Chapter 5

1. 3 Chapter 5

2. Theme of OS Design? Management of processes and threads Multiprogramming Multiprocessing Distributed processing

3. Currency • Communication among processes • Sharing resources • Synchronization of multiple processes • Allocation of processor time

4. Concurrency in different contexts • Multiple applications • Multiprogramming – time sharing • Structured application • Application can be a set of concurrent processes • Operating-system structure • Operating system is a set of processes or threads Unit of concurrency : process / thread

5. Chapter 5 • What is mutual exclusion? • How to implement mutual exclusion • Busy waiting? Software/Hardware • Semaphore, monitor, massage passing

6. Difficulties with Concurrency • Sharing global resources, global variables • Management of allocation of resources • Optimally?, deadlock? • Programming errors difficult to locate • Relative speed of execution can not be predicted • Other process, interrupt handling, scheduling

7. A Simple Example void echo() { chin = getchar(); chout = chin; putchar(chout); } //uniprocessor Block from entering

8. A Simple Example Process P1 Process P2 . . in = getchar(); . . in = getchar(); chout = chin; chout = chin; putchar(chout); . . putchar(chout); . . //in processor A & processor B Control access to the shared resource.

9. Operating System Concernsissues • Keep track of active processes • Allocate and deallocate resources • Processor time • Memory • Files • I/O devices • Protect data and resources –chapter 15 • Result of process must be independent of the speed of execution of other concurrent processes – this chapter PCB

10. Process Interaction • Processes unaware of each other –multiprograming of multiplr independent process – competition for disk, file or printer • Processes indirectly aware of each other –not by the PID, but by sharing some object such as I/O buffer • Process directly aware of each other –by PID and can communicate each other - cooperation

12. Processes : P1, P2 Resources: R1, R2 P1 hold R1 and waiting R2 P2 hold R2 and waiting R1 Processes : P1, P2, P3 Resources: R P1 hold R and P2, P3 are waiting P3 – P1 – P3 …. Competition Among Processes for Resources • Mutual Exclusion (non shareable resource) • Critical sections – portion of program • Only one program at a time is allowed in its critical section • Example only one process at a time is allowed to send command to the printer • Deadlock • Starvation

13. Mutual exclusion mechanism in abstract terms Const int n = // number of processes; void P(int i){ while (true) { entercritical(i); //critical section; exitcritical(i); } } Void main() { parbegin(P(R1), P(R2), …,P(Rn)) }

14. Cooperation Among Processes by Sharing • Aware each other with shared data • Writing must be mutually exclusive- • data coherence is required • Critical sections are used to provide data integrity

15. In a bookkeeping application a = b P1: a = a + 1; b = b + 1; P2: b = 2 * b; a = a * 2; Start w/ a = b = 1 A = 4 , b = 3 -> cs is important!

16. Cooperation Among Processes by Communication • Communication • A way of synchronization or coordination • Messages are passes – no sharing • Mutual exclusion is not a control requirement • Possible to have deadlock • Each process waiting for a message from the other process • Possible to have starvation • Two processes sending message to each other while another process waits for a message

17. 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 non-critical section must do so without interfering with other processes • No deadlock or starvation

18. 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

19. Ways • Each process takes responsibility • Software approach • High processing overhead, error prone • Special purpose machine instn • Hardware approach • Special purpose • O/S or PL supports

20. First Attempt • Global memory - turn • Busy Waiting • Process is always checking to see if it can enter the critical section • Process can do nothing productive until it gets permission to enter its critical section

21. //process 0 … While (turn != 0) //do nothing; //critical section turn = 1 //process 1 … While (turn != 1) //do nothing; //critical section turn = 0 Drawback ; Speed is dictated by slower process One fails inside or outside cs

22. Coroutine • Designed to be able to pass execution control back and forth between themselves • Inadequate to support concurrent processing

23. Second Attempt • Each process can examine the other’s status but cannot alter it • When a process wants to enter the critical section is checks the other processes first • If no other process is in the critical section, it sets its status for the critical section • Each process can check the flags and then proceed to enter the critical section at the same time

24. Boolean flag[2] = false, false //process 0 … While (flag[1]) //do nothing; flag[0] = true; //critical section Flag[0] = false; //process 1 … While (flag[0]) //do nothing; flag[1] = true; //critical section Flag[1] = false;

25. drawbacks • If processes fails inside its cs • It does not guarantee mutual exclusion • P0 finds flag[1] set to false. • P1 finds flag[0] set to false. • P0 set flag[0] and enter cs • P1 set flag[1] and enter cs

26. Third attempt //process 0 … Flag[0] = true While (flag[1]) //do nothing; //critical section Flag[0] = false; //process 1 … Flag[1] = true While (flag[0]) //do nothing; //critical section Flag[1] = false;

27. Third Attempt • Set flag to enter critical section before check other processes • If another process is in the critical section when the flag is set, the process is blocked until the other process releases the critical section • Deadlock is possible when two process set their flags to enter the critical section. Now each process must wait for the other process to release the critical section

28. Fourth Attempt • A process sets its flag to indicate its desire to enter its critical section but is prepared to reset the flag • Other processes are checked. If they are in the critical region, the flag is reset and later set to indicate desire to enter the critical region. This is repeated until the process can enter the critical region.

29. 4th attempt //process 1 … Flag[1] = true While (flag[0]){ flag[0] = false; Delay; Flag[0] = true}; //critical section Flag[1] = false; //process 0 … Flag[0] = true While (flag[1]){ flag[0] = false; Delay; Flag[0] = true}; //critical section Flag[0] = false;

30. Fourth Attempt • It is possible for each process to set their flag, check other processes, and reset their flags. This scenario will not last very long so it is not deadlock. It is undesirable

31. livelock • P0 sets flag[0] to true • P1 sets flag[1] to true • P0 checks flag[1] • P1 checks flag[0] • P0 sets flag[0] to false • P1 sets flag[1] to false • P0 sets flag[0] to true • P1 sets flag[1] to true

32. Correct Solution • Each process gets a turn at the critical section • If a process wants the critical section, it sets its flag and may have to wait for its turn • Set flag and check other’s flag, if set consults turn -> eventually get its turn

33. Fig 5.3 Dekker’s algorithm Void P1(){ while (true) { flag[1] = true; while(flag[0]) if (turn == 1){ flag[1] = false; while (turn == 0) //do nothing; flag[1] = true; } //cs turn = 0; flag[1] = false }} Boolean flag 2; Int turn; Void P0(){ while (true) { flag[0] = true; while(flag[1]) if (turn == 1){ flag[0] = false; while (turn == 1) //do nothing; flag[0] = true; } //cs turn = 1; flag[0] = false }} Void main(){ Flag[0] = false; Flag[1] = false; Turn = 1; Parbegin(P0,P1) }

34. Fig 5.3 Peterson’s algorithm Void P1(){ while (true) { flag[1] = true; turn = 0 while(flag[1] && turn == 1) //do nothing; //cs; flag[1] = false; } } Boolean flag 2; Int turn; Void P0(){ while (true) { flag[0] = true; turn = 1 while(flag[1] && turn == 1) //do nothing; //cs; flag[0] = false; } } Void main(){ Flag[0] = false; Flag[1] = false; Parbegin(P0,P1) }

35. Mutual Exclusion:Hardware Support • Interrupt Disabling • A process runs until it invokes an operating-system service or until it is interrupted • Disabling interrupts guarantees mutual exclusion • Processor is limited in its ability to interleave programs • Multiprocessing • disabling interrupts on one processor will not guarantee mutual exclusion

36. Mutual Exclusion:Hardware Support • Special Machine Instructions • Performed in a single instruction cycle • Not subject to interference from other instructions • Reading and writing • Reading and testing

37. Mutual Exclusion:Hardware Support • Test and Set Instruction boolean testset (int i) { if (i == 0) { i = 1; return true; } else { return false; } }

38. Mutual Exclusion:Hardware Support • Exchange Instruction void exchange(int register, int memory) { int temp; temp = memory; memory = register; register = temp; }

39. Const int = n //number of processes int bolt; void P(int i) { while (true) { while(!testset(bolt)) //do nothing; //critical section; bolt = 0; } } void main() { bolt = 0; parbegin(P(1),P(2), …,P(n)); }

40. Const int = n //number of processes int bolt; void P(int i) { int keyi; while (true) { keyi = 1; while(keyi != 0) exchange(keyi,bolt); //critical section; exchange(keyi,bolt); bolt = 0; } } void main() { bolt = 0; parbegin(P(1),P(2), …,P(n)); }

41. Mutual Exclusion Machine Instructions • 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

42. Mutual Exclusion Machine Instructions • Disadvantages • Busy-waiting consumes processor time • Starvation is possible when a process leaves a critical section and more than one process is waiting. • Deadlock • If a low priority process has the critical region and a higher priority process needs, the higher priority process will obtain the processor to wait for the critical region

43. O.S / PL • Semaphore • Monitors • Message passing

44. Semaphores • Special variable called a semaphore is used for signaling • If a process is waiting for a signal, it is suspended until that signal is sent • Wait and signal operations cannot be interrupted • Queue is used to hold processes waiting on the semaphore

45. Semaphores • Semaphore is a variable that has an integer value • May be initialized to a nonnegative number • Wait operation decrements the semaphore value • Signal operation increments semaphore value • signal(s): P, wait(s): V

46. Const int n //number of processes Semaphore s = 1; Void P(int I) { while(true) { wait(s); //cs; signal(s); } } Void main() { Parbegin(P(1),P(2), …P(n)) }

47. A B C 1 Wait(s) 0 Wait(s) B Wait(s) B C signal(s)

48. Producer/Consumer Problem • 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 • Power & pitfalls of semaphore!

49. Producer producer: while (true) { /* produce item v */ b[in] = v; in++; }

50. Consumer consumer: while (true) { while (in <= out) /*do nothing */; w = b[out]; out++; /* consume item w */ }