Module 6: Process Synchronization

1. Module 6: Process Synchronization

2. Module 6: Process Synchronization Background The Critical-Section Problem Peterson’s Solution Synchronization Hardware Semaphores Classic Problems of Synchronization Monitors Synchronization Examples Atomic Transactions

3. Objectives To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared data To present solutions of the critical-section problem

4. Background Concurrent access to shared data may result in data inconsistency Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes Suppose that we wanted to provide a solution to the consumer-producer problem that fills all the buffers. We can do so by having an integer count that keeps track of the number of full buffers. Initially, count is 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 (count == BUFFER.SIZE) ; // do nothing // add an item to the buffer buffer[in] = item; in = (in + 1)%BUFFER.SIZE; ++count;

6. Consumer while (count == 0) ; // do nothing // remove an item from the buffer item = buffer[out]; out = (out + 1)%BUFFER.SIZE; --count;

7. Race Condition count++ could be implemented asregister1 = count register1 = register1 + 1 count = register1 count-- could be implemented asregister2 = count register2 = register2 - 1 count = register2 Consider this execution interleaving with “count = 5” initially: T0: producer execute register1 = count {register1 = 5}T1: producer execute register1 = register1 + 1 {register1 = 6} T2: consumer execute register2 = count {register2 = 5} T3: consumer execute register2 = register2 - 1 {register2 = 4} T4: producer execute count = register1 {count = 6 } T5: consumer execute count = register2 {count = 4}

8. Race Condition Incorrect value of count may be generated because both processes manipulate count concurrently. It is needed to ensure that only one process can manipulating the variable count at a time. When several processes access and manipulate the same data concurrently and the outcome of the execution depends on the order in which the access takes place, we have a race condition.

9. Critical-Section Problem Say a system consists of n processes {P0, P1, …, Pn-1}. Each process has a segment of code, called critical section (CS), in which the process maybe changing common variables, updating a table, writing a file etc. When one process is executing in its CS, no other process can be allowed to execute in its CS (mutual exclusion). The CS problem is to design a protocol that the processes can use to cooperate.

10. Structure of a Typical Process

11. Solution to Critical-Section Problem Mutual Exclusion - If process Pi is executing in its critical section, then no other processes can be executing in their critical sections. 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. 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 the N processes

12. Two Process Solutions Algorithm 1 do { while turn ≠ i do noop; Critical section turn = j; Remainder section } while (TRUE); Here Pi and Pj share an integer variable turn initialized to 0. If turn = i, Pi can execute in its critical section. Mutual exclusion is assured but progress requirement is not since it requires strict alternation of processes in the execution of the critical section. E.g. if turn = i and Pi does not want to enter the CS and Pj wants to enter the CS, then it can’t.

13. Two Process Solutions Algorithm 2 Pi and Pj share an array: boolean flag[2]; flag[0] and flag[1] are initialized to false. If flag[i] is true, then it indicates Pi is ready to enter its critical section. do { flag[i] = true; while flag[j] do noop; Critical section flag[i] = false; Remainder section } while (TRUE); Here too the progress requirement is not met even though mutual exclusion is satisfied. Need to remember that P0 and P1 are executing concurrently. Say, T0: P0 sets flag[0] to true T1: P1 sets flag[1] to true Now both P0 and P1 are looping infinitely in their while loops.

14. Peterson’s Solution Two process solution Assume that the LOAD and STORE 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 flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process Piis ready!

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

16. Peterson’s Solution contd.. • This algorithm meets all 3 conditions that a solution to CS problem must meet. • Say Pi wants to enter its CS. It can be prevented only if flag[j] is true and turn = j. If Pj is not ready to enter its CS, then flag[j] is false and Pi will enter its CS. • If Pj has set flag[j] to true and is also executing in its while loop, then either turn = i or turn = j (depends on whether Pi or Pj accessed turn last) and so mutual exclusion is assured. • If say Pj enters it CS then upon exiting it will set flag[j] to false so Pi can now enter its CS. • If Pj resets flag[j] to true if it wants to enter CS again, then it will also turn = i. So Pi will enter its CS (progress is assured) after at most 1 entry by Pj (bounded waiting is assured).

17. Semaphore • A semaphore S is a global variable that is accessed by only process Pi at a time. • It is an integer variable that, apart from initialization, is accessed only through two atomic operations: wait and signal. • Two standard operations modify S: wait () and signal () • Originally called P() andV() wait (s): while S ≤ 0 do noop; S = S – 1; signal (s): S = S + 1; wait and signal are executed indivisibly (while one process is modifying the semaphore, no other process can simultaneously modify it.)

18. Semaphore Usage – Mutual Exclusion • Semaphores are used to deal with the n-process critical section problem. • n processes can share a semaphore, mutex, initialized to 1. • Each Pi executes as follows: do { wait (mutex); Critical section signal (mutex); Remainder section } while (TRUE);

19. Semaphore Usage - Synchronization • Semaphores can also be used to solve synchronization problems. • Say there are 2 concurrently executing processes: P1 with a statement S1 and P2 with S2. S2 needs to be executed only after S1. • Solution: Let P1 and P2 share a semaphore, synch, initialized to 0. Process P1: S1 signal (synch); Process P2: wait (synch); S2; Because synch is initialized to 0, P2 will execute S2 only after P1 has invoked signal (synch) after executing S1.

20. Disadvantage of these mutual exclusion solutions • These solutions all require busy waiting i.e. while a process is in its CS, all other processes trying to enter CS must loop continuously in the entry code (wait on mutex). Thus CPU cycles are wasted. This is also called spinlock (Pi “spins” waiting on mutex lock). • Advantage of spin lock is that no context switch is required. So this can be used where applications need to hold the lock for a short duration. • Note that applications may spend lots of time in critical sections and therefore this may not be a good solution.

21. Semaphore Implementation with no Busy waiting • When a process executes wait (s) and finds the semaphore to be non-positive, it blocks itself. • The block operation places a process into a waiting queue associated with semaphore and the state of the process is changed to the waiting state. • A blocked process is restarted when another process executes a signal operation. This is done by a wakeup operation. • Two operations: • block – place the process invoking the operation on the appropriate semaphore waiting queue. • wakeup – remove one of processes in the waiting queue and place it in the ready queue.

22. Semaphore Implementation with no Busy waiting (Cont.) • Semaphore definition: • An integer value (initialized to 1) and a list of processes • Implementation of wait(): • Implementation of signal(): • Note: If the semaphore value is negative, its magnitude is the # of processes waiting on that semaphore (because of switching of order of decrement and test in wait(s)).

23. Deadlocks • The implementation of a semaphore with a waiting queue can result in a deadlock. • Let S and Q be two semaphores initialized to 1 P0 P1 wait (S); wait (Q); wait (Q); wait (S); : : signal (S); signal (Q); signal (Q); signal (S);

24. Classical Problems of Synchronization Bounded-Buffer Problem Readers and Writers Problem Dining-Philosophers Problem

25. Bounded-Buffer Problem using Semaphores N buffers, each can hold one item Semaphore mutex initialized to the value 1 to provide mutual exclusion for access to the buffer pool. Semaphore full initialized to the value 0 to count the # of full buffers. Semaphore empty initialized to the value N to count the # of empty buffers.

26. Bounded-Buffer Problem using Semaphores Producer process: do { produce an item in nextp; wait (empty); wait (mutex); Add nextp to buffer; signal (mutex); signal (full); } while (TRUE);

27. Bounded-Buffer Problem using Semaphores Consumer process: do { wait (full); wait (mutex); Remove an item from buffer to nextc; signal (mutex); signal (empty); consume the item nextc; } while (TRUE);

28. Readers-Writers Problem A data set (file or record or variable) is shared among a number of concurrent processes Readers – only read the data set; they do not perform any updates Writers – can both read and write Problem – allow multiple readers to read at the same time. Mutual exclusion is needed to prevent a writer and some other process from simultaneously accessing the shared object (e.g. airline reservation record). This synchronization is called the readers and writers problem. No reader should be kept waiting unless a writer has already obtained permission to use the shared object (could lead to writer starvation.)

29. Readers-Writers Problem Shared Data Semaphore mutex initialized to 1 (ensures mutual exclusion when the variable readCount is updated) Semaphore wrt initialized to 1 (common to both readers and writers. It is used to provide mutual exclusion to writers. It is also used by the first or last reader that enters or exits the CS) Integer readCount initialized to 0 (the # of readers currently reading the object)

30. Readers-Writers Problem Writer process: do { wait (wrt); …. writing is performed; …. signal (wrt); } while (TRUE);

31. Readers-Writers Problem Reader process: do { wait (mutex); readCount++; // # of readers if (readCount == 1) wait (wrt); signal (mutex); … reading is performed; wait (mutex); readCount--; if (readCount == 0) signal (wrt); signal (mutex); } while (TRUE);

32. Dining-Philosophers Problem Problem represents the need to allocate several resources among several processes in a deadlock and starvation free manner. Solution: Represent each chopstick by a semaphore. A philosopher tries to grab a chopstick by executing a wait() on that semaphore. A chopstick is released by a signal operation. Semaphore chopStick [5] initialized to 1

33. Dining-Philosophers Problem (Cont.) The structure of Philosopher i: do { wait (chopstick[i]); wait (chopstick[i+1 mod 5]); … eat … signal(chopstick[i]); signal (chopstick[i+1 mod 5]); … think; … } while (TRUE); Disadvantage: while this solution guarantees that no two neighbors are eating simultaneously, it might create a deadlock if say all 5 philosophers grab their left chopsticks. Now no philosopher can get the chopstick to the right.

34. Java Synchronization • Java provides synchronization at the language-level. • Each Java object has an associated lock. • This lock is acquired by invoking a synchronized method. • This lock is released when exiting the synchronized method. • Threads waiting to acquire the object lock are placed in the entry set for the object lock.

35. Java Synchronization • Each object has an associated entry set.

36. Java Synchronization • Synchronized insert() and remove() methods – Incorrect!

37. Java Synchronization wait/notify() • When a thread invokes wait():1. The thread releases the object lock;2. The state of the thread is set to Blocked; 3. The thread is placed in the wait set for the object. • When a thread invokes notify():1. An arbitrary thread T from the wait set is selected;2. T is moved from the wait to the entry set;3. The state of T is set to Runnable.

38. Java Synchronization • Entry and wait sets

39. Java Synchronization – wait/notify • Synchronized insert() method – Correct!

40. Java Synchronization – wait/notify • Synchronized remove() method – Correct!

41. Java Synchronization - Bounded Buffer

42. Java Synchronization • The call to notify() selects an aribitrary thread from the wait set. It is possible the selected thread is in fact not waiting upon the condition for which it was notified. • The call notifyAll() selects all threads in the wait set and moves them to the entry set. • In general, notifyAll() is a more conservative strategy than notify().

43. Java Synchronization notify() may not notify the correct thread!

44. Java Synchronization - Readers-Writers

45. Java Synchronization - Readers-Writers • Methods called by readers

46. Java Synchronization - Readers-Writers • Methods called by writers

47. Java Synchronization • Rather than synchronizing an entire method, Block synchronization allows blocks of code to be declared as synchronized

48. Java Synchronization • Block synchronization using wait()/notify()

49. Concurrency Features in Java 5 • Prior to Java 5, the only concurrency features in Java were Using synchronized/wait/notify. • Beginning with Java 5, new features were added to the API: • Reentrant Locks • Semaphores • Condition Variables

50. Concurrency Features in Java 5 • Reentrant Locks