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1. Silberschatz, Galvin and Gagne ©2013Operating System Concepts Essentials – 2
nd
Edition
UNIT 2 Chapter 1: Processes

2. 3.2 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
UNIT 2 Chapter 1: Processes
Process Concept
Process Scheduling
Operations on Processes
Interprocess Communication
Communication in Client-Server Systems

3. 3.3 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Objectives
To introduce the notion of a process -- a program in
execution, which forms the basis of all computation
To describe the various features of processes, including
scheduling, creation and termination, and communication
To explore interprocess communication using shared memory
and message passing
To describe communication in client-server systems

4. 3.4 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Concept
Textbook uses the terms job and process almost interchangeably
Process – a program in execution; process execution must
progress in sequential fashion
Multiple parts
Text section is also called as program code section which stores
instructions of the program.
program counter is a processor register which contains
address of next instruction to be executed.
Data section containing global variables and static variables.
Heap section contains dynamically memory allocated
variables.
Stack is also called as function stack which is used for storing
function parameters, local variables or temporary variables,
return variables.

5. 3.5 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process in Memory

6. 3.6 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Concept (Cont.)
Program is passive entity stored on disk (executable file),
process is active
Program becomes process when executable file loaded into
memory
Execution of program started via GUI mouse clicks, command
line entry of its name, etc
One program can be several processes
Consider multiple users executing the same program

7. 3.7 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process State transition diagram
As a process executes, it changes state
new: when ever a process is created then it is in new state.
ready: ready state contains a list of processes which are ready
for execution. The process is waiting to be assigned to a
processor. scheduler picks one process from ready state and
allocate it to the CPU.
running: running state contains the process that is currently
being executed by CPU.
a)When CPU receives interrupt, then the currently running
process will be moved to ready state.
b)If currently executing process needs an I/O operation
then that process will be moved to waiting state.
waiting: The process is waiting for some I/O operation. If I/O
operation completed, then that process will be moved to ready
state.
terminated: The process has finished execution

8. 3.8 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
process state transition diagram

9. 3.9 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Control Block (PCB)
A process is represented in OS using
PCB.
Every process has its own PCB.
Information associated with each
process (also called task control
block)
The PCB is used by operating system
to manage information about a running
process.
Process state – The state may be new,
ready, running, waiting, terminated or
halted.
Process number:- A unique
identification number assigned to each
process in the system. This allows the
OS to distinguish between different
processes.

10. 3.10 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Control Block (PCB)
Program counter – The address of the
next instruction to be executed .
CPU registers – used for storing the
data. CPU uses Different registers like
Program counter, Instruction register,
Accumulator, Index register, Base
register, limit register, PSW(program
status word)
CPU scheduling information-
Parameters related to process
scheduling like priority levels,
scheduling queues, and time-related
information.
Memory-management information –
This information includes values of
base register, limit register, page table,
segmentation table.

11. 3.11 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Control Block (PCB)
Accounting Information:- This
information includes the amount of CPU
time used, and other metrics that can
be useful for performance analysis.
I/O status information:- This information
includes list of I/O devices allocated to
the process, a list of open files.

12. 3.12 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Context Switch
Switching the CPU from one process to another process is
known as context switching.
When CPU switches from one process to another process, the
Operating 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 CPU does no useful work
while switching
The more complex the OS and the PCB  the longer the
context switch

13. 3.13 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
CPU Switch From Process to Process

14. 3.14 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Scheduling
The objective of multiprogramming is to have some process
always running, to Maximize CPU utilization.
The objective of time sharing is to switch the CPU among
processes so frequently that users can interact with each
program while it is running
To meet these objectives Process scheduler decides the order
in which the processes are to be executed.
Process scheduler Maintains scheduling queues of processes
Job queue –It contains set of all processes in the system
Ready queue – It contains a list of processes which are
ready for execution. The process is waiting to be assigned to
a processor. scheduler picks one process from ready queue
and allocate it to the CPU. This queue is generally stored as
a linked list.
Device queues or I/O queues – set of processes waiting for
I/O Operations.
Processes migrate among the various queues

15. 3.15 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Representation of Process Scheduling
Queueing diagram represents queues, resources, flows

16. 3.16 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Schedulers
Short-term scheduler (or CPU scheduler) – selects which process should
be executed next and allocates CPU
Sometimes the only scheduler in a system
Short-term scheduler is invoked frequently (milliseconds)  (must be
fast)
Long-term scheduler (or job scheduler) – Loads a process from
secondary memory to main memory i.e., it selects which processes should
be brought into the ready queue
Long-term scheduler is invoked 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
Long-term scheduler strives for good process mix

17. 3.17 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Medium Term Scheduling
Medium-term scheduler can be added if degree of multiple
programming needs to decrease
Remove process from memory, store on disk, bring back in
from disk to continue execution: swapping

18. 3.18 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Operations on Processes
System must provide mechanisms for:
 process creation,
 process termination

19. 3.19 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
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)
Resource sharing options
Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources
Execution options
Parent and children execute concurrently
Parent waits until children terminate

20. 3.20 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
A Tree of Processes in Linux

21. 3.21 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Creation (Cont.)
Address space of child process
Child duplicate of parent
Child has a new process 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

22. 3.22 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
C Program Forking Separate Process

23. 3.23 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Process Termination
Process executes last statement and then asks the operating
system to delete it using the exit() system call.
Parent may terminate the execution of children processes using
the abort() system call. Some reasons for doing so:
Child has exceeded allocated resources
Task assigned to child is no longer required
Some operating systems do not allow child to exists if its parent
has terminated . If a process terminates, then all its children must
also be terminated i.e., cascading termination. All children,
grandchildren, etc. are terminated.
The termination is initiated by the operating system.
. .

24. 3.24 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Interprocess Communication
Processes within a system may be independent or cooperating
Cooperating process can affect or be affected by other processes,
including sharing data
Reasons for cooperating processes:
Information sharing
Computation speedup
Modularity
Convenience
Cooperating processes need interprocess communication (IPC)
Two mechanisms or models to implement IPC
Shared memory
Message passing

25. 3.25 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
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

26. 3.26 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Communications Models
(a) Message passing. (b) shared memory.

27. 3.27 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
SHARED MEMORY
The best example for shared memory is producer – consumer problem.
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

28. 3.28 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Bounded-Buffer – Shared-Memory Solution
Shared data
#define BUFFER_SIZE 10
typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
Solution is correct, but can only use BUFFER_SIZE-1 elements
in specifies next free position
out specifies first full position

29. 3.29 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Bounded-Buffer – Producer
item next_produced;
while (true) {
/* produce an item in next produced */
while (((in + 1) % BUFFER_SIZE) == out) ; /* do nothing */
buffer[in] = next_produced;
in = (in + 1) % BUFFER_SIZE;
}

30. 3.30 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Bounded Buffer – Consumer
item next_consumed;
while (true) {
while (in == out) ; /* do nothing */
next_consumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
/* consume the item in next consumed */
}

31. 3.31 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Interprocess Communication – Message Passing
3 ISSUES
1.Naming
a. Direct Communication
b. Indirect Communication
2. Synchronization
a. Blocking Sender and Receiver
b. Non-Blocking Sender and Non-Blocking Receiver
3. Buffering
a. Zero capacity
b. Bounded or finite capacity
c. unbounded or infinite capacity

32. 3.32 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Naming - 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

33. 3.33 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
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
Operations
create a new mailbox (port)
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

34. 3.34 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Synchronization
Message passing may be either blocking or non-blocking
Blocking is considered synchronous
Blocking send -- the sender is blocked until the message is
received
Blocking receive -- the receiver is blocked until a message
is available
Non-blocking is considered asynchronous
Non-blocking send -- the sender sends the message and
continue
Non-blocking receive -- the receiver receives:
 A valid message, or
 Null message

35. 3.35 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Buffering
Queue of messages attached to the link.
implemented in one of three ways
1.Zero capacity – no messages are queued on a link.
Sender must wait for receiver
2.Bounded capacity – finite length of n messages
Sender must wait if link full
3.Unbounded capacity – infinite length
Sender never waits

36. 3.36 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Communications in Client-Server Systems
Client sends a request to server; server process the request
and delivers a response to the client.
Client – Server communication is done in 3 ways.
1. Sockets
2. Remote Procedure Calls (RPC)
3. Pipes

37. 3.37 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Sockets
Sockets are useful for communication between 2 processes
Socket is a Concatenation of IP address and port
The socket 161.25.19.8:1625 refers to port 1625 on host
161.25.19.8
Communication consists between a pair of sockets
All ports below 1024 are well known, used for standard
services
Special IP address 127.0.0.1 (loopback) to refer to system on
which process is running
FTP port no is 21, Telnet port no is 23, HTTP port no is 80.

38. 3.38 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Socket Communication

39. 3.39 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Remote Procedure Calls
Calling a procedure located at remote systems.
Client calls client-side proxy or client stub for the actual
procedure on the server.
Client stub locates the server and packs parameters into a
message, and this is called marshalling.
Client sends message to the server using system call.
Server calls server stub or server proxy.
server stub receives this message, unpacks the marshalled
parameters, and executes the procedure on the server

40. 3.40 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Pipes
 Pipes also used to communicate between 2 processes.
Ordinary pipes – Ordinary pipes are unidirectional
communication channels between two related processes,
typically a parent and its child process..
Named pipes – Named pipes are also communication
channels, but they are bidirectional and can be used for
communication between unrelated processes.

41. 3.41 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Ordinary Pipes
Ordinary Pipes allow communication in standard producer-consumer
style
Producer writes to one end (the write-end of the pipe)
Consumer reads from the other end (the read-end of the pipe)
Ordinary pipes are therefore unidirectional
Require parent-child relationship between communicating processes

42. 3.42 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts Essentials – 2
nd
Edition
Named Pipes
Named Pipes are more powerful than ordinary pipes
Communication is bidirectional
No parent-child relationship is necessary between the
communicating processes
Several processes can use the named pipe for communication
Provided on both UNIX and Windows systems

43. Silberschatz, Galvin and Gagne ©2013Operating System Concepts Essentials – 2
nd
Edition
End of Chapter 1