Processes, Threads, and IPC: Understanding the Basics

1. Processes and Threads 2.1 Processes 2.2 Threads 2.3 Interprocess communication 2.4 Classical IPC problems 2.5 Scheduling Chapter 2

2. ProcessesThe Process Model • Multiprogramming of four programs • Conceptual model of 4 independent, sequential processes • Only one program active at any instant

3. Process Creation Principal events that cause process creation • System initialization • Execution of a process creation system • User request to create a new process • Initiation of a batch job

4. Process Termination Conditions which terminate processes • Normal exit (voluntary) • Error exit (voluntary) • Fatal error (involuntary) • Killed by another process (involuntary)

5. Process Hierarchies • Parent creates a child process, child processes can create its own process • Forms a hierarchy • UNIX calls this a "process group" • Windows has no concept of process hierarchy • all processes are created equal

6. Process States (1) • Possible process states • running • blocked • ready • Transitions between states shown

7. Process States (2) • Lowest layer of process-structured OS • handles interrupts, scheduling • Above that layer are sequential processes

8. Implementation of Processes (1) Fields of a process table entry

9. Implementation of Processes (2) Skeleton of what lowest level of OS does when an interrupt occurs

10. ThreadsThe Thread Model (1) (a) Three processes each with one thread (b) One process with three threads

11. The Thread Model (2) • Items shared by all threads in a process • Items private to each thread

12. The Thread Model (3) Each thread has its own stack

13. Thread Usage (1) A word processor with three threads

14. Thread Usage (2) A multithreaded Web server

15. Thread Usage (3) • Rough outline of code for previous slide (a) Dispatcher thread (b) Worker thread

16. Thread Usage (4) Three ways to construct a server

17. Implementing Threads in User Space A user-level threads package

18. Implementing Threads in the Kernel A threads package managed by the kernel

19. Hybrid Implementations Multiplexing user-level threads onto kernel- level threads

20. Scheduler Activations • Goal – mimic functionality of kernel threads • gain performance of user space threads • Avoids unnecessary user/kernel transitions • Kernel assigns virtual processors to each process • lets runtime system allocate threads to processors • Problem: Fundamental reliance on kernel (lower layer) calling procedures in user space (higher layer)

21. Pop-Up Threads • Creation of a new thread when message arrives (a) before message arrives (b) after message arrives

22. Making Single-Threaded Code Multithreaded (1) Conflicts between threads over the use of a global variable

23. Making Single-Threaded Code Multithreaded (2) Threads can have private global variables

24. Interprocess CommunicationRace Conditions Two processes want to access shared memory at same time

25. Critical Regions (1) Four conditions to provide mutual exclusion • No two processes simultaneously in critical region • No assumptions made about speeds or numbers of CPUs • No process running outside its critical region may block another process • No process must wait forever to enter its critical region

26. Critical Regions (2) Mutual exclusion using critical regions

27. Algorithm 1 • Shared variables: • int turn;initially turn = 0 • turn - i Pi can enter its critical section • Process Pi do { while (turn != i) /*infinite loop */; critical section turn = j; reminder section } while (1);

28. Mutual Exclusion with Busy Waiting (1) Proposed solution to critical region problem (a) Process 0. (b) Process 1.

29. Mutual Exclusion with Busy Waiting (2) Peterson's solution for achieving mutual exclusion

30. Synchronization Hardware • Test and modify the content of a word atomically. boolean TestAndSet(boolean &target) { boolean rv = target; tqrget = true; return rv; }

31. Mutual Exclusion with Test-and-Set • Shared data: boolean lock = false; • Process Pi do { while (TestAndSet(lock)) ; critical section lock = false; remainder section }

32. Mutual Exclusion with Busy Waiting (3) Entering and leaving a critical region using the TSL instruction

33. Synchronization Hardware • Atomically swap two variables. void Swap(boolean &a, boolean &b) { boolean temp = a; a = b; b = temp; }

34. Mutual Exclusion with Swap • Shared data (initialized to false): boolean lock; • Process Pi do { key = true; while (key == true) Swap(lock,key); critical section lock = false; remainder section }

35. Semaphores • Synchronization tool that does not require busy waiting. • Semaphore S – integer variable • can only be accessed via two indivisible (atomic) operations wait/down (S): while S 0 do no-op;S--; signal/up (S): S++;

36. Critical Section of n Processes • Shared data: semaphore mutex; //initially mutex = 1 • Process Pi: do { wait(mutex);critical section signal(mutex); remainder section} while (1);

37. Semaphore Implementation • Define a semaphore as a record typedef struct { int value; struct process *L; } semaphore; • Assume two simple operations: • block suspends the process that invokes it. • wakeup(P) resumes the execution of a blocked process P.

38. Implementation • Semaphore operations now defined as wait/down(S): S.value--; if (S.value < 0) { add this process to S.L; block; } signal/up(S): S.value++; if (S.value <= 0) { remove a process P from S.L; wakeup(P); }

39. Mutexes Implementation of mutex_lock and mutex_unlock

40. Semaphore as a General Synchronization Tool • Execute B in Pj only after A executed in Pi • Use semaphore flag initialized to 0 • Code: Pi Pj   Await/down(flag) signal/up(flag) B

41. 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 and Q be 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.

42. Two Types of Semaphores • Counting semaphore – integer value can range over an unrestricted domain. • Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement. • Can implement a counting semaphore S as a binary semaphore.

43. Implementing S as a Binary Semaphore • Data structures: binary-semaphore S1, S2; int C: • Initialization: S1 = 1 S2 = 0 C = initial value of semaphore S

44. Implementing S • wait operation wait(S1); C--; if (C < 0) { signal(S1); wait(S2); } signal(S1); • signal operation wait(S1); C ++; if (C <= 0) signal(S2); else signal(S1);

45. Sleep and Wakeup Producer-consumer problem with fatal race condition

46. Semaphores The producer-consumer problem using semaphores

47. Semaphores The producer-consumer problem using semaphores

48. Monitors (1) Example of a monitor

49. Monitors (2) • Outline of producer-consumer problem with monitors • only one monitor procedure active at one time • buffer has N slots

50. Monitors (3) Solution to producer-consumer problem in Java (part 1)