Processes and Threads

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. Agenda • 2.1 Processes • 2.2 Threads • 2.3 Interprocess communication • 2.4 Classical IPC problems • 2.5 Scheduling

3. Process • The most central concept in any OS • An abstraction of a running program • Modern Computers • Can do more than one thing at the same time • Can run user programs • Can read disk and work with user terminal • In a multiprogramming system • More than one user program can be scheduled • Each may run for tens of msecs.

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

5. 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

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

7. 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

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

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

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

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

12. Agenda • 2.1 Processes • 2.2 Threads • 2.3 Interprocess communication • 2.4 Classical IPC problems • 2.5 Scheduling

13. Threads • In traditional OS, each process has • An address space • A single tread of control • In modern OS, each process may have • Multiple threads of control • Same address spare • Running in quasi-parallel • Thread or a Lightweight Process has • a program counter, registers, stack

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

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

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

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

18. Thread Usage (2) A multithreaded Web server

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

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

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

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

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

24. 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) • Called upcall

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

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

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

28. Agenda • 2.1 Processes • 2.2 Threads • 2.3 Interprocess communication • 2.4 Classical IPC problems • 2.5 Scheduling

29. Interprocess Communication • Processes may need to communicate • E.g., output of one goes to input of the other one • Issues • How to pass information • different address spaces • Making critical activities • Two process try to grab the last 1MB of memory • Proper sequencing • One process is producing data for the other one

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

31. Critical Regions (1) • Critical Regions or Critical Sections • The part of the program where shared memory is accessed. • 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

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

33. Mutual Exclusion with Busy Waiting (0) • Mutual Exclusion • Only one process can be in the critical section. • Disabling Interrupts (HW Solution) • Dangerous: May lead to the end of the system • Lock Variables (SW Solution) • Fatal Flow: Similar to Spooler Directory • Strict Alternation (SW Solution) • Busy Waiting, Violating Condition 3 • Peterson’s Solution (SW Solution) • Busy Waiting • TSL Instruction (HW/SW Solution) • Busy Waiting, but Faster

34. Mutual Exclusion with Busy Waiting (1)Strict Alternation (SW Solution) Proposed solution to critical region problem (a) Process 0. (b) Process 1.

35. Mutual Exclusion with Busy Waiting (2)Peterson’s Solution (SW Solution)

36. Mutual Exclusion with Busy Waiting (3)Using TSL Instruction (HW/SW Solution) Entering and leaving a critical region using the TSL instruction

37. Mutual Exclusion with Sleep and Wakeup (0)Producer-Consumer Problem • Priority Inversion Problem • busy waiting with priority! • Solution with Fatal Race Condition • Waking up a consumer that is not asleep yet! • Semaphore • An integer that counts the number of wake-up calls. • Mutexes • Binary semaphores, good for mutual exclusion. • Monitors • Easier to program (Synchronized in Java). • Message Passing • N messages used for communication coordination. • Barriers • Synchronization of N processes/threads.

38. Mutual Exclusion with Sleep and Wakeup (1)Fatal Race Condition

39. Mutual Exclusion with Sleep and Wakeup (2)Using Semaphores

40. Mutual Exclusion with Sleep and Wakeup (3)Using Mutexes Implementation of mutex_lock and mutex_unlock

41. Mutual Exclusion with Sleep and Wakeup (4)Using Monitors (1) Example of a monitor

42. Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (2) • Outline of producer-consumer problem with monitors • only one monitor procedure active at one time • buffer has N slots

43. Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (3) Solution to producer-consumer problem in Java (part 1)

44. Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (4) Solution to producer-consumer problem in Java (part 2)

45. Mutual Exclusion with Sleep and Wakeup (5) Message Passing The producer-consumer problem with N messages

46. Barriers • Use of a barrier • processes approaching a barrier • all processes but one blocked at barrier • last process arrives, all are let through

47. Agenda • 2.1 Processes • 2.2 Threads • 2.3 Interprocess communication • 2.4 Classical IPC problems • 2.5 Scheduling

48. Dining Philosophers (1) • Philosophers eat/think • Eating needs 2 forks • Pick one fork at a time • How to prevent deadlock

49. Dining Philosophers (2) A nonsolution to the dining philosophers problem

50. Dining Philosophers (3) Solution to dining philosophers problem (part 1)