating tail Consumer notifies Producer by updating head Data copied in before notifying consumer Ultimately, our solution will be: mutex and condition Data copied out before notifying producer variable What about out-of-order writes? 11 12 15-213 Lectures 22 and 23: Synchronization Carnegie Mellon Carnegie Mellon Naïve Solution What If Resource Is Unavailable? /* protected access to buffer structures */ /* protected access to buffer structures */ mutex_t mutex; mutex_t mutex; Mutex_init(&mutex); Mutex_init(&mutex); /* get a buffer */ /* get a buffer */ Mutex_lock(&mutex); Mutex_lock(&mutex); Find a suitable buffer while (! Find_a_suitable_buffer ()) { Mutex_unlock(&mutex); Mutex_unlock(); Mutex_lock(); } /* return a buffer */ Reserve the suitable buffer Mutex_lock(&mutex); Mutex_unlock(&mutex); Return buffer to pool Mutex_unlock(&mutex); /* return a buffer */ Mutex_lock(&mutex); Return buffer to pool Notes: Mutex_unlock(&mutex); mutex_lock is functionally similar to P() mutex_unlock is similar to V() Bad: spins rather than yielding processor to threads Analog of Mutex_init() is initializing semaphore to 1 holding the very resources we are waiting for! 13 14 Carnegie Mellon Carnegie Mellon What If Resource Is Unavailable? (cont) How About Waiting for a Notification? /* protected access to buffer structures */ /* protected access to buffer structures */ mutex_t mutex; mutex_t mutex; Mutex_init(&mutex); Mutex_init(&mutex); int waiting = FALSE; /* get a buffer */ sem_t rsrc; Mutex_lock(&mutex); Sem_init(&rsrc, 0, 0); while (! Find_a_suitable_buffer ()) { Mutex_unlock(); Yield(); Mutex_lock(); /* get a buffer */ } Mutex_lock(&mutex); Reserve the suitable buffer while (! Find_a_suitable_buffer ()) { Mutex_unlock(&mutex); waiting = TRUE; Mutex_unlock(&mutex); P(&rsrc); Mutex_lock(&mutex); /* return a buffer */ } Mutex_lock(&mutex); Reserve the suitable buffer Return buffer to pool Mutex_unlock(&mutex); Mutex_unlock(&mutex); /* return a buffer */ Mutex_lock(&mutex); Better, but not really acceptable. Polls until buffer is Return Better, buffer but to not pool really acceptable. Polls until buffer is available. if available.(waiting) { waiting = FALSE; V(&rsrc); } Mutex_unlock(&mutex); 15 16 Carnegie Mellon Carnegie Mellon How About Waiting for a Notification? Condition Variable /* protected access to buffer structures */ mutex_t mutex; Yet another synchronization mechanism Mutex_init(&mutex); Not a semaphore, not a mutex int waiting = FALSE; sem_t rsrc; Not really a “variable” Problems:Sem_init(&rsrc, 0, 0); Essentially a queue of waiting/sleeping/suspended threads • This will not work with multiple waiting threads /* get a buffer */ Operations: Mutex_lock(&mutex);• waiting is only a Boolean while (! Find_a_suitable_buffer ()) { Cond_wait puts thread on the queue • Hardwaiting to =analyze TRUE; Cond_signal wakes up one thread on the queue (if any) Mutex_unlock(&mutex); P(&rsrc); Mutex_lock(&mutex); } • Need a more general approach Cond_broadcast wakes up all threads on the queue Reserve the suitable buffer Special feature of Condition Variables: Mutex_unlock(&mutex);• Special-case “hacks” invite obscure bugs Cond_wait atomically puts thread on queue and releases a mutex lock /* return a buffer */ Waking up automatically reacquires the lock (!) Mutex_lock(&mutex); Return Better, buffer but to not pool really acceptable. Polls until buffer is if available.(waiting) { waiting = FALSE; V(&rsrc); } Mutex_unlock(&mutex); 17 18 15-213 Lectures 22 and 23: Synchronization Carnegie Mellon Carnegie Mellon Sharing with Condition Variable Another Problem: Readers and Writers /* protected access to buffer
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