Understanding Process Synchronization Concepts

1. Chapter 5: Process Synchronization

2. Chapter 5: Process Synchronization • Background • The Critical-Section Problem • Peterson’s Solution • Synchronization Hardware • Mutex Locks • Semaphores • Classic Problems of Synchronization • Monitors • Synchronization Examples • Alternative Approaches

3. Objectives • To present the concept of process synchronization. • To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared data • To present both software and hardware solutions of the critical-section problem • To examine several classical process-synchronization problems • To explore several tools that are used to solve process synchronization problems

4. Background • Processes can execute concurrently • May be interrupted at any time, partially completing execution • Concurrent access to shared data may result in data inconsistency • Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes • Illustration of the problem:Suppose that we wanted to provide a solution to the consumer-producer problem that fills allthe buffers. We can do so by having an integer counterthat keeps track of the number of full buffers. Initially, counteris set to 0. It is incremented by the producer after it produces a new buffer and is decremented by the consumer after it consumes a buffer.

5. Producer while (true) { /* produce an item in next produced */ while (counter == BUFFER_SIZE) ; /* do nothing */ buffer[in] = next_produced; in = (in + 1) % BUFFER_SIZE; counter++; }

6. Consumer while (true) { while (counter == 0) ; /* do nothing */ next_consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; counter--; /* consume the item in next consumed */ }

7. Race Condition • counter++ could be implemented asregister1 = counter register1 = register1 + 1 counter = register1 • counter--could be implemented asregister2 = counter register2 = register2 - 1 counter = register2 • Consider this execution interleaving with “count = 5” initially: S0: producer execute register1 = counter{register1 = 5}S1: producer execute register1 = register1 + 1 {register1 = 6} S2: consumer execute register2 = counter{register2 = 5} S3: consumer execute register2 = register2 – 1 {register2 = 4} S4: producer execute counter = register1 {counter = 6 } S5: consumer execute counter = register2 {counter = 4}

8. Critical Section Problem • Consider system of nprocesses {p0, p1, … pn-1} • Each process has critical section segment of code • Process may be changing common variables, updating table, writing file, etc • When one process in critical section, no other may be in its critical section • Critical section problem is to design protocol to solve this • Each process must ask permission to enter critical section in entry section, may follow critical section with exit section, then remainder section

9. Critical Section • General structure of process Pi

10. Algorithm for Process Pi do { while (turn == j); critical section turn = j; remainder section } while (true);

11. Solution to Critical-Section Problem 1. Mutual Exclusion - If process Piis executing in its critical section, then no other processes can be executing in their critical sections 2. Progress- If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely 3. Bounded Waiting - A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted • Assume that each process executes at a nonzero speed • No assumption concerning relative speed of thenprocesses

12. Critical-Section Handling in OS Two approaches depending on if kernel is preemptive or non- preemptive • Preemptive– allows preemption of process when running in kernel mode • Non-preemptive – runs until exits kernel mode, blocks, or voluntarily yields CPU • Essentially free of race conditions in kernel mode

13. Peterson’s Solution • Good algorithmic description of solving the problem • Two process solution • Assume that the loadand store machine-language instructions are atomic; that is, cannot be interrupted • The two processes share two variables: • int turn; • Boolean flag[2] • The variable turn indicates whose turn it is to enter the critical section • The flagarray is used to indicate if a process is ready to enter the critical section. flag[i] = trueimplies that process Pi is ready!

14. Algorithm for Process Pi do { flag[i] = true; turn = j; while (flag[j] && turn = = j); critical section flag[i] = false; remainder section } while (true);

15. Peterson’s Solution (Cont.) • Provable that the three CS requirement are met: 1. Mutual exclusion is preserved Pienters CS only if: either flag[j] = false or turn = i 2. Progress requirement is satisfied 3. Bounded-waiting requirement is met note: since Pi does not change the value of the variable turn while executing the while statement, Pi will enter the critical section (progress) after at most one entry by Pj (bounded waiting).

16. Synchronization Hardware • Many systems provide hardware support for implementing the critical section code. • All solutions below based on idea of locking • Protecting critical regions via locks • Uniprocessors – could disable interrupts • Currently running code would execute without preemption • Generally too inefficient on multiprocessor systems • Operating systems using this not broadly scalable • Modern machines provide special atomic hardware instructions • Atomic = non-interruptible • Either test memory word and set value • Or swap contents of two memory words

17. Concurrent Programming:

18. Race condition and Sharing of Resources This problem is called a race condition and can always happen if multiple threads access a shared resource without making sure that one thread is finished operating on a resource before another one begins accessing it.

19. Atomic Operations = non-interruptible • Atomic Operations • Operations on shared data • An update operation consists of a "read, update, write" sequence • Hardware prevents two primitive operations (read/write) at once, but sequences of instructions must be synchronized to maintain atomic operation integrity • Atomic meaning that other threads\processes cannot interrupt until it is finished. Operating Systems use atomic operations to implement sychronization - locks, mutexes, semaphores, and critical sections.

20. Solution to Critical-section Problem Using Locks do { acquire lock critical section release lock remainder section } while (TRUE);

21. test_and_set Instruction Definition: boolean test_and_set (boolean *target) { boolean rv = *target; *target = TRUE; return rv: // rv(return value) } • Executed atomically (noninterrupt) • Returns the original value of passed parameter • Set the new value of passed parameter to “TRUE”. Note: if two test and set() instructions are executed simultaneously (each on a different CPU), they will be executed sequentially in some arbitrary order

22. Solution using test_and_set() • Shared Boolean variable lock, initialized to FALSE • Solution: do { while (test_and_set(&lock)) ; /* do nothing */ /* critical section */ lock = false; /* remainder section */ } while (true);

23. compare_and_swap Instruction Definition: int compare _and_swap(int *value, int expected, int new_value) { int temp = *value; if (*value == expected) *value = new_value; return temp; } • Executed atomically • Returns the original value of passed parameter “value” • Set the variable “value” the value of the passed parameter “new_value” but only if “value” ==“expected”. That is, the swap takes place only under this condition.

24. Solution using compare_and_swap • Shared integer “lock” initialized to 0; • Solution: do { while (compare_and_swap(&lock, 0, 1) != 0) ; /* do nothing */ /* critical section */ lock = 0; /* remainder section */ } while (true);

25. Mutual Exclusion Mutual exclusive access means that only one thread at a time gets access to a certain resource. In order to ensure this, each thread that wants to access a resource first needs to acquire a mutex lock on it. Once it has finished its operation, it releases the lock, so that other threads get a chance to access it. Note: Mutex locks solve the problem of race conditions

26. Bounded-waiting Mutual Exclusion with test_and_set • i ,j represent process Pi , pj respectively • These data structures are initialized to false. • To prove that the mutual-exclusion requirement is met, we note that process P i can enter its critical • section only if either • waiting[i] == false or key == false. • The value of key can become false only if the test and set() is executed. The first process to execute the test and set() will find key == false; all others must wait. • The variable waiting[i] can become false only if another process leaves its critical section; only one waiting[i] is set to false, maintaining the mutual-exclusion requirement. • n= array waiting in the cyclic ordering • a process exiting the critical section either sets lock to false or sets waiting[j] to false. Both allow a process that is waiting to enter its critical section to proceed. do { waiting[i] = true; key = true; while (waiting[i] && key) key = test_and_set(&lock); waiting[i] = false; /* critical section */ j = (i + 1) % n; while ((j != i) && !waiting[j]) j = (j + 1) % n; if (j == i) lock = false; else waiting[j] = false; /* remainder section */ } while (true); To prove that the bounded-waiting requirement is met, we note that, when a process leaves its critical section, it scans the array waiting in the cyclic ordering (i + 1, i + 2, ..., n − 1, 0, ..., i − 1). It designates the first process in this ordering that is in the entry section (waiting[j] == true) as the next one to enter the critical section. Any process waiting to enter its critical section will thus do so within n − 1 turns.

27. Mutex Locks(cont) • Previous solutions are complicated and generally inaccessible to application programmers • OS designers build software tools to solve critical section problem • Simplest is mutex lock • Protect a critical section by first acquire()a lock then release()the lock • Boolean variable indicating if lock is available or not • Calls to acquire()and release()must be atomic • Usually implemented via hardware atomic instructions • But this solution requires busy waiting While a process is in its critical section, any other process that tries to enter its critical section must loop continuously in the call to acquire() • Mutex Locks This lock therefore called a spinlock

28. acquire() and release() • acquire() { while (!available) ; /* busy wait */ available = false;; } • release() { available = true; } • do { acquire lock critical section release lock remainder section } while (true);

29. semaphores

30. semaphores • semaphore is a value that indicates the status of common recourses. A semaphore, in its most basic form, is a protected integer variable that can facilitate and restrict access to shared resources in a multi-processing environment. The two most common kinds of semaphores are counting semaphores and binary semaphores. Counting semaphores represent multiple resources, while binary semaphores, as the name implies, represents two possible states (generally 0 or 1; locked or unlocked) Semaphores can be looked at as a representation of a limited number of resources

31. semaphores • A semaphore can only be accessed using the following operations: wait() and signal(). • wait() is called when a process wants access to a resource.that process must wait until it becomes available. • signal() is called when a process is done using a resource,

32. Semaphore • Synchronization tool that provides more sophisticated ways (than Mutex locks) for process to synchronize their activities. • Semaphore S – integer variable • Can only be accessed via two indivisible (atomic) operations • wait()and signal() • Originally called P()and V() • Definition of the wait() operation wait(S) { while (S <= 0) ; // busy wait S--; } • Definition of the signal() operation signal(S) { S++; }

33. Semaphore • The Semaphore Management function group is used to manage and protect access to shared resources. • with a Semaphore the access to a group of identical peripherals can be managed • The parameter count specifies the number of available resources. • Each time a Semaphore token is obtained with semaphore wait the semaphore count is decremented. • When the semaphore count is 0, no Semaphore token can be obtained. • Semaphores are released with semaphore release (signal) ; this function increments the semaphore count.

34. How a semaphore coordinates concurrent access • When process 5 needs access to a record, index, or other shared resource already locked by process 8, process 5 decrements its semaphore. When the process holding the lock (process 8) releases the resource, it notifies the waiting process (process 5) by incrementing the semaphore. • By default, each database has an array of semaphores,

35. Semaphore Usage • Counting semaphore – integer value can range over an unrestricted domain • Binary semaphore – integer value can range only between 0 and 1 • Same as a mutex lock • Can solve various synchronization problems • Consider P1 and P2 that require S1to happen before S2 Create a semaphore “synch” initialized to 0 P1: S1; signal(synch); P2: wait(synch); S2; • Can implement a counting semaphore S as a binary semaphore do{ wait(s); // critical section signal(s); // remainder section} while(1);

36. Semaphore Implementation • Must guarantee that no two processes can execute the wait() and signal() on the same semaphore at the same time • Thus, the implementation becomes the critical section problem where the wait and signal code are placed in the critical section • Could now have busy waitingin critical section implementation • But implementation code is short • Little busy waiting if critical section rarely occupied • Note that applications may spend lots of time in critical sections and therefore this is not a good solution Disadvantage As shown in the examples above, processes waiting on a semaphore must constantly check to see if the semaphore is not zero. This continual looping is clearly a problem in a real multiprogramming system (where often a single CPU is shared among multiple processes). This is called busy waiting and it wastes CPU cycles. When a semaphore does this, it is called a spinlock.

37. Semaphore Implementation with no Busy waiting • To avoid busy waiting, a semaphore may use an associated queue of processes that are waiting on the semaphore, allowing the semaphore to block() the process and then wakeup it when the semaphore is incremented. • With each semaphore there is an associated waiting queue • Each entry in a waiting queue has two data items: • value (of type integer) • pointer to next record in the list • Two operations: • block– place the process invoking the operation on the appropriate waiting queue • wakeup– remove one of processes in the waiting queue and place it in the ready queue • typedef struct{ int value; struct process *list; } semaphore; In a slightly modified implementation, it would be possible for a semaphore's value to be less than zero. When a process executes wait(), the semaphore count is automatically decremented. The magnitude of the negative value would determine how many processes were waiting on the semaphore:

38. Implementation with no Busy waiting (Cont.) If a process calls wait() on a semaphore with a value of zero, the process is added to the semaphore’s queue and then blocked. The state of the process is switched to the waiting state, and control is transferred to the CPU scheduler, which selects another process to execute. When another process increments the semaphore by calling signal() and there are tasks on the queue, one is taken off of it and resumed.

39. Deadlock and Starvation • Deadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes • Let S andQbe two semaphores initialized to 1 P0P1 wait(S); wait(Q); wait(Q); wait(S); ... ... signal(S); signal(Q); signal(Q); signal(S); • Starvation– indefinite blocking • A process may never be removed from the semaphore queue in which it is suspended • Priority Inversion– Scheduling problem when lower-priority process holds a lock needed by higher-priority process • Solved via priority-inheritance protocol Suppose that P0 executes wait(S) and then P1 executes wait(Q). When P0 executes wait(Q), it must wait until P1 executes signal(Q). Similarly, when P1 executes wait(S),

40. Dead Locks Mutex locks solve the problem of race conditions, but unfortunately they also introduce a new problem (amongst others) at the same time: Dead lock. A dead lock occurs when multiple threads are waiting on each other to finish and get stuck.

41. Classical Problems of Synchronization • Classical problems used to test newly-proposed synchronization schemes • Bounded-Buffer Problem • Readers and Writers Problem • Dining-Philosophers Problem

42. Bounded-Buffer Problem • n buffers, each can hold one item • Semaphore mutex initialized to the value 1 • Semaphore full initialized to the value 0 • Semaphore emptyinitialized to the value n

43. Bounded-Buffer Problem (cont) • A producer must wait until the buffer is not full, deposit its data, and then notify the consumers that the buffer is not empty. • A consumer, on the other hand, must wait until the buffer is not empty, retrieve a data item, and then notify the producers that the buffer is not full. • Of course, before a producer or a consumer can have access to the buffer, it must lock the buffer. After a producer and consumer finishes using the buffer, it must unlock the buffer. Combined these activities together, we have the following diagram:

44. The Producer/Consumer (or Bounded-Buffer) Problem • in summary, we need a semaphore to block producers when the buffer is full, • a semaphore to block consumers when the buffer is empty, • a binary semaphore to guarantee mutex exclusion when the buffer is accessed. • Note that the first semaphore is signaled (by a consumer) when the buffer is not full, and the second is signaled (by a producer) when the buffer is not empty.

45. Bounded Buffer Problem (Cont.) We assume that the pool consists of n buffers, each capable of holding one item. The mutex semaphore provides mutual exclusion for accesses to the buffer pool. the producer producing full buffers • The structure of the producer process do { ... /* produce an item in next_produced */ ... wait(empty); wait(mutex); ... /* add next produced to the buffer */ ... signal(mutex); signal(full); } while (true);

46. Bounded Buffer Problem (Cont.) the consumer or as the consumer producing empty buffers for the producer. • The structure of the consumer process Do { wait(full); wait(mutex); ... /* remove an item from buffer to next_consumed */ ... signal(mutex); signal(empty); ... /* consume the item in next consumed */ ... } while (true);

47. Readers-Writers Problem • A data set is shared among a number of concurrent processes • Readers – only read the data set; they do notperform any updates • Writers – can both read and write = (update) • Problem – allow multiple readers to read at the same time • Only one single writer can access the shared data at the same time • Several variations of how readers and writers are considered – all involve some form of priorities • Shared Data • Data set • Semaphorerw_mutexinitialized to 1 • Semaphore mutexinitialized to 1 • Integer read_count initialized to 0 first readers–writers problem, requires that no reader be kept waiting unless a writer has already obtained permission to use the shared object. The second readers –writers problem requires that, once a writer is ready, that writer perform its write as soon as possible.

48. Readers-Writers Problem (Cont.) • The structure of a writer process do { wait(rw_mutex); ... /* writing is performed */ ... signal(rw_mutex); } while (true); semaphore rw mutex is common to both reader and writer processes. The mutex semaphore is used to ensure mutual exclusion when the variable read count is updated.

49. Readers-Writers Problem (Cont.) The read count variable keeps track of how many processes are currently reading the object. The semaphore rw mutex functions as a mutual exclusion semaphore for the writers. It is also used by the first or last reader that enters or exits the critical section. It is not used by readers who enter or exit while other readers are in their critical sections. if a writer is in the critical section and n readers are waiting, then one reader is queued on rw mutex, and n − 1 readers are queued on mutex. Also observe that, when a writer executes signal(rw mutex), we may resume the execution of either the waiting readers or a single waiting writer. The selection is made by the scheduler. • The structure of a reader process do { wait(mutex); read_count++; if (read_count == 1) wait(rw_mutex); signal(mutex); ... /* reading is performed */ ... wait(mutex); read count--; if (read_count == 0) signal(rw_mutex); signal(mutex); } while (true);

50. Readers-Writers Problem Variations • Firstvariation – no reader kept waiting unless writer has permission to use shared object • Secondvariation – once writer is ready, it performs the write ASAP • Both may have starvation leading to even more variations • Problem is solved on some systems by kernel providing reader-writer locks