1. Lecture 3
Process

2. Process Concept An operating system executes a variety of programs:
Batch system – jobs
Time-shared systems – user programs or tasks
The terms job and process almost interchangeably

3. Process Concept 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

4. Process Concept
Process – a program in execution; process execution must progress in sequential fashion

5. 1. An executable (i.e., code) 2. Associated data needed by
A process consists of
1. An executable (i.e., code)
2. Associated data needed by
the program (global data,
dynamic data, shared data)
3. Execution context (or state) of the program, e.g.,
Contents of data registers
Program counter, stack pointer
Memory allocation
Open file (pointers)

6. Process in Memory

7. Process Program : Passive entity (executable file)
Process: Active entity (progam counter specify next instruction).
A program becomes process when executable file is loaded into memory

8. Process State As a process executes, it changes state
new: The process is being created
running: Instructions are being executed
waiting: The process is waiting for some event to occur
ready: The process is waiting to be assigned to a processor
terminated: The process has finished execution

9. Diagram of Process State

10. Process Control Block (PCB)
Information associated with each process
Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information

11. Process Control Block (PCB)

12. PROCESSES Process State PROCESS CONTROL BLOCK:
CONTAINS INFORMATION ASSOCIATED WITH EACH PROCESS:
It's a data structure holding:
PC, CPU registers,
memory management information,
accounting ( time used, ID, ... )
I/O status ( such as file resources ),
scheduling data ( relative priority, etc. )
Process State (so running, suspended, etc. is simply a field in the PCB ).
3: Processes

13. Example Execution

14. Trace of Process

15. CPU Switch From Process to Process

16. Process scheduling

17. A new process is initially put in the ready queue
A new process is initially put in the ready queue. It waits there until it is selected for execution, or is dispatched.

18. Queuing Diagram

19. Once the process is allocated the CPU and is executing, one of several events could occur:
The process could issue an I/O request and then be placed in an I/O queue.
The process could create a new subprocess and wait for the subprocess’s termination.
The process could be removed forcibly from the CPU as a result of an interrupt, and be put back in the ready queue.

20. Process Scheduling Queues
Job queue – set of all processes in the system
Ready queue – set of all processes residing in main memory, ready and waiting to execute
Device queues – set of processes waiting for an I/O device Processes migrate among the various queues

21. Representation of Process Scheduling

22. Queuing Processes Not-running ready to execute Not-running block
Dispatcher must scan list to find process not running, ready, and in queue the longest

23. Ready Queue And Various I/O Device Queues

24. scheduler How to select the process from process queue ?
Using scheduler

25. The operating system must select, for scheduling purposes, processes from these queues in some fashion. The selection process is carried out by the appropriate scheduler.

27. Schedulers
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU

28. Addition of Medium Term Scheduling

29. Schedulers (Cont)
Short-term scheduler is invoked very frequently (milliseconds)  (must be fast)
Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow)

30. Schedulers (Cont)
The long-term scheduler controls the degree of multiprogramming
Processes can be described as either:
I/O-bound process – spends more time doing I/O than computations, many short CPU bursts
CPU-bound process – spends more time doing computations; few very long CPU bursts

31. scheduler What happen if processes always in I/O bound?
(read queue will almost empty)
(short-term have little wor)
What happen if processes in CPU bunds?
(I/O queue almost will be empty)
System unbalance

32. Context Switch
Interrupt occurs  system need to save the current context of the running process  to be restored when resuming.
Context in PCB (cpu registers, process state , memory management information).

33. 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 via a context switch
Context of a process represented in the PCB
Context-switch time is overhead; the system does no useful work while switching
Time dependent on hardware support

34. Operation on processes
Process creation
Process termination

35. Process Creation Created by OS to provide a service
a process to control printing
a process to control network connection

36. Process Creation
Parent process create children processes, which, in turn create other processes, forming a tree of processes
Generally, process identified and managed via a process identifier (pid)
Process will need certain resources

37. Process Creation (con)
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

38. Process Creation (Cont)
Address space
Child duplicate of parent
Child has a program loaded into it
UNIX examples
fork system call creates new process
exec system call used after a fork to replace the process’ memory space with a new program

39. Process Termination Batch job issues Halt instruction User logs off
Quit an application
e.g., word processing
Error and fault conditions

40. Reasons for Process Termination
Normal completion
Time limit exceeded
Memory unavailable
Bounds violation
Protection error
example : write to read-only file
Arithmetic error
Time overrun
process waited longer than a specified maximum for an event

41. Reasons for Process Termination
I/O failure
Invalid instruction
happens when try to execute data
Privileged instruction
Data misuse
Operating system intervention
such as when deadlock occurs
Parent terminates so child processes terminate
Parent request

42. Process Creation

43. C Program Forking Separate Process
int main() { pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); exit(-1); } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); else { /* parent process */ /* parent will wait for the child to complete */ wait (NULL); printf ("Child Complete"); exit(0);

44. A tree of processes on a typical Solaris

45. Process Termination
Process executes last statement and asks the operating system to delete it (exit)
Output data from child to parent (via wait)
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

46. Interprocess Communication
Processes within a system may be independent or cooperating
Independent process cannot affect or be affected by the execution of another process
Cooperating process can affect or be affected by other processes, including sharing data

47. Interprocess Communication
Reasons for cooperating processes:
Information sharing
Computation speedup… (multi processing elements)
Modularity s… function separation processes or threads
Convenience … user work in parrallel
Cooperating processes need interprocess communication (IPC)
Allow exchange data and information
Two models of IPC
Shared memory
Message passing

48. Communications Models

49. Inter process communication: shared memory
Every process has address space
Other process wish to communicates must attach the shared memory address to their address space
They an change data by reading and writing data to shared memory
Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process
unbounded-buffer places no practical limit on the size of the buffer
bounded-buffer assumes that there is a fixed buffer size

50. Interprocess Communication – Message Passing
Mechanism for processes to communicate without shared memory
Like chat programs
IPC facility provides two operations:
send(message) – message size fixed or variable
receive(message)

51. Interprocess Communication – Message Passing
If P and Q wish to communicate, they need to:
establish a communication link between them
exchange messages via send/receive

52. Communications in Client-Server Systems
Sockets
Remote Procedure Calls
Remote Method Invocation (Java)

53. Sockets A socket is defined as an endpoint for communication
Concatenation of IP address and port
The socket :1625 refers to port 1625 on host
Communication consists between a pair of sockets

54. Socket Communication