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Reg. No.
SNS COLLEGE OF TECHNOLOGY
A
(An Autonomous Institution)
INTERNAL ASSESSMENT EXAMINATION-I
ACADEMIC YEAR 2016 – 17 (Even Semester)
FOURTH SEMESTER
B.E – COMPUTER SCIENCE AND ENGINEERING
CS203– OPERATING SYSTEMS
TIME: 1 Hour 30 Mins
Maximum: 50 Marks
PART – A
(5 x 1 = 5 MARKS)
ANSWER ALL QUESTIONS
1.
Which of the following do not belong to queues for processes?
a) Job Queue b) PCB Queue c) Device Queue
d) Ready Queue
2.
An IPC facility provides at least two operations
3.
a) write message and read
c) send message and receive
A process can be
4.
a) single threaded
b) multithreaded
c) both (a) and (b)
d) none of the mentioned
Semaphore is a/an _______ to solve the critical section problem.
5.
a) hardware for a system
b) special program for a system
c) integer variable
d) None of these
The two atomic operations permissible on semaphores are
a) wait
b) stop and hold
b) delete message
d) read message
c) signal
d) Both a) and c)
PART – B
(5 x 2 = 10 MARKS)
ANSWER ALL QUESTIONS
6.
7.
8.
Define operating system.
A collection of programs and utilities, it is the interface between user and
computer. It creates the user friendly environment.
Define resource allocation graph.
A resource allocation graph is a graph used to represent the deadlock. It
consisting of two symbols and edges. A square is used to represent resources
and the circle is used to represent the process. An edge from a process to
resource is called requesting edge. An edge from a resource to process is
called the assignment edge.
List the services of operating system
 Program execution
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9.
10.
Accounting
Program execution
Controlled access to files
System access
Error detection and response
Access to I/O devices.
Define process control block.
The process control block, it is the central data structure in an operating
system. Each process maintains one PCB. Each PCB contains all the
information about a process that is needed by the operating system.
List the three requirements that must satisfy by the critical-section
problem.
 Mutual Exclusion
 Progress
 Bounded waiting
PART – C
(14 + 14 + 7 = 35 Marks)
ANSWER ALL QUESTIONS
11. a)
Define process. Explain states of process with neat sketch and discuss the
process state transition with a neat diagram.
A process is basically a program in execution. The execution of a
process must progress in a sequential fashion.
Process Life Cycle
When a process executes, it passes through different states. These stages may
differ in different operating systems, and the names of these states are also not
standardized. In general, a process can have one of the following five states at a
time.
State & Description
1. Start: This is the initial state when a process is first started/created.
2. Ready: The process is waiting to be assigned to a processor. Ready processes are
waiting to have the processor allocated to them by the operating system so that
they can run. Process may come into this state after Start state or while running it
by but interrupted by the scheduler to assign CPU to some other process.
3. Running: Once the process has been assigned to a processor by the OS
scheduler, the process state is set to running and the processor executes its
instructions.
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4. Waiting: Process moves into the waiting state if it needs to wait for a resource,
such as waiting for user input, or waiting for a file to become available.
5. Terminated or Exit: Once the process finishes its execution, or it is terminated
by the operating system, it is moved to the terminated state where it waits to be
removed from main memory.
Process States
Information associated with each process
 Process state – new ,ready, running, waiting, halted
 Program counter – address of the next instruction
 CPU registers – accumulators ,index registers, stack pointers
 CPU scheduling information – process priority, pointers to scheduling queues
 Memory-management information – base and limit registers, page tables, segment tables
 Accounting information – amount of CPU and real time used, time limits, account
numbers, job or process numbers
 I/O status information –list of I/O devices , list of open files
Process Control Block (PCB)
CPU Switch From Process to Process
b)
OR
Consider the following set of processes, with the length of the CPU-burst (14
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time given in milliseconds:
Process Burst Time
Priority
P1
10
3
P2
1
1
P3
2
3
P4
1
4
P5
5
2
The processes are arrived in the order P1, P2, P3, P4, P5, ALL AT
TIME 0.
(1) Draw four Gantt charts illustrating the execution of these processes
using FCFS, SJF, a non preemptive priority (a smaller priority number
implies a higher priority), and RR (quantum=1) scheduling.
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(2) What is the turnaround time and waiting time of each process for each of
the scheduling algorithms in part a?
(3) Which of the schedules in part a results in the minimal average waiting
time (overall processes)?
FCFS:
WAITING TIME: 9.6,
TURN AROUND TIME: 13.4
SJF:
WAITING TIME: 9.6,
TURN AROUND TIME: 7
PRIORITY:
WAITING TIME: 9.6,
TURN AROUND TIME: 5.2
ROUND ROBIN:
WAITING TIME: 9.6,
TURN AROUND TIME: 9
12. a)
Describe the Inter Process communication in client-server systems.
Mechanism for processes to communicate and to synchronize their actions
Message system – processes communicate with each other without resorting to
shared variables
i)
Message passing system
 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)
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
several methods of implementing a link
-direct /indirect communication
-symmetric/asymmetric communication
-automatic /explicit buffering
-send by copy/send by reference
-fixed sized /variable sized msg
ii) Naming
A) 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
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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
B) 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
 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
 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.
iii) 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
iv) 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
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Client-Server Communication
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Sockets
Remote Procedure Calls
Remote Method Invocation (Java)
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i)
Sockets
 A socket is defined as an endpoint for communication
 pair of sockets – one for each process
 socket made up of Concatenation of IP address and port number
 The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8
 Communication consists between a pair of sockets
Socket Communication
java provides 3 different types of sockets
- connection oriented (TCP)
- connection less(UDP)
- multicast socket
ii) Remote Procedure Calls
 Remote procedure call (RPC) abstracts procedure calls between processes on
networked systems.
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Stubs – client-side proxy for the actual procedure on the server.
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The client-side stub locates the server and marshalls the parameters.
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The server-side stub receives this message, unpacks the marshalled parameters,
and peforms the procedure on the server.
Execution of RPC
iii)
Remote Method Invocation
Remote Method Invocation (RMI) is a Java mechanism similar to RPCs.
RMI allows a Java program on one machine to invoke a method on a remote
object.
 the RMI implements the remote object using stubs and skeletons
 stub is a proxy for the remote object - it resides with the client
 client side stub is responsible for creating a parcel consisting the name of the
method to be invoked on the server and the marshaled parameters for the
method
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the skeleton is responsible for unmarshalling the parameters and invoking the
desired method on the server.
Marshalling Parameters
b)
OR
Consider the following Snapshot of a system.
Allocation
Max
Available
ABCD
ABCD
ABCD
P0
0012
0012
1520
P1
1000
1750
P2
1354
2356
P3
0632
0652
P4
0014
0656
Answer the following questions using the banker’s algorithm.
a. What is the content of the matrix Need?
b. Is the system in a safe state?
c. If a request from process P1 arrives for (0,4,2,0), can the request be (14
granted immediately?
)
a)NEED MATRIX:
0 0 0 0
0 7 5 0
1 0 0 2
0 0 2 0
0 6 4 2
b) THE SYSTEM IS IN SAFE STATE: P0,P2,P3,P4,P1
c) YES THE REQUEST CAN BE GRANTED
13. a)
Illustrate the various deadlock prevention methods.
Restrain the ways request can be made.
 Mutual Exclusion – not required for sharable resources; must hold for
nonsharable resources.
 Hold and Wait – must guarantee that whenever a process requests a resource, it
does not hold any other resources.
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Require process to request and be allocated all its resources before it
begins execution, or allow process to request resources only when the
process has none.
 Low resource utilization; starvation possible.
 No Preemption –
 If a process that is holding some resources requests another resource that
cannot be immediately allocated to it, then all resources currently being
held are released.
 Preempted resources are added to the list of resources for which the
process is waiting.
 Process will be restarted only when it can regain its old resources, as well
as the new ones that it is requesting.
 Circular Wait – impose a total ordering of all resource types, and require that
each process requests resources in an increasing order of enumeration.
Deadlock Avoidance
Requires that the system has some additional a priori information
available.
 Simplest and most useful model requires that each process declare the maximum
number of resources of each type that it may need.
 The deadlock-avoidance algorithm dynamically examines the resourceallocation state to ensure that there can never be a circular-wait condition.
 Resource-allocation state is defined by the number of available and allocated
resources, and the maximum demands of the processes.
Safe State
 When a process requests an available resource, system must decide if
immediate allocation leaves the system in a safe state.
 System is in safe state if there exists a sequence <P1, P2, …, Pn> of ALL the
processes is the systems such that for each Pi, the resources that Pi can still
request can be satisfied by currently available resources + resources held by all
the Pj, with j < i.
 That is:
 If Pi resource needs are not immediately available, then Pi can wait until
all Pj have finished.
 When Pj is finished, Pi can obtain needed resources, execute, return
allocated resources, and terminate.
 When Pi terminates, Pi +1 can obtain its needed resources, and so on.
Basic Facts
 If a system is in safe state  no deadlocks.
 If a system is in unsafe state  possibility of deadlock.
Avoidance  ensure that a system will never enter an unsafe state
Safe, Unsafe , Deadlock State
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OR
b)
Explain in detail about reader writer problem
A data set is shared among a number of concurrent processes
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Readers – only read the data set; they do not perform any updates
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Writers – can both read and write.
Problem – allow multiple readers to read at the same time. Only one
single writer can access the shared data at the same time.
Shared Data
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Data set
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Semaphore mutex initialized to 1.
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Semaphore wrt initialized to 1.
Integer readcount initialized to 0.
The structure of a writer process
while (true) {
wait (wrt) ;
// writing is performed
signal (wrt) ;
}
The structure of a reader process
while (true) {
wait (mutex) ;
readcount ++ ;
if (readcount == 1) wait (wrt) ;
signal (mutex)
// reading is performed
wait (mutex) ;
readcount - - ;
if (readcount == 0) signal (wrt) ;
signal (mutex) ;
}
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