1. Chapter 3: Processes

2. Chapter 3: Processes Process Concept Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
Communication in Client-Server Systems

3. Process Concept An operating system executes a variety of programs:
Batch system – jobs
Time-shared systems – user programs or tasks
Textbook uses the terms job and process almost interchangeably
Process – a program in execution; process execution must progress in sequential fashion
A process includes:
program counter which represents current activity
Stack which holds temporary data such as function parameters, local variables and return addresses.
data section which contains global variables
Heap which contains memory that is dynamically allocated during process runtime.
Text includes the application's code, might be shared by a number of processes?
The process's address space.

4. Process in Memory

5. 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

6. Diagram of Process State

7. Process Control Block (PCB)
Information associated with each process
Process state
Program counter indicates the address of next instruction.
CPU registers
CPU scheduling information includes process priority and pointers to scheduling queues.
Memory-management information includes value of limit and base registers
Accounting information includes amount of CPU time used, time limits etc.
I/O status information includes the list of I/O devices allocated to processes, list of open files

8. Process Control Block (PCB)

9. General Purpose Registers
PCB
OS Data Structures
Process Address Space
RAM
CPU
Kernel
User
PSW
State
Memory
Files
Accounting
Priority
User
CPU register storage
text IR . PC
data SP
heap
General Purpose Registers
stack

10. CPU Switch From Process to Process

11. 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.
It is stored as a linked list. It contains pointers to the first and final PCB’s.
Each PCB includes a pointer field that points to the next PCB in the ready queue.
Device queues – set of processes waiting for an I/O device
Processes migrate among the various queues

12. Ready Queue And Various I/O Device Queues

13. Representation of Process Scheduling

14. 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
Medium term Scheduler (Swaper)

15. Addition of Medium Term Scheduling

16. 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)
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
If all processes are I/O bound,
The ready queue will almost always be empty
If all processes are CPU bound,
The I/O waiting queue will almost always be empty, devices will go unused
System will again be unbalanced

17. 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
Part of OS responsible for switching the processor among the processes is called Dispatcher

18. Two-state process model
Processor
Queue
Enter
Dispatch
Exit
Pause
Dispatcher is now redefined:
Moves processes to the waiting queue
Remove completed/aborted processes
Select the next process to run

19. How process state changes1
Ready
Ready
Ready
 a := b := a c := b read a file a := b - c c := c * b b := 0
 a := read a file b := a c := b a := b - c c := c * b b := 0
 a := b := a c := b a := b - c c := c * b b := c := 0

20. How process state changes2
Running
Ready
Ready
 a := b := a c := b read a file a := b - c c := c * b b := 0
 a := read a file b := a c := b a := b - c c := c * b b := 0
 a := b := a c := b a := b - c c := c * b b := c := 0
a := 1  b := a c := b read a file a := b - c c := c * b b := 0
a := b := a + 1  c := b read a file a := b - c c := c * b b := 0
Timeout

21. How process state changes3
Ready
Running
Ready
a := b := a c := b + 1  read a file a := b - c c := c * b b := 0
 a := read a file b := a c := b a := b - c c := c * b b := 0
 a := b := a c := b a := b - c c := c * b b := c := 0
a := 1  read a file b := a c := b a := b - c c := c * b b := 0
I/O

22. How process state changes4
Ready
Blocked
Running
a := b := a c := b + 1  read a file a := b - c c := c * b b := 0
a := 1  read a file b := a c := b a := b - c c := c * b b := 0
 a := b := a c := b a := b - c c := c * b b := c := 0
a := 1  b := a c := b a := b - c c := c * b b := c := 0
a := b := a + 1  c := b a := b - c c := c * b b := c := 0
Timeout

23. How process state changes5
Running
Blocked
Ready
a := b := a c := b + 1  read a file a := b - c c := c * b b := 0
a := 1  read a file b := a c := b a := b - c c := c * b b := 0
a := b := a c := b + 1  a := b - c c := c * b b := c := 0
I/O

24. How process state changes6
Blocked
Blocked
Running
The Next Process to Run cannot be simply selected from the front
a := b := a c := b + 1  read a file a := b - c c := c * b b := 0
a := 1  read a file b := a c := b a := b - c c := c * b b := 0
a := b := a c := b + 1  a := b - c c := c * b b := c := 0

25. How process state changes7
Blocked
Blocked
Running
a := b := a c := b + 1  read a file a := b - c c := c * b b := 0
a := 1  read a file b := a c := b a := b - c c := c * b b := 0
a := b := a c := b + 1  a := b - c c := c * b b := c := 0
a := b := a c := b a := b - c  c := c * b b := c := 0
Suppose the Green process finishes I/O

26. How process state changes8
Blocked
Ready
Running
a := b := a c := b + 1  read a file a := b - c c := c * b b := 0
a := read a file  b := a c := b a := b - c c := c * b b := 0
a := b := a c := b + 1  a := b - c c := c * b b := c := 0
a := b := a c := b a := b - c  c := c * b b := c := 0
a := b := a c := b a := b - c c := c * b  b := c := 0
Timeout

27. How process state changes9
Blocked
Running
Ready
a := b := a c := b + 1  read a file a := b - c c := c * b b := 0
a := read a file  b := a c := b a := b - c c := c * b b := 0
a := b := a c := b a := b - c c := c * b b := 0  c := 0
a := read a file b := a + 1  c := b a := b - c c := c * b b := 0
a := read a file b := a c := b + 1  a := b - c c := c * b b := 0
Timeout

28. Process Creation Reasons to create a process
Submit a new batch job/Start program
User logs on to the system
OS creates on behalf of a user (printing)
Spawned by existing process
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

29. 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

30. Process Creation

31. 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);

32. The fork() system call
At the end of the system call there is a new process waiting to run once the scheduler chooses it
A new data structure is allocated
The new process is called the child process.
The existing process is called the parent process.
The parent gets the child’s pid returned to it.
The child gets 0 returned to it.
Both parent and child execute at the same point after fork() returns

33. Unix Process Control
The fork syscall returns a zero to the child and the child process ID to the parent
int pid;
int status = 0;
if (pid = fork()) {
/* parent */ ……
pid = wait(&status);
} else {
/* child */
exit(status); }
Parent uses wait to sleep until the child exits; wait returns child pid and status.
Wait variants allow wait on a specific child, or notification of stops and other signals
Fork creates an exact copy of the parent process
Child process passes status back to parent on exit, to report success/failure

34. Process Termination Batch job issues Halt instruction User logs off
Process executes a service request to terminate
Parent terminates so child processes terminate
Operating system intervention
such as when deadlock occurs
Error and fault conditions
E.g. memory unavailable, protection error, arithmetic error, I/O failure, invalid instruction

35. 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

36. Inter process Communication
Processes executing concurrently in the operating system may be either independent processes or 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

37. Advantages of Process Cooperation
Information sharing: Allow concurrent access to same piece of information that several users may be interested in it.
Computation speed-up: break a task to subtasks, each of which will be executing in parallel with the others. (Should be multiple processing elements such as CPUs)
Modularity: divided the system functions into separate processes or threads.
Convenience: for user, user may work on many tasks at the same time.

38. MODELS OF IPC
Cooperating processes require an inter process communication mechanism that will allow them to exchange data and information.
Shared Memory
Message Passing

39. SHARED MEMORY
Region of memory that is shared by cooperating processes is established.
Processes can then exchange information by reading and writing data to shared region

40. MESSAGE PASSING
Communication takes place by means of messages exchanged between the cooperating processes.

41. Communications Models

42. Advantages & Disadvantages of Message Passing & Shared Memory
Message passing is useful for exchanging smaller amounts of data
Message Passing is easier to implement than shared memory for IPC
Shared Memory allows maximum speed and convenience as it can be done at memory speeds within a computer
Shared memory is faster than message passing as message passing is implemented using system calls.

43. Contd….
Message passing requires the more time consuming task of kernel intervention.
Shared memory system calls are required only to establish shared memory regions, no assistance from the kernel is required.

44. Producer-Consumer Problem
Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process
A compiler may produce assembly code, which is consumed by an assembler. The assembler, in turn, may produce object modules, which are consumed by the loader
A buffer of items that can be filled by the producer and emptied by the consumer. should be available
A producer can produce one item while the consumer is consuming another item.
The producer and consumer must be synchronized
the consumer does not try to consume an item that has not yet been produced.
the consumer must wait until an item is produced

45. Producer-Consumer Problem
unbounded-buffer places no practical limit on the size of the buffer
the consumer may have to wait for new items, but
the producer can always produce new items
bounded-buffer assumes that there is a fixed buffer size
the consumer must wait if the buffer is empty, and
the producer must wait if the buffer is full.

46. Bounded-Buffer – Shared-Memory Solution
Shared data
#define BUFFER_SIZE 10
Typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0; % in: the next free position in the buffer
int out = 0; % out: the first full position in the buffer
The buffer is empty when in == out ;
The buffer is full when ((in + 1) % BUFFERSIZE) == out
Solution is correct, but can only use BUFFER_SIZE-1 elements

47. Bounded-Buffer – Insert() Method
The producer process has a local variable nextproduced in which the new item to be produced is stored:
while (true) { /* Produce an item */
while (((in = (in + 1) % BUFFER SIZE count) == out)
; /* do nothing -- no free buffers */
buffer[in] = item;
in = (in + 1) % BUFFER SIZE; {

48. Bounded Buffer – Remove() Method
The consumer process has a local variable nextconsumed in which the item to be consumed is stored:
while (true) {
while (in == out)
; // do nothing -- nothing to consume
// remove an item from the buffer
item = buffer[out];
out = (out + 1) % BUFFER SIZE;
return item; {

49. Message Passing
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) e.g. send(), receive()

50. Implementation Questions
How are links established?
Can a link be associated with more than two processes?
How many links can there be between every pair of communicating processes?
What is the capacity of a link?
Is the size of a message that the link can accommodate fixed or variable?
Is a link unidirectional or bi-directional?

51. Methods for logical implementation of link
Direct or Indirect Communication
Synchronous and Asynchronous Communication
Automatic or explicit Buffering

52. 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 because the processes need to know only the identities to communicate.
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

53. 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 communication links
Link may be unidirectional or bi-directional

54. 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

55. Indirect Communication
Mailbox sharing
P1, P2, and P3 share mailbox A
P1, sends; P2 and P3 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.

56. Indirect Communication
If the mailbox is owned by a process (that is, the mailbox is part of the address space of the process),
then we distinguish between the owner (who can only receive messages through this mailbox) and the user (who can only send messages to the mailbox).
Since each mailbox has a unique owner, there can be
no confusion about who should receive a message sent to this mailbox.
When a process that owns a mailbox terminates, the mailbox disappears.
Any process that subsequently sends a message to this mailbox must be notified that the mailbox no longer exists.

57. Indirect Communication
If a mailbox owned by the operating system is independent and is not attached to any particular process:
The operating system then must provide a mechanism that allows a process to do the following:
Create a new mailbox.
Send and receive messages through the mailbox.
Delete a mailbox
Note: the ownership and receive privilege may be passed to other processes through appropriate system calls.

58. 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

59. Buffering
Queue of messages attached to the link; implemented in one of three ways
1. Zero capacity – 0 messages Sender must wait for receiver (rendezvous)
2. Bounded capacity – finite length of n messages Sender must wait if link full
3. Unbounded capacity – infinite length Sender never waits

60. Client-Server Communication
Sockets
Remote Procedure Calls
Remote Method Invocation (Java)

61. 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

62. Socket Communication

63. Remote Procedure Calls
Remote procedure call (RPC) abstracts procedure calls between processes on networked systems.
Stubs – client-side proxy for the actual procedure on the server.
The client-side stub locates the server and marshalls the parameters.
The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server.
A stub is a small program routine that substitutes for a longer program, possibly to be loaded later or that is located remotely. For example, a program that uses Remote Procedure Calls ( RPC ) is compiled with stubs that substitute for the program that provides a requested procedure. The stub accepts the request and then forwards it (through another program) to the remote procedure. When that procedure has completed its service, it returns the results or other status to the stub which passes it back to the program that made the request.

64. Execution of RPC

65. End of Chapter 3