Semaphores. To avoid busy waiting: when a process has to wait, it will be put in a blocked queue of processes waiting for the same event

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1 Semaphores Synchronization tool (provided by the OS) that do not require busy waiting A semaphore S is an integer variable that, apart from initialization, can only be accessed through 2 atomic and mutually exclusive operations: wait(s) also P(S) signal(s) also V(S) To avoid busy waiting: when a process has to wait, it will be put in a blocked queue of processes waiting for the same event 1

2 Semaphores Hence, in fact, a semaphore is a record (structure): typedef struct General_Semaphore { count: integer; queue: list of process } semaphore; semaphore S; When a process must wait for a semaphore S, it is blocked and put on the semaphore s queue The signal operation removes (acc. to a fair policy like FIFO) one process from the queue and puts it in the list of ready processes 2

3 Semaphore s operations P(S)or wait(s)[ if (S.count <=0) then //wait until positive {block this process and place this process in S.queue } S.count--; //decrement ] V(S) or signal(s)[ S.count++; //increment remove one process P, if any, from S.queue; place this process P on ready list ] S.count must be initialized to a non-negative integer value (depending on the application) 3

4 Semaphores: observations When a semaphore is created, its value can be initialized to any non-negative integer (0,1,2,3..) When S.count >=0: the number of processes that can execute wait(s) without being blocked = S.count Atomicity and mutual exclusion: no 2 process can be in wait(s) and signal(s) (on the same S) at the same time (even with multiple CPUs) Hence the blocks of code defining wait(s) and signal(s) are, in fact, critical sections No other operations are allowed on a semaphore 4

5 Semaphores: observations The critical sections defined by wait(s) and signal(s) are very short: typically 10 instructions Solutions: uniprocessor: disable interrupts during these operations (ie: for a very short period). This does not work on a multiprocessor machine. multiprocessor: use previous software or hardware schemes. The amount of busy waiting should be small. 5

6 Binary semaphores Semaphores that allow arbitrary non-negative values are called counting semaphores or general semaphores We also have binary semaphores similar to counting semaphores except that count is Boolean valued (0 or 1) counting semaphores can be implemented by binary semaphores... generally more difficult to use than counting semaphores (eg: they cannot be initialized to an integer k > 1) 6

7 Binary semaphores waitb(s):[ if (S.value == 0 ) { block this process; place this process in S.queue; } S.value=0; ] signalb(s):[ S.value=1; remove one process P, if any from S.queue place this process P on ready list; ] Binary Semaphore S; {init 0 or 1} 7

8 Binary sempahore using busy wait S initially 0 or 1 Wait(S): [while (S == 0 )do skip; S = 0;] Signal(S): [S = 1;] 8

9 Using semaphores for solving critical section problems For n processes Initialize S.count to 1 Then only 1 process is allowed into CS (mutual exclusion) To allow k processes into CS, we initialize S.count to k Process Pi: repeat wait(s); CS signal(s); RS forever wait(s): wait until semaphore becomes +ve; atomically decrement signal(s): atomically increment semaphore value; wake up one process if any 9

10 Mutual exclusion Only 1 train can cross the pass at a time Each train will execute wait(s) prior to entering Each train will execute signal(s) after leaving the pass 10

11 Using semaphores to synchronize processes We have 2 processes: Parent (P1) and child (P2) Statement S2 in P2 needs to be performed before statement S1 in P1 Then define a semaphore synch Initialize synch to 0 Proper synchronization is achieved by having in P2: Thread_exit();{ S2; signal(synch);} And having in P1: Thread_join(){ wait(synch); S1;} 11

12 Hiking trail Child has to wait for parent to first cross the ridge Child executes wait(s) S initialized to 0 Parent executes signal(s) 12

13 Binary semaphore in pthreads Binary semaphore initialized to 1 also called mutex Type pthread_mutex_t #include <pthread.h> pthread_mutex_t lock; int pthread_mutex_lock(pthread_mutex_t *lock); int pthread_mutex_unlock(pthread_mutex_t *lock); 13

14 Implementing general semaphore using binary semaphores Binary semaphore entry,mutex =1; Binary semaphore delay=0; Int c ={non-negative integer}; wait(c)[ wait(entry); wait(mutex); c=c-1 If c<0 { signal(mutex); wait(delay); } else signal(mutex); signal(entry); ] signal(c)[ wait(mutex) c++; If c==0 {signal (delay);} signal(mutex) ] Why is entry semaphore needed? 14

15 The producer/consumer problem A producer process produces information that is consumed by a consumer process Ex1: a print program produces characters that are consumed by a printer Ex2: a compiler produces object modules from a series of steps pipelined gcc : cpp cc as ld We need a buffer to hold items that are produced and eventually consumed A common paradigm for cooperating processes 15

16 P/C: unbounded buffer We assume first an unbounded buffer consisting of a linear array of elements in points to the next item to be produced out points to the next item to be consumed 16

17 P/C: unbounded buffer We need a semaphore S to perform mutual exclusion on the buffer: only 1 process at a time can access the buffer We need another semaphore N to synchronize producer and consumer on the number N (= in - out) of items in the buffer an item can be consumed only after it has been created 17

18 P/C: unbounded buffer The producer is free to add an item into the buffer at any time: it performs wait(s) before appending and signal(s) afterwards to prevent customer access It also performs signal(n) after each append to increment N The consumer must first do wait(n) to see if there is an item to consume and use wait(s)/ signal(s) to access the buffer 18

19 Producer consumer problem int c; bin semaphore mutex=1; gen semaphore empty=0; int buf[]; int in, out=0; Producer Process do { read v; wait(mutex); produce(v); signal(mutex); signal (empty); } while (TRUE); produce(v){ b[in]=v; in++; } Consumer Process do { wait(empty); wait(mutex;) w=consume(); signal (mutex); } while (TRUE); consume(){ W=b[out]; out++; return w;} 19

20 P/C: unbounded buffer Remarks: Putting signal(empty) inside the CS of the producer (instead of outside) has no effect since the consumer must always wait for both semaphores before proceeding The consumer must perform wait(empty) before wait(mutex), otherwise deadlock occurs if consumer enter CS while the buffer is empty Using semaphores is a difficult art... 20

21 P/C: finite circular buffer of size k can consume only when number N of (consumable) items is at least 1 (now: N!=in-out) can produce only when number E of empty spaces is at least 1 21

22 P/C: finite circular buffer of size k As before: we need a semaphore S to have mutual exclusion on buffer access we need a semaphore N to synchronize producer and consumer on the number of consumable items In addition: we need a semaphore E to synchronize producer and consumer on the number of empty spaces 22

23 Producer consumer problem int const k= 50; bin semaphore mutex=1; gen semaphore empty=0; gen semaphore full=k; int buf[k]; int in, out=0; Producer Process do { wait(full); read v; wait(mutex); produce(v); signal(mutex); signal (empty); } while (TRUE); produce(v){ b[in]=v; in= (in+1) mod k; } Consumer Process do { wait(empty); wait(mutex;) w=consume(); signal (mutex); signal(full); } while (TRUE); consume(){ W=b[out]; out=(out +1) mod k; return w;} 23

24 Readers and writers problem Many readers and many writer processes Constraint: many readers can be in the CS (no other writer) Only 1 writer in the CS (no other reader or writer) 24

25 Readers writers problem semaphore wrt, mutex=1; int No_Of_readers = 0; Writer Process wait(wrt); Write signal (wrt); Reader Process wait(mutex); Entry condition. signal (mutex); Read wait(mutex); Exit condition. signal(mutex); 25

26 The Dining Philosophers Problem 5 philosophers who only eat and think each need to use 2 forks for eating we have only 5 forks A classical synchron. problem Illustrates the difficulty of allocating resources among process without deadlock and starvation 26

27 The Dining Philosophers Problem Each philosopher is a process One semaphore per fork: fork: array[0..4] of semaphores Initialization: fork[i].count:=1 for i:=0..4 A first attempt: Deadlock if each philosopher start by picking his left fork! Process Pi: repeat think; wait(fork[i]); wait(fork[i+1 mod 5]); eat; signal(fork[i+1 mod 5]); signal(fork[i]); forever 27

28 The Dining Philosophers Problem A solution: admit only 4 philosophers at a time that tries to eat Then 1 philosopher can always eat when the other 3 are holding 1 fork Hence, we can use another semaphore T that would limit at 4 the numb. of philosophers sitting at the table Initialize: T.count:=4 Process Pi: repeat think; wait(t); wait(fork[i]); wait(fork[i+1 mod 5]); eat; signal(fork[i+1 mod 5]); signal(fork[i]); signal(t); forever 28

29 Problems with semaphores semaphores provide a powerful tool for enforcing mutual exclusion and coordinate processes But wait(s) and signal(s) are scattered among several processes. Hence, difficult to understand their effects Usage must be correct in all the processes One bad (or malicious) process can fail the entire collection of processes 29

30 Monitors Are high-level language constructs that provide equivalent functionality to that of semaphores but are easier to control Found in many concurrent programming languages Concurrent Pascal, Modula-3, C++, Java... Can be implemented by semaphores... 30

31 Monitor Is a software module containing: one or more procedures an initialization sequence local data variables Characteristics: local variables accessible only by monitor s procedures a process enters the monitor by invoking one of it s procedures only one process can be in the monitor at any one time 31

32 Monitor The monitor ensures mutual exclusion: no need to program this constraint explicitly Hence, shared data are protected by placing them in the monitor The monitor locks the shared data on process entry Process synchronization is done by the programmer by using condition variables that represent conditions a process may need to wait for before executing in the monitor 32

33 Schematic view of a Monitor

34 Condition Variables condition x, y; Two operations are allowed on a condition variable: x.wait() a process that invokes the operation is suspended until x.signal() x.signal() resumes one of processes (if any) that invoked x.wait() If no x.wait() on the variable, then it has no effect on the variable

35 Monitor with Condition Variables Condition Variable: a queue of threads waiting for something inside a critical section Key idea: make it possible to go to sleep inside critical section by atomically releasing lock prior to block Contrast to semaphores: Can t wait inside critical section

36 Producer/Consumer problem Two types of processes: producers consumers Synchronization is now confined within the monitor append(.) and take(.) are procedures within the monitor: are the only means by which P/C can access the buffer If these procedures are correct, synchronization will be correct for all participating processes ProducerI: repeat produce v; Append(v); forever ConsumerI: repeat Take(v); consume v; forever 36

37 Monitor for the bounded P/C problem Monitor needs to hold the buffer: buffer: array[0..k-1] of items; needs two condition variables: notfull: csignal(notfull) indicates that the buffer is not full notempty: csignal(notempty) indicates that the buffer is not empty needs buffer pointers and counts: nextin: points to next item to be appended nextout: points to next item to be taken count: holds the number of items in buffer 37

38 Monitor for the bounded P/C problem Monitor boundedbuffer: const k 50; buffer: array[k] of items; nextin:=0, nextout:=0, count:=0: integer; notfull, notempty: condition; Append(v): if (count==k) cwait(notfull); buffer[nextin]= v; nextin:= nextin+1 mod k; count++; csignal(notempty); Take(v): if (count==0) cwait(notempty); v:= buffer[nextout]; nextout= nextout+1 mod k; count--; csignal(notfull); 38

39 Condition Variables Choices If process P invokes x.signal(), and process Q is suspended in x.wait(), what should happen next? Both Q and P cannot execute in parallel. If Q is resumed, then P must wait Options include Signal and wait P waits until Q either leaves the monitor or it waits for another condition Signal and continue Q waits until P either leaves the monitor or it waits for another condition Both have pros and cons language implementer can decide Monitors implemented in Concurrent Pascal compromise P executing signal immediately leaves the monitor, Q is resumed Implemented in other languages including Mesa, C#, Java

40 Review: Mesa vs. Hoare Monitors Need to be careful about precise definition of signal and wait. Consider a piece of our producer code: We did this. What assumptions we had to make? if (count==k) { cwait(notfull); // If full, sleep } buffer[nextin]=v; // Get next item sometimes we may need to do this while (count==k) { cwait(notfull); // If full, sleep } buffer[nextin]=v; // Get next item Answer: depends on the type of scheduling Hoare-style (most textbooks): Signaler gives up lock, CPU to waiter; waiter runs immediately Waiter gives up lock, processor back to signaler when it exits critical section or if it waits again Mesa-style (most real operating systems): Signaler keeps lock and processor Waiter placed on ready queue with no special priority Practically, need to check condition again after wait

41 Pthread condition variables pthread_condition_t xc; pthread_mutex_t lock; Pthread_cond_wait(xc,lock); Pthread_cond_signal(xc); Pthread_mutex_t Pthread_cond_t cv_mutex; waitfor_dad_cv; Dad() {. pthread_cond_signal(&waitfor_dad_cv) } Child() { pthread_cond_wait(&cv_mutex,&waitfor_dad_cv) }; 41

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