Chapter 5 Concurrency: Mutual Exclusion and Synchronization

1. Chapter 5Concurrency: Mutual Exclusion and Synchronization • Principals of Concurrency • Mutual Exclusion: Hardware Support • Semaphores • Readers/Writers Problem

2. Multiple Processes • Central to the design of modern Operating Systems is managing multiple processes • Multiprogramming • Multiprocessing • Distributed Processing • Big Issue is Concurrency • Managing the interaction of all of these processes

3. Interleaving and Overlapping Processes • Earlier (Ch2) we saw that processes may be interleaved on uniprocessors

4. Interleaving and Overlapping Processes • And not only interleaved but overlapped on multi-processors • Both interleaving and overlapping present the same problems in concurrent processing

5. Difficulties of Concurrency • Sharing of global resources • consider two processes perform reads and writes on the same global variable • Optimally managing the allocation of resources is difficult • Difficult to locate programming errors as results are not deterministic and reproducible.

6. What would happen if P1 is interrupted here by P2? A Simple Example Suppose echo is a shared procedure and P1 echoes ‘x’ and P2 echoes ‘y’ void echo() { // send a keyboard-input character to display chin = getchar(); chout = chin; putchar(chout); } What would happen if only one process is permitted at a time to be in the procedure?

7. A Simple Example: On a Multiprocessor Process P1 Process P2 . . chin = getchar(); . . chin = getchar(); chout = chin; chout = chin; putchar(chout); . . putchar(chout); . .

8. Enforce Single Access • If we enforce a rule that only one process may enter the function at a time then: • P1 & P2 run on separate processors • P1 enters echo first, • P2 tries to enter but is blocked • P1 completes execution • P2 resumes and executes echo Solution: Control access to shared resource

9. Race Condition • A race condition occurs when • Multiple processes or threads read and write data items • They do so in a way where the final result depends on the order of execution of the processes • The output depends on who finishes the race last (the loser) • However, the processes and outputs must be independent of the processing speed

10. Process Interaction Example: multiprogramming of multiple independent processes Example: processes share access to some object, such as an I/O buffer Example: Processes designed to work jointly on some activity

11. Competition among Processes for Resources Three main control problems: • Need for Mutual Exclusion • Critical resource: nonsharable resource, e.g., printer • Critical section: portion of the program that uses a critical resource • Deadlock • Starvation

12. Requirements for Mutual Exclusion • Only one process at a time is allowed in the critical section for a resource • A process that halts in its noncritical section must do so without interfering with other processes • No deadlock or starvation

13. Requirements for Mutual Exclusion • A process must not be delayed access to a critical section when there is no other process using it • No assumptions are made about relative process speeds or number of processes • A process remains inside its critical section for a finite time only

14. Roadmap • Principals of Concurrency • Mutual Exclusion: Hardware Support • Semaphores • Readers/Writers Problem

15. Disabling Interrupts • Uniprocessors only allow interleaving • Interrupt Disabling • A process runs until it invokes an operating system service or until it is interrupted • Disabling interrupts guarantees mutual exclusion because the critical section cannot be interrupted

16. Pseudo-Code while (true) { /* disable interrupts */; /* critical section */; /* enable interrupts */; /* remainder */; } •  Reduced interleaving •  Will not work in multiprocessor architecture

17. Special MachineInstructions • Use of atomic action: instruction is treated as a single step that cannot be interrupted • Compare&Swap Instruction int compare_and_swap (int *word, int testval, int newval) { /* checks a memory location (*word) against a test value (testval) */ int oldval; oldval = *word; if (oldval == testval) *word = newval; return oldval; }

18. Mutual Exclusion (fig 5.2) • The only process that may enter its critical section is one that finds bolt equal to 0 • All other processes go into a busy waiting mode

19. Hardware Mutual Exclusion: Advantages • Applicable to any number of processes on either a single processor or multiple processors sharing main memory • It is simple and therefore easy to verify • It can be used to support multiple critical sections

20. Hardware Mutual Exclusion: Disadvantages • Busy-waiting consumes processor time • Starvation is possible when a process leaves a critical section and more than one process is waiting • Some process could indefinitely be denied access because selection of a waiting process is arbitrary • Deadlock is possible • Example: P1 enters its critical section and is then preempted by higher priority P2 which will go into a busy waiting loop

21. Roadmap • Principals of Concurrency • Mutual Exclusion: Hardware Support • Semaphores • Readers/Writers Problem

22. Semaphore • Semaphore: • An integer value used for signalling among processes • Only three operations may be performed on a semaphore, all of which are atomic: • initialize (to a nonnegative integer value) • decrement (semWait) • increment (semSignal)

23. Semaphore Primitives

24. Binary Semaphore Primitives

25. Strong/WeakSemaphore • A queue is used to hold processes waiting on the semaphore • In what order are processes removed from the queue? • Strong Semaphores use FIFO • Weak Semaphores don’t specify the order of removal from the queue

26. Mutual Exclusion Using Semaphores • The first process that executes a semWait will be able to enter the critical section immediately, setting the value of s to 0 • Any other processes attempting to enter the critical section will find it busy and will be blocked

27. Processes Accessing Shared Data UsingSemaphore Three processes (A,B,C) access a shared resource protected by the semaphore lock.

28. Mutual Exclusion Using Semaphores • The semaphore can be initialized to a specified value to allow more than one process in its critical section at a time • s.count0: the number of processes that can execute semWait(s) without suspension • s.count<0: the magnitude is the number processes suspended in s.queue

29. Producer/Consumer Problem • General Situation: • One or more producers are generating data and placing these in a buffer • A single consumer is taking items out of the buffer one at time • Only one producer or consumer may access the buffer at any one time • The Problem: • Ensure that the Producer can’t add data into full buffer and consumer can’t remove data from empty buffer Producer/Consumer Animation

30. Functions • Assume an infinite buffer b with a linear array of elements

31. Buffer The consumer must make sure that the producer has advanced beyond it (in > out) before proceeding The producer can generate items and store them in the buffer at its own pace. Each time, in is incremented

32. n is equal to the number of items in the buffer Prevent the consumer and any other producer from accessing the buffer during the append operation The consumer must wait on both semaphores before proceeding Semaphores

33. Bounded Buffer The buffer is treated as a circular storage; pointer values are expressed modulo the size of the buffer

34. Functions in a Bounded Buffer in and out are initialized to 0 and n is the size of the buffer

35. e keeps track of the number of empty spaces Semaphores

36. DemonstrationAnimations • Producer/Consumer • Illustrates the operation of a producer-consumer buffer. • Bounded-Buffer Problem Using Semaphores • Demonstrates the bounded-buffer consumer/producer problem using semaphores.

37. Roadmap • Principals of Concurrency • Mutual Exclusion: Hardware Support • Semaphores • Readers/Writers Problem

38. Readers/Writers Problem • A data area (e.g., a file) is shared among many processes • Some processes (readers) only read the data area, some (writers) only write to the area • Conditions to satisfy: • Multiple readers may simultaneously read the file • Only one writer at a time may write • If a writer is writing to the file, no reader may read it interaction of readers and writers.

39. readcount keeps track of the number of readers x is used to assure that readcount is updated properly the first reader that attempts to read should wait on wsem wsem is used to enforce mutual exclusion: as long as one writer is accessing the shared data area, no other writers and no readers may access it Readers have Priority

40. Readers/Writers Problem Once a single reader has begun to access the data area, it is possible for readers to retain control of the data area as long as there is at least one reader reading. Therefore, writers are subject to starvation An alternative solution: no new readers are allowed access to the data area once at least one writer wants to write

41. y controls the updating of writecount writecount controls the setting of rsem rsem inhibits all readers while there is at least one writer desiring access to the data area Writers have Priority

42. Writers have Priority only one reader is allowed to queue on rsem, with any additional readers queuing on z

43. Writers have Priority

44. Key Terms