Chapter 2: Process/Thread

1. Chapter 2: Process/Thread Instructor: Hengming Zou, Ph.D. In Pursuit of Absolute Simplicity求于至简，归于永恒

2. Content • Processes • Threads • Inter-process communication • Classical IPC problems • Scheduling

3. Definition of A Process • Informal • A program in execution • A running piece of code along with all the things the program can read/write • Formal • One or more threads in their own address space • Note that process != program

4. The Need for Process • What is the principle motivation for inventing process? • To support multiprogramming

5. The Process Model • Conceptual viewing of the processes • Concurrency • Multiple processes seem to run concurrently • But in reality only one active at any instant • Progress • Every process makes progress

6. Multiprogramming of 4 programs Programs in memory Conceptual view Time line view

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

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

9. Process Hierarchies • Parent creates a child process • Child processes can create its own process • Processes creation forms a hierarchy • UNIX calls this a "process group" • Windows has no concept of such hierarchy, i.e. all processes are created equal

10. Process States • Possible process states • running, blocked, ready Running Process blocks for input Scheduler picks another process Scheduler picks this process Blocked Ready Input becomes available

11. Process Space • Also called Address Space • All the data the process uses as it runs • Passive (acted upon by the process) • Play analogy: • all the objects on the stage in a play

12. Process Space • Is the unit of state partitioning • Each process occupies a different state of the computer • Main topic: • How multiple processes spaces can share a single physical memory efficiently and safely

13. Manage Process & Space • Who manages process & space? • The operating systems • How does OS achieve it? • By maintain information about processes • i.e. use process tables

14. Fields of A Process Table • Registers, Program counter, Status word • Stack pointer, Priority, Process ID • Parent group, Process group, Signals • Time when process started • CPU time used, Children’s CPU time, etc.

15. Problems with Process • While supporting multiprogramming on shared hardware • Itself is single threaded! • i.e. a process can do only one thing at a time • blocking call renders entire process unrunnable

16. Threads • Invented to support multiprogramming on process level • Manage OS complexity • Multiple users, programs, I/O devices, etc. • Each thread dedicated to do one task

17. Thread • Sequence of executing instructions from a program • i.e. the running computation • Play analogy: one actor on stage in a play

18. Threads • Processes decompose mix of activities into several parallel tasks (columns) • Each job can work independently of others job1 job2 job3 Thread 1 Thread 2 Thread 3

19. The Thread Model 3 processes each with 1 thread Proc 1 Proc 2 Proc 3 Process User space Thread Thread Kernel Kernel Kernel space 1 process with 3 threads

20. The Thread Model • Some items shared by all threads in a process • Some items private to each thread

21. Shared and Private Items

22. Shared and Private Items Each thread has its own stack

23. A Word Processor w/3 Threads Display thread Backup thread Input thread

24. Implementation of Thread • How many options to implement thread? • Implement in kernel space • Implement in user space • Hybrid implementation

25. Kernel-Level Implementation • Completely implemented in kernel space • OS acts as a scheduler • OS maintains information about threads • In additions to processes

26. Kernel-Level Implementation

27. Kernel-Level Implementation • Advantage: • Easier to programming • Blocking threads does not block process • Problems: • Costly: need to trap into OS to switch threads • OS space is limited (for maintaining info) • Need to modifying OS!

28. User-Level Implementation • Completely implemented in user space • A run-time system acts as a scheduler • Threads voluntarily cooperate • i.e. yield control to other threads • OS need not know existence of threads

29. User-Level Implementation

30. User-Level Implementation • Advantage: • Flexible, can be implemented on any OS • Faster: no need to trap into OS • Problems: • Programming is tricky • blocking threads block process!

31. User-Level Implementation • How do we solve the problem of blocking thread blocks the whole process? • Modifying system calls to be unblocking • Write a wrap around blocking calls • i.e. call only when it is safe to do so

32. Scheduler Activations • A technique that solves the problem of blocking calls in user-level threads • Method: use up-call • Goal – mimic functionality of kernel threads • gain performance of user space threads

33. Scheduler Activations • Kernel assigns virtual processors to process • Runtime sys. allocate threads to processors • Blocking threads are handled by OS upcall • i.e. OS notify runtime system about blocking calls

34. Scheduler Activations • Problem: • Reliance on kernel (lower layer) calling procedures in user space (higher layer) • Violates layered structure of OS design

35. Hybrid Implementation • Can we have the best of both worlds • i.e. kernel-level and user-level implementations • While avoiding the problems of either? • Hybrid implementation

36. Hybrid Implementation • User-level threads are managed by runtime systems • Kernel-level threads are managed by OS • Multiplexing user-level threads onto kernel- level threads

37. Hybrid Implementation

38. Multiple Threads • Can have several threads in a single address space • That is what thread is invented for • Play analogy: several actors on a single set • Sometimes interact (e.g. dance together) • Sometimes do independent tasks

39. Multiple Threads • Private state for a thread vs. global state shared between threads • What private state must a thread have? • Other state is shared between all threads in a process

40. Multiple Threads

41. Multiple Threads • Many programs are written in single-threaded processes • Make them multithreaded is very tricky • Can cause unexpected problems

42. Multiple Threads Conflicts between threads over the use of a global variable

43. Multiple Threads • Many solutions: • Prohibit global variables • Assign each thread its private global variables

44. Multiple Threads Threads can have private global variables

45. Cooperating Threads • Often we create threads to cooperate • Each thread handles one request • Each thread can issue a blocking disk I/O, • wait for I/O to finish • then continue with next part of its request

46. Cooperating Threads • Ordering of events from different threads is non-deterministic • e.g. after 10 seconds, different threads may have gotten differing amounts of work done • thread A ---------------------------------> • thread B - - - - > • thread C - - - - - - - - - - - >

47. Cooperating Threads • Example • thread A: x=1 • thread B: x=2 • Possible results? • Is 3 a possible output? • yes

48. Cooperating Threads • 3 is possible because assignment is not atomic and threads can interleave • if assignment to x is atomic • then only possible results are 1 and 2

49. Atomic Operations • Atomic: indivisible • Either happens in its entirety without interruption or has yet to happen at all • No events from other threads can happen in between start & end of an atomic event

50. Atomic Operations • On most machines, memory load & store are atomic • But many instructions are not atomic • double-precision floating on a 32-bit machine • two separate memory operations