1. 1 Process Description and Control Chapter 2

2. 2 Process A program in execution An instance of a program running on a computer The entity that can be assigned to and executed on a processor A unit of activity characterized by the execution of a sequence of instructions, a current state, and an associated set of system instructions

3. 3 Process Management A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity. Process needs resources to accomplish its task  CPU, memory, I/O, files  Initialization data Process termination requires reclaim of any reusable resources

4. 4 Process Management Single-threaded process has one program counter specifying location of next instruction to execute  Process executes instructions sequentially, one at a time, until completion Multi-threaded process has one program counter per thread Typically system has many processes, some user, some operating system running concurrently on one or more CPUs  Concurrency by multiplexing the CPUs among the processes / threads

5. 5 Process Elements Identifier State Priority Program counter Memory pointers Context data I/O status information Accounting information

6. 6 Process Elements Identifier – unique to the process State – executing/running or not running Priority – relative to other processes Program counter – address of next instruction in the process Memory pointers – to program code and data Context data – data in the registers while process is executing I/O status information – outstanding I/O requests, I/O devices assigned to the process, files used. Accounting information – amount of processor time & clock time used, time limits, account numbers, etc.

7. 7 Process Control Block Contains the process elements Created and managed by the operating system Allows support for multiple processes

8. 8  Process Control Block

9. 9 Trace of Process Sequence of instruction that execute for a process Dispatcher switches the processor from one process to another

10. 10 Example Execution 

11. 11 Trace of Processes I/O operation 5000 = starting address of program of Process A, with 12 instructions 8000 = starting address of program of Process B, with 4 instructions, then perform I/O operation 12000 = starting address of program of Process C, with 12 instructions

12. 12 Assume OS only allows execution of 6 instructions before interrupt Dispatcher 5000 = starting address of dispatcher program, with 6 instructions

13. 13 CPU Switch From Process to Process

14. 14 Context Switch When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process Context-switch time is overhead; the system does no useful work while switching Time dependent on hardware support

15. 15 Change of Process State Save context of processor including program counter and other registers Update the process control block of the process that is currently in the Running state Move process control block to appropriate queue – ready; blocked; ready/suspend Select another process for execution Update the process control block of the process selected Update memory-management data structures Restore context of the selected process

16. 16 When to Switch a Process Clock interrupt  process has executed for the maximum allowable time slice I/O interrupt Memory fault  memory address is in virtual memory so it must be brought into main memory

17. 17 When to Switch a Process Trap  error or exception occurred  may cause process to be moved to Exit state Supervisor call  such as file open

18. 18 Two-State Process Model Process may be in one of two states  Running  Not-running

19. 19 Not-Running Process in a Queue

20. 20 Processes Not-running  Ready to execute  Blocked - waiting for I/O Dispatcher cannot just select the process that has been in the queue the longest because it may be blocked

21. 21 A Five-State Model Running Ready Blocked New Exit Not-running

22. 22 A Five-State Model Running – currently executing Ready – prepared to execute Blocked – cannot execute until some event occurs, e.g. I/O operation New – just created, typically not loaded into MM Exit – released from the pool of executable processes, either halt (completed) or aborted.

23. 23 Five-State Process Model

24. 24 Five State Model Transition NULL----NEW: A new process is created due to any of the four reasons described in the creation of processes (New batch Job, Interactive logon, To provide service & Spawning). NEW-----READY: Operating system moves a process from new state to ready state, when it is prepared to accept an additional process. There could be limit on number of processes to be admitted to the ready state. READY---RUNNING: Any process can be moved from ready to running state when ever it is scheduled.

25. 25 Five State Model Transition RUNNING----EXIT: The currently running process is terminated if it has signaled its completion or it is aborted. RUNNING----READY: The most commonly known situation is that currently running process has taken its share of time for execution (Time out). Also, in some events a process may have to be admitted from running to ready if a high priority process has occurred.

26. 26 Five State Model Transition RUNNING----BLOCKED: A process is moved to the blocked state, if it has requested some data for which it may have to wait. For example the process may have requested a resource such as data file or shared data from virtual memory, which is not ready at that time. BLOCKED---READY: A process is moved to the ready state, if the event for which it is waiting has occurred.

27. 27 Five State Model Transition There are two more transition but are not shown for clarity. READY----EXIT: This is the case for example a parent process has generated a single or multiple children processes & they are in the ready state. Now during the execution of the process it may terminate any child process, therefore, it will directly go to exit state. BLOCKED----EXIT: Similarly as above, during the execution of a parent process any child process waiting for an event to occur may directly go to exit if the parent itself terminates.

28. 28 Process States

29. 29 Using Two Queues

30. 30 Multiple Blocked Queues

31. 31 Ready Queue And Various I/O Device Queues

32. 32 Suspended Processes Processor is faster than I/O so all processes could be waiting for I/O Swap these processes to disk to free up more memory Blocked state becomes suspend state when swapped to disk Two new states  Blocked/Suspend  Ready/Suspend

33. 33 One Suspend State

34. 34 Reasons for Process Suspension

35. 35 Processes and Resources

36. 36 OS Control Structures Information about the current status of each process and resource Tables are constructed for each entity the operating system manages  Memory tables  I/O tables  File tables  Process tables

37. 37 OS Control Structures

38. 38 Memory Tables Includes: Allocation of main memory to processes Allocation of secondary memory to processes Protection attributes for access to shared memory regions Information needed to manage virtual memory

39. 39 I/O Tables I/O device is available or assigned Status of I/O operation Location in main memory being used as the source or destination of the I/O transfer

40. 40 File Tables Existence of files Location on secondary memory Current Status Attributes Sometimes this information is maintained by a file management system

41. 41 Process Table Where process is located Attributes in the process control block  Program  Data  Stack

42. 42 Process Image (in Memory) Contains temporary data (function var, return address, local var) (optional) Memory dynamically allocated during process runtime Contains global var Program code

43. 43 Process Image

44. 44 Process Control Block Process identification  Identifiers Numeric identifiers that may be stored with the process control block include  Identifier of this process  Identifier of the process that created this process (parent process)  User identifier

45. 45 Process Control Block Processor State Information  User-Visible Registers A user-visible register is one that may be referenced by means of the machine language that the processor executes while in user mode. Typically, there are from 8 to 32 of these registers, although some RISC implementations have over 100.

46. 46 Process Control Block Processor State Information  Control and Status Registers These are a variety of processor registers that are employed to control the operation of the processor. These include Program counter: Contains the address of the next instruction to be fetched Condition codes: Result of the most recent arithmetic or logical operation (e.g., sign, zero, carry, equal, overflow) Status information: Includes interrupt enabled/disabled flags, execution mode

47. 47 Process Control Block Processor State Information  Stack Pointers Each process has one or more last-in-first-out (LIFO) system stacks associated with it. A stack is used to store parameters and calling addresses for procedure and system calls. The stack pointer points to the top of the stack.

48. 48 Process Control Block Process Control Information  Scheduling and State Information This is information that is needed by the operating system to perform its scheduling function. Typical items of information: Process state: defines the readiness of the process to be scheduled for execution (e.g., running, ready, waiting, halted). Priority: One or more fields may be used to describe the scheduling priority of the process. In some systems, several values are required (e.g., default, current, highest-allowable) Scheduling-related information: This will depend on the scheduling algorithm used. Examples are the amount of time that the process has been waiting and the amount of time that the process executed the last time it was running. Event: Identity of event the process is awaiting before it can be resumed

49. 49 Process Control Block Process Control Information  Data Structuring A process may be linked to other process in a queue, ring, or some other structure. For example, all processes in a waiting state for a particular priority level may be linked in a queue. A process may exhibit a parent-child (creator-created) relationship with another process. The process control block may contain pointers to other processes to support these structures.

50. 50 Process Control Block Process Control Information  Interprocess Communication Various flags, signals, and messages may be associated with communication between two independent processes. Some or all of this information may be maintained in the process control block.  Process Privileges Processes are granted privileges in terms of the memory that may be accessed and the types of instructions that may be executed. In addition, privileges may apply to the use of system utilities and services.

51. 51 Process Control Block Process Control Information  Memory Management This section may include pointers to segment and/or page tables that describe the virtual memory assigned to this process.  Resource Ownership and Utilization Resources controlled by the process may be indicated, such as opened files. A history of utilization of the processor or other resources may also be included; this information may be needed by the scheduler.

52. 52 Processor State Information Contents of processor registers  User-visible registers  Control and status registers  Stack pointers Program status word (PSW)  contains status information  Example: the EFLAGS register on Pentium machines

53. 53 Process Creation Assign a unique process identifier Allocate space for the process Initialize process control block Set up appropriate linkages  Ex: add new process to linked list used for scheduling queue Create or expand other data structures  Ex: maintain an accounting file

54. 54 Process Creation

55. 55 Process Creation Parent process create children processes, which, in turn create other processes, forming a tree of processes Resource sharing  Parent and children share all resources  Children share subset of parent’s resources  Parent and child share no resources Execution  Parent and children execute concurrently  Parent waits until children terminate

56. 56 Process Termination Process executes last statement and asks the operating system to delete it (exit)  Process’ resources are deallocated by operating system Parent may terminate execution of children processes (abort)  Child has exceeded allocated resources  Task assigned to child is no longer required  If parent is exiting Some operating system do not allow child to continue if its parent terminates  All children terminated - cascading termination

57. 57 Process Termination Reasons for process termination

58. 58 Reasons for process termination Process Termination

59. 59 Cooperating Processes Independent process cannot affect or be affected by the execution of another process Cooperating process can affect or be affected by the execution of another process Advantages of process cooperation  Information sharing  Computation speed-up  Modularity  Convenience

60. 60 Interprocess Communication (IPC) Mechanism for processes to communicate and to synchronize their actions Message system – processes communicate with each other without resorting to shared variables IPC facility provides two operations:  send(message) – message size fixed or variable  receive(message) If P and Q wish to communicate, they need to:  establish a communication link between them  exchange messages via send/receive Implementation of communication link  physical (e.g., shared memory, hardware bus)  logical (e.g., logical properties)

61. 61 Direct Communication Processes must name each other explicitly:  send (P, message) – send a message to process P  receive(Q, message) – receive a message from process Q Properties of communication link  Links are established automatically  A link is associated with exactly one pair of communicating processes  Between each pair there exists exactly one link  The link may be unidirectional, but is usually bi- directional

62. 62 Indirect Communication Messages are directed and received from mailboxes (also referred to as ports)  Each mailbox has a unique id  Processes can communicate only if they share a mailbox Properties of communication link  Link established only if processes share a common mailbox  A link may be associated with many processes  Each pair of processes may share several comm. links  Link may be unidirectional or bi-directional

63. 63 Indirect Communication Operations  create a new mailbox  send and receive messages through mailbox  destroy a mailbox Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A

64. 64 Indirect Communication Mailbox sharing  P 1, P 2, and P 3 share mailbox A  P 1, sends; P 2 and P 3 receive  Who gets the message? Solutions  Allow a link to be associated with at most two processes  Allow only one process at a time to execute a receive operation  Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.

65. 65 Synchronization Message passing may be either blocking or non-blocking Blocking is considered synchronous  Blocking send has the sender block until the message is received  Blocking receive has the receiver block until a message is available Non-blocking is considered asynchronous  Non-blocking send has the sender send the message and continue  Non-blocking receive has the receiver receive a valid message or null

66. 66 Threads Concept of process – two characteristics:  Resource ownership  Scheduling/execution

67. 67 Threads Resource ownership - process includes a virtual address space to hold the process image Scheduling/execution- follows an execution path that may be interleaved with other processes These two characteristics are treated independently by the operating system Dispatching is referred to as a thread or lightweight process Resource ownership is referred to as a process or task

68. 68 Multithreading Operating system supports multiple threads of execution within a single process MS-DOS supports a single thread UNIX supports multiple user processes but only supports one thread per process Windows, Solaris, Linux, Mach, and OS/2 support multiple threads

69. 69

70. 70 Process Have a virtual address space which holds the process image Protected access to processors, other processes, files, and I/O resources

71. 71 Thread An execution state (running, ready, etc.) Saved thread context when not running Has an execution stack Some per-thread static storage for local variables Access to the memory and resources of its process  all threads of a process share this

72. 72

73. 73 Benefits of Threads Takes less time to create a new thread than a process Less time to terminate a thread than a process Less time to switch between two threads within the same process Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel

74. 74 Uses of Threads in a Single- User Multiprocessing System Foreground to background work  E.g: A spreadsheet program: one thread display menus and read user input while another thread executes user commands and updates the spreadsheet Asynchronous processing  E.g: protection against power failure: One thread writes the RAM buffer to disk every minute – for periodic backup and schedules itself directly with the OS

75. 75 Uses of Threads in a Single- User Multiprocessing System Speed of execution  Compute one batch of data while reading the next batch from a device  On multiprocessor system, multiple threads may execute simultaneously. Thus, if one thread is blocked for an I/O, another thread may be executing. Modular program structure  Easier to design and implement (using threads) programs involving a variety of activities or a variety of sources/destinations of I/O.

76. 76 Threads Suspending a process involves suspending all threads of the process since all threads share the same address space Termination of a process, terminates all threads within the process

77. 77 Thread States States associated with a change in thread state  Spawn Spawn another thread  Block  Unblock  Finish Deallocate register context and stacks

78. 78 Example: Remote Procedure Call Using Single Thread Assume a program needs to make 2 RPCs to 2 different hosts to get the combined results. Using single thread:  The result is obtained in sequence  The program has to wait for response from each host/server in turn.

79. 79 Remote Procedure Call Using Single Thread

80. 80 Remote Procedure Call Using Two Threads

81. 81 Multithreading

82. 82 User-Level Threads All thread management is done by the application The kernel is not aware of the existence of threads

83. 83 User-Level Threads Threads library contains codes for:  creating and destroying threads  passing messages and data between threads  scheduling thread execution  saving and restoring thread context

84. 84 Kernel-Level Threads Windows is an example of this approach Kernel maintains context information for the process and the threads Scheduling is done on a thread basis

85. 85 User-Level Threads Advantages:  Thread switching do not require mode switching  less overhead  Thread scheduling is application specific, without disturbing the underlying OS scheduler  ULTs can run on any OS, need no change to the underlying kernel to support ULT – use threads library Disadvantages:  If one thread blocks, the whole process blocks  Cannot take advantage of multiprocessing – 1 process to 1 processor at a time, i.e. only 1 thread running at a time

86. 86 Kernel-Level Threads This approach overcome the two principal drawbacks of ULT approach The kernel can simultaneously schedule threads on different processors If one thread of a process is blocked it can schedule another thread of the same process The disadvantage is Transfer of control from one thread to another thread within the same process requires mode switching

87. 87 Combined Approaches Example is Solaris Thread creation done in the user space Bulk of scheduling and synchronization of threads within application

88. 88 Combined Approaches -Adv Multiple threads within the same application can run concurrently on a number of processors. Blocking system calls need not block the entire process.

89. 89 Relationship Between Threads and Processes