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1 Classical Synchronization Problems Copyright : University of Illinois CS 241 Staff 1 1

2 This lecture Goals: Introduce classical synchronization problems Topics Producer-Consumer Problem Reader-Writer Problem Dining Philosophers Problem Sleeping Barber s Problem Copyright : University of Illinois CS 241 Staff 2

3 1. Producer-Consumer Chefs cook items and put them on a conveyer belt Customers pick items off the belt corn clip art credit: wikimedia User:Spedona Copyright : University of Illinois CS 241 Staff 3

4 Problem Producers insert items Consumers remove items Shared bounded buffer Chef Customer = Producer = Consumer Efficient implementation: circular buffer with an insert and a removal pointer. Copyright : University of Illinois CS 241 Staff 4

5 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 5

6 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 6

7 Chef = Producer Customer = Consumer Copyright : University of Illinois CS 241 Staff 7

8 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 8

9 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 9

10 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 10

11 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 11

12 Chef Customer = Producer = Consumer BUFFER FULL: Producer must be blocked! Copyright : University of Illinois CS 241 Staff 12

13 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 13

14 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 14

15 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 15

16 Chef = Producer Customer = Consumer Copyright : University of Illinois CS 241 Staff 16

17 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 17

18 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 18

19 Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 19

20 BUFFER EMPTY: Consumer must be blocked! Chef Customer = Producer = Consumer Copyright : University of Illinois CS 241 Staff 20

21 Problem Producer inserts items. Updates insertion pointer. Consumer executes destructive reads on the buffer. Updates removal pointer. Both update information about how full/ empty the buffer is. Solution should allow multiple producers and consumers Copyright : University of Illinois CS 241 Staff 21

22 Challenges Prevent buffer overflow Prevent buffer underflow Mutual exclusion when modifying the buffer data structure Copyright : University of Illinois CS 241 Staff 22

23 Solution Prevent overflow: block producer when full! Counting semaphore to count #free slots 0 block producer Prevent underflow: block consumer when empty! Counting semaphore to count #items in buffer 0 block consumer Mutex to protect accesses to shared buffer & pointers. Copyright : University of Illinois CS 241 Staff 23

24 Solution Prevent overflow: block producer when full! Counting semaphore to count #free slots 0 block producer Prevent underflow: block consumer when empty! Counting semaphore to count #items in buffer 0 block consumer Mutex to protect accesses to shared buffer & pointers. Copyright : University of Illinois CS 241 Staff 24

25 Assembling the solution sem_wait(slots), sem_signal(slots) sem_wait(items), sem_signal(items) mutex_lock(m), mutex_unlock(m) insertptr = (insertptr+1) % N removalptr = (removalptr+1) % N Initialize semaphore slots to size of buffer Initialize semaphore items to zero. Copyright : University of Illinois CS 241 Staff 25

26 Pseudocode getitem() For consumer Error checking/eintr handling not shown sem_wait(items); mutex_lock(mutex); result = buffer[ ]; = ( +1) % N; mutex_unlock(mutex); sem_signal(slots); Copyright : University of Illinois CS 241 Staff 26

27 Pseudocode putitem(data) For producer Error checking/eintr handling not shown sem_wait(slots); mutex_lock(mutex); buffer[ ] = data; = ( + 1) % N; mutex_unlock(mutex); sem_signal(items); Copyright : University of Illinois CS 241 Staff 27

28 II. Reader-Writer Problem A reader: read data A writer: write data Rules: Multiple readers may read the data simultaneously Only one writer can write the data at any time A reader and a writer cannot access data simultaneously Locking table: whether any two can be in the critical section simultaneously Reader Writer Reader OK No Writer No No Copyright : University of Illinois CS 241 Staff 28

29 Reader-writer solution * * semaphore mutex = 1; / * controls access to rc * / semaphore db = 1; / * controls access to the data base * / int rc = 0; / * # of processes reading or wanting to * / void reader(void) { 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); use data read(); } } [From Tanenbaum, Modern Operating Systems] void writer(void) { while (TRUE) { think up data(); down(&db); write data base(); up(&db); } } Note: down means semwait up means semsignal Copyright : University of Illinois CS 241 Staff This solution can starve the writer! 29

30 Better R-W solution idea Idea: serve requests in order once a writer requests access, any entering readers have to block until the writer is done Advantage? Disadvantage? Copyright : University of Illinois CS 241 Staff 30

31 Summary Classic synchronization problems Producer-Consumer Problem Reader-Writer Problem Saved for next time: Sleeping Barber s Problem Dining Philosophers Problem Copyright : University of Illinois CS 241 Staff 31

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