UNIT II

PROCESS MANAGEMENT

Processes: Concept – Scheduling - Operations on

Processes - Interprocess Communication -Communication in

Client-Server Systems. Process Scheduling-Scheduling

Criteria – Scheduling Algorithms - Multiple-Processor

Scheduling.

Process Concept

Process Concept

Process – a program in execution.

Batch system – 1 job

Time-shared systems – multiple processes

Program – passive entity, process – active entity

Same program – multiple process

A process includes:

Text section – program code

program counter - address of next instruction to be

executed

Stack – temporary data (parameters, return address, local

variables, etc..,)

data section (global variables)

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 process.

terminated: The process has finished execution.

Diagram of Process State

Process Control Block (PCB)

Process Control Block (PCB)

Information associated with each process.

Pointer – points to next PCB in queue

Process state – new, ready, running, waiting, terminated

Program counter – addr of next instruction to be exec.

CPU registers – accumulators, index registers, stack

pointers, general purpose registers.

CPU scheduling information – priority, pointer to

scheduling queue, scheduling parameters, etc.,

Memory-management information – base, limit register

values, page table, segment table, etc.,

Accounting information – CPU time used, time limit,

process no., etc.,

I/O status information – I/O devices allocated, files

opened, etc.,

CPU Switch From Process to Process

Process Scheduling

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.

Process migration between the various queues.

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

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.

Addition of Medium Term Scheduling

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, maintains process mix.

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.

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.

Operations on Processes

Process Creation

Parent process create children processes, which, in turn

create other processes, forming a tree of processes (refer

fig.)

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.

Process Creation (Cont.)

Address space

Child duplicate of parent.

Child has a program loaded into it.

UNIX examples

fork system call creates new process

Processes Tree on a UNIX System

fork system call

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.

Parent is exiting.

Operating system does not allow child to continue if its

parent terminates.

Cascading termination.

Cooperating Processes

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

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.

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

Bounded-Buffer – Producer Process

item nextProduced;

while (1) {

while (((in + 1) % BUFFER_SIZE) == out)

; /* do nothing */

buffer[in] = nextProduced;

in = (in + 1) % BUFFER_SIZE;

}

Bounded-Buffer – Consumer Process

item nextConsumed;

while (1) {

while (in == out)

; /* do nothing */

nextConsumed = buffer[out];

out = (out + 1) % BUFFER_SIZE;

}

Interprocess Communication (IPC)

Interprocess Communication (IPC)

Mechanism for processes to communicate and to

synchronize their actions.

Message Passing system – processes communicate with

each other without resorting to shared data.

Useful in distributed environment.

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

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.

Direct Communication

Processes must name each other explicitly (Symmetric):

send (P, message) – send a message to process P

receive(Q, message) – receive a message from process Q

Sender Processes must name Receiver explicitly:

send (P, message) – send a message to process P

Receive(id, message) – receive a message from any

process. Later, process from which message was received

is stored in „id‟.

Limitations:

Changing name of process => rename all references

Indirect Communication

Messages are directed and received from mailboxes (also

referred to as ports).

Mailbox – object in which messages are placed and

removed by processes

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.

Indirect Communication

Mailbox owned by

Process (mailbox in its own address space)

OS

Operations for mailbox in OS

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

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.

Synchronization

Message passing may be either blocking or non-blocking.

Blocking is considered synchronous

Non-blocking is considered asynchronous

send and receive primitives may be either blocking or

non-blocking.

Blocking-send: sender blocked until msg sent

Non-Blocking-send: sender sends and resumes process

Blocking-receive: receiver waits until a msg is received

Non-Blocking-receive: receiver receives msg or resumes

process

Buffering

Queue of messages attached to the link; implemented in

one of three ways.

1. Zero capacity – 0 messages

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.

Communication in Client-Server Systems

Client-Server Communication

Sockets

Remote Procedure Calls

Remote Method Invocation (Java)

Sockets

A socket is defined as an endpoint for communication.

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.

Socket Communication

Sockets – Example programTCP - Server

import java.io.*;

import java.net.*;

public class server {

public static void main(String[] args) throws IOException

{

ServerSocket listener = new ServerSocket(2000);

try {

while (true) {

Socket socket = listener.accept();

try {

BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

PrintWriter out =new PrintWriter(socket.getOutputStream(), true);

out.println("Hello, client");

String input = in.readLine();

System.out.println(input);

}

finally {socket.close(); }

}

}

finally { listener.close(); }

}

}

Sockets – Example programTCP – Client

import java.io.*;

import java.net.*;

public class client

{

public static void main(String[] args) throws IOException

{

Socket socket = new Socket("localhost", 2000);

BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

String input = in.readLine();

System.out.println(input);

out.println("Hello, server");

}

}

Sockets – Example programUDP – Server

import java.net.*;

import java.io.*;

public class UDPEchoServer

{

public static void main(String args[]) throws SocketException, IOException

{

DatagramSocket aSocket = new DatagramSocket(2000);

try

{

byte[] buffer = new byte[1000];

while(true)

{

DatagramPacket request = new DatagramPacket(buffer,buffer.length);

aSocket.receive(request);

DatagramPacket reply = new

DatagramPacket(request.getData(),request.getLength(),request.getAddress(),request.getPort());

aSocket.send(reply);

}

}

finally { aSocket.close(); }

}

}

Sockets – Example programUDP- Client

import java.net.*;

import java.io.*;

public class UDPEchoClient

{

public static void main(String args[]) throws SocketException, IOException

{

DatagramSocket aSocket = new DatagramSocket();

try

{

String m = "Hello UDPEchoServer";

InetAddress aHost = InetAddress.getLocalHost();

DatagramPacket request = new DatagramPacket(m.getBytes(), m.length(), aHost,

2000);

aSocket.send(request);

byte[] buffer = new byte[1000];

DatagramPacket reply = new DatagramPacket(buffer, buffer.length);

aSocket.receive(reply);

System.out.println("Reply: " + new String(reply.getData()).trim());

}

finally { aSocket.close(); }

}

}

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 peforms the procedure on

the server.

Execution of RPC

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.

Marshalling Parameters

WORKING OF RMI

CLIENT VIRTUAL

MACHINE

REGISTRY VIRTUAL

MACHINE

SERVER VIRTUAL

MACHINE

CLIENT

SERVER

STUB

REMOTE

OBJECT

NAME,REF

REMOTE

OBJECT

SKELETON

76

5

43

2

1

2. Server registers

remote object

4. Registry returns

remote reference &

stub

5. Client invokes stub

1. Server creates

remote object

3. Client requests

object from registry

6. Stub talks to skeleton7. Skeleton invokes

remote object method

RMI – Example programAdd.java

import java.rmi.*;

public interface Add extends Remote

{

public int getSum() throws RemoteException;

}

RMI – Example programAddImpl.java

import java.rmi.*;

import java.rmi.server.UnicastRemoteObject;

public class AddImpl extends UnicastRemoteObject implements Add

{

public AddImpl() throws RemoteException

{

super();

}

public int getSum() throws RemoteException

{

System.out.println("Calculating the sum..");

int a=10;

int b=15;

int sum;

sum=a+b;

return sum;

}

}

RMI – Example programAddServer.java

import java.net.*;

import java.rmi.*;

public class AddServer

{

public static void main(String[] args)

{

try

{

AddImpl f = new AddImpl();

Naming.rebind("add", f);

System.out.println("Addition Server ready.");

}

catch (RemoteException rex)

{

System.out.println("Exception in AddImpl.main: " + rex);

}

catch (MalformedURLException ex)

{

System.out.println("MalformedURLException " + ex);

}

}

}

RMI – Example programAddClient

import java.rmi.*;

import java.net.*;

public class AddClient

{

public static void main(String args[])

{

try

{

Object o = Naming.lookup("add");

Add calculator = (Add) o;

int f = calculator.getSum();

System.out.println("The sum is: "+f);

}

catch (NotBoundException ex) {

System.err.println("Could not find the requested remote object on the server");

}

catch (RemoteException ex) {

System.err.println("Could not find the requested remote object on the server");

}

catch (MalformedURLException ex) {

System.err.println("Could not find the requested remote object on the server");

}

}

}

CPU Scheduling - Basic Concepts

CPU–I/O Burst Cycle

Process execution consists of a cycle of CPU execution

and I/O wait.

CPU burst distribution helps to choose CPU scheduling

algorithm

CPU And I/O Bursts (Alternating Sequence)

CPU-burst Times (Histogram)

CPU Scheduler

Selects from among the processes in memory that are

ready to execute, and allocates the CPU to one of them.

CPU scheduling decisions may take place when a

process:

1. Switches from running to waiting state.

2. Switches from running to ready state.

3. Switches from waiting to ready.

4. Terminates.

Scheduling under 1 and 4 is nonpreemptive (i.e. process

forced to release CPU).

All other scheduling is preemptive (i.e. process releases

CPU voluntarily).

Dispatcher

Dispatcher module gives control of the CPU to the

process selected by the short-term scheduler; this

involves:

switching context

switching to user mode

Dispatch latency – time it takes for the dispatcher to stop

one process and start another running.

Scheduling Criteria

Scheduling Criteria

CPU utilization – keep the CPU as busy as possible

Throughput – No. of processes that complete their

execution per time unit

Turnaround time – amount of time to execute a

particular process i.e. from the time submitted till the time

of completion (JobQ + ReadyQ + CPUburst + DeviceQ +

IOburst)

Waiting time – amount of time a process has been

waiting in the ready queue

Response time – amount of time it takes from when a

request was submitted until the first response is produced

i.e. 1st Waiting Time.

Optimization Criteria

Max CPU utilization

Max throughput

Min turnaround time

Min waiting time

Min response time

Scheduling Algorithms

First-Come, First-Served (FCFS) Scheduling

Process Burst Time

P1 24

P2 3

P3 3

Suppose that the processes arrive in the order: P1 , P2 , P3

The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300

FCFS Scheduling (Cont.)

Suppose that the processes arrive in the order

P2 , P3 , P1 .

The Gantt chart for the schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3

Average waiting time: (6 + 0 + 3)/3 = 3

Much better than previous case.

Convoy effect - short process behind long process i.e.

other processes wait for one big process to get off CPU.

P1P3P2

63 300

Shortest-Job-First (SJF) Scheduling

Associate with each process the length of its next CPU

burst. Use these lengths to schedule the process with the

shortest time.

Two schemes:

nonpreemptive – once CPU given to the process it cannot

be preempted until completes its CPU burst.

preemptive – if a new process arrives with CPU burst length

less than remaining time of current executing process,

preempt. This scheme is know as the

Shortest-Remaining-Time-First (SRTF).

SJF is optimal – gives minimum average waiting time for

a given set of processes.

Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 6

P3 4.0 1

P4 5.0 4

SJF (non-preemptive)

Average waiting time = (0 + 10 + 3 + 3)/4 = 4

Example of Non-Preemptive SJF

P1 P3 P4

73 180

P2

8 12

Example of Preemptive SJF

Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

16

Determining Length of Next CPU Burst

Can only estimate the length.

Can be done by using the length of previous CPU bursts,

using exponential averaging.

Priority Scheduling

A priority number (integer) is associated with each

process

The CPU is allocated to the process with the highest

priority (smallest integer highest priority).

Preemptive

nonpreemptive

SJF is a priority scheduling where priority is the predicted

next CPU burst time.

Problem Starvation – low priority processes may never

execute.

Solution Aging – as time progresses increase the

priority of the process.

Process Arrival Time Burst Time Priority

P1 0.0 7 3

P2 2.0 4 4

P3 4.0 1 1

P4 5.0 4 2

Priority (non-preemptive)

Average waiting time = (0 + 10 + 3 + 3)/4 = 4

Example of Non-Preemptive Priority

Scheduling

P1 P3 P4

73 180

P2

8 12

Example of Preemptive Priority

Scheduling

Process Arrival Time Burst Time Priority

P1 0.0 7 4

P2 2.0 4 2

P3 4.0 1 1

P4 5.0 4 3

Priority (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

16

Round Robin (RR)

Each process gets a small unit of CPU time (time

quantum), usually 10-100 milliseconds. After this time

has elapsed, the process is preempted and added to the

end of the ready queue.

If there are n processes in the ready queue and the time

quantum is q, then each process gets 1/n of the CPU time

in chunks of at most q time units at once. No process

waits more than (n-1)q time units.

Performance

q large FIFO

q small q must be large with respect to context switch,

otherwise overhead is too high.

Example of RR with Time Quantum = 20

Process Burst Time

P1 53

P2 17

P3 68

P4 24

The Gantt chart is:

Typically, higher average turnaround than SJF, but better

response.

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

Time Quantum and Context Switch Time

Multilevel Queue

Ready queue is partitioned into separate queues

Each queue has its own internal scheduling algorithm,

RR, FCFS, etc.,

Each queue is again scheduled using a scheduling

algorithm

Low scheduling overhead but inflexible.

Multilevel Queue Scheduling

Multilevel Feedback Queue

A process can move between the various queues; aging

can be implemented this way.

Multilevel-feedback-queue scheduler defined by the

following parameters:

number of queues

scheduling algorithms for each queue

method used to determine when to upgrade a process

method used to determine when to demote a process

method used to determine which queue a process will enter

when that process needs service

Example of Multilevel Feedback Queue

Three queues:

Q0 – time quantum 8 milliseconds

Q1 – time quantum 16 milliseconds

Q2 – FCFS

Scheduling

The scheduler first executes all processes in queue 0.

Only when queue 0 is empty will it execute processes in

queue 1.

Similarly, processes in queue 2 will be executed only if

queues 0 and 1 are empty.

A process that arrives for queue 1 will preempt a process in

queue 2.

A process that arrives for queue 0 will, in turn, preempt a

process in queue 1.

Example of Multilevel Feedback Queue

Scheduling

A process entering the ready queue is put in queue 0. A

process in queue 0 is given a time quantum of 8

milliseconds. If it does not finish within this time, it is moved

to the tail of queue 1.

If queue 0 is empty, the process at the head of queue 1 is

given a quantum of 16 milliseconds. If it does not complete,

it is preempted and is put into queue 2.

Processes in queue 2 are run on an FCFS basis, only when

queues 0 and 1 are empty.

Multilevel Feedback Queues

Multiple-Processor Scheduling

Multiple-Processor Scheduling

CPU scheduling is more complex when multiple CPUs

are available.

Let us assume Homogeneous processors within a

multiprocessor.

2 approaches:

Separate ready queue for each processor: Drawback is that,

some processor might be idle if its ready queue alone is

empty.

Common ready queue for all processor:

Approach 1: Each processor choose its own process.

Drawback is that, 2 processor could choose the same

process.

Approach 2 (Master-Slave): Master assigns, slave

executes.

Syllabus

UNIT II

PROCESS MANAGEMENT

Synchronization: The Critical-Section Problem -

Semaphores – Classic Problems of Synchronization.

The Critical-Section Problem

Background

Concurrent access to shared data may result in data

inconsistency.

Maintaining data consistency requires mechanisms to

ensure the orderly execution of cooperating processes.

Suppose that we modify the producer-consumer code by

adding a variable counter, initialized to 0 and incremented

each time a new item is added to the buffer

Bounded-Buffer

Shared data

#define BUFFER_SIZE 10

typedef struct {

. . .

} item;

item buffer[BUFFER_SIZE];

int in = 0;

int out = 0;

int counter = 0;

Bounded-Buffer

Producer process

item nextProduced;

while (1) {

while (counter == BUFFER_SIZE)

; /* do nothing */

buffer[in] = nextProduced;

in = (in + 1) % BUFFER_SIZE;

counter++;

}

Bounded-Buffer

Consumer process

item nextConsumed;

while (1) {

while (counter == 0)

; /* do nothing */

nextConsumed = buffer[out];

out = (out + 1) % BUFFER_SIZE;

counter--;

}

Bounded Buffer

The statements

counter++;

counter--;

must be performed atomically.

Atomic operation means an operation that completes in

its entirety without interruption.

Bounded Buffer

The statement “count++” may be implemented in

machine language as:

register1 = counter

register1 = register1 + 1

counter = register1

The statement “count—” may be implemented as:

register2 = counter

register2 = register2 – 1

counter = register2

Bounded Buffer

If both the producer and consumer attempt to update the

buffer concurrently, the assembly language statements

may get interleaved.

Interleaving depends upon how the producer and

consumer processes are scheduled.

Bounded Buffer

Assume counter is initially 5. One interleaving of

statements is:

producer: register1 = counter (register1 = 5)

producer: register1 = register1 + 1 (register1 = 6)

consumer: register2 = counter (register2 = 5)

consumer: register2 = register2 – 1 (register2 = 4)

producer: counter = register1 (counter = 6)

consumer: counter = register2 (counter = 4)

The value of count may be either 4 or 6, where the

correct result should be 5.

Race Condition

Race condition: The situation where several processes

access – and manipulate shared data concurrently. The

final value of the shared data depends upon which

process finishes last.

To prevent race conditions, concurrent processes must

be synchronized.

Definitions

Process Synchronization:

Orderly execution of co-operating processes to maintain

consistency

Race Condition:

Several processes access and manipulate the same data

concurrently and the outcome of the execution depends on the

particular order in which the access takes place, is called a race

condition.

The Critical-Section

Consider a system consisting of n processes {Po,P1, ...,

P,-1).

Each process has a segment of code, called a critical

section, in which the process may be changing common

variables, updating a table, writing a file, and so on.

The critical-section problem is to design a protocol that

the processes can use to cooperate.

Each process must request permission to enter its critical

section. The section of code implementing this request is

the entry section.

The critical section may be followed by an exit section.

The remaining code is the remainder section.

The Critical-Section Problem

n processes all competing to use some shared data

Each process has a code segment, called critical section,

in which the shared data is accessed.

Problem – ensure that when one process is executing in

its critical section, no other process is allowed to execute

in its critical section.

Solution to Critical-Section Problem

1. Mutual Exclusion. If process Pi is executing in its critical

section, then no other processes can be executing in their

critical sections.

2. Progress. f no process is executing in its critical section

and some processes wish to enter their critical sections,

then only those processes that are not executing in their

remainder section can participate in the decision on

which will enter its critical section next, and this selection

cannot be postponed indefinitely.

3. Bounded Waiting. A bound must exist on the number of

times that other processes are allowed to enter their

critical sections after a process has made a request to

enter its critical section and before that request is

granted.

Initial Attempts to Solve Problem

Only 2 processes, P0 and P1

General structure of process Pi (other process Pj)

do {

entry section

critical section

exit section

reminder section

} while (1);

Processes may share some common variables to

synchronize their actions.

Algorithm 1

Shared variables:

int turn;initially turn = 0

turn = 0 P0 can enter its critical section

turn = 1 P1 can enter its critical section

Process P0

do {

while (turn != 0) ;

critical section

turn = 1;

reminder section

} while (1);

Process P1

do {

while (turn != 1) ;

critical section

turn = 0;

reminder section

} while (1);

Algorithm 2

The problem with algorithm 1 is that it does not retain sufficientinformation about the state of each process; it remembers onlywhich process is allowed to enter its critical section. To remedy thisproblem, we can replace the variable turn with the following array:boolean flag [2];

Shared variables

boolean flag[2];initially flag [0] = flag [1] = false.

flag [0] = true P0 ready to enter its critical section

flag [1] = true Pi ready to enter its critical section

Process P0

do {

flag[0] := true;while (flag[1]) ;

critical section

flag [0] = false;

remainder section

} while (1);

Process P1

do {flag[1] := true;while (flag[0]) ;critical section

flag [1] = false;remainder section

} while (1);

Algorithm 3

Combined shared variables of algorithms 1 and 2.

Process P0

do {

flag [0]:= true;turn = 1;while (flag [1] and turn = 1) ;

critical section

flag [0] = false;

remainder section

} while (1);

Meets all three requirements; solves the critical-section problem for two processes.

Process P1

do {flag [1]:= true;

turn = 0;while (flag [0] and turn=0) ;

critical sectionflag [1] = false;

remainder section} while (1);

Bakery Algorithm

Before entering its critical section, process receives a

number. Holder of the smallest number enters the critical

section.

If processes Pi and Pj receive the same number, if i < j,

then Pi is served first; else Pj is served first.

Critical section for n processes

Bakery Algorithm

Notation < lexicographical order (ticket #, process id #)

(a,b) < (c,d) if a < c or if a == c and b < d

max (a0,…, an-1) is a number, k, such that k ai for i = 0,

…, n – 1

Shared data

boolean choosing[n];

int number[n];

Data structures are initialized to false and 0 respectively

Bakery Algorithm

Process Pi

do {

choosing[i] = true;

number[i] = max(number[0], number[1], …, number [n – 1])+1;

for (j = 0; j < n; j++) {

while (choosing[j]) ;

while ((number[j] !=0) && ((number[j],j) < (number[i],i))) ;

}

critical section

number[i] = 0;

remainder section

} while (1);

Semaphores

Semaphores

Semaphore S – integer variable

can only be accessed via two indivisible (atomic)

operations

wait (S):

while S 0 do no-op;

S--;

signal (S):

S++;

Critical Section of n Processes

Shared data:

semaphore mutex; //initially mutex = 1

Process Pi:

do {wait(mutex);

critical section

signal(mutex);remainder section

} while (1);

Drawback: Spin until lock (spinlock semaphore), CPU wasted

Semaphore Implementation

Define a semaphore as a record

typedef struct {

int value;

struct process *L;

} semaphore;

Assume two simple operations:

block suspends the process that invokes it (i.e.) running to

wait state

wakeup(P) resumes the execution of a blocked process P

(i.e.) wait to ready state

Implementation

Semaphore operations now defined as

wait(S):S.value--;

if (S.value < 0) {

add this process to S.L;block;

}

signal(S): S.value++;

if (S.value <= 0) {

remove a process P from S.L;wakeup(P);

}

Semaphore value can be negative

Magnitude = no. of process waiting in the list

Operating System Concepts

Deadlock and Starvation

Deadlock – two or more processes are waiting indefinitely for

an event that can be caused by only one of the waiting

processes.

Let S and Q be two semaphores initialized to 1

P0 P1

wait(S); wait(Q);

wait(Q); wait(S);

signal(S); signal(Q);

signal(Q) signal(S);

Starvation – indefinite blocking. A process may never be

removed from the semaphore queue in which it is suspended.

Two Types of Semaphores

Counting semaphore – integer value can range over

an unrestricted domain.

Binary semaphore – integer value can range only

between 0 and 1; can be simpler to implement.

Classical Problems of Synchronization

Classical Problems of Synchronization

Bounded-Buffer Problem

Readers and Writers Problem

Dining-Philosophers Problem

Bounded-Buffer Problem

Shared data

semaphore full, empty, mutex;

full – no. of full buffers

empty – no. of empty buffers

mutex – no. of process that can enter a critical section

simultaneously

Initially:

full = 0, empty = n, mutex = 1

Bounded-Buffer Problem - Producer

Process

do {

…

produce an item in nextp

…

wait(empty);

wait(mutex);

…

add nextp to buffer

…

signal(mutex);

signal(full);

} while (1);

Bounded-Buffer Problem - Consumer

Process

do {

wait(full)

wait(mutex);

…

remove an item from buffer to nextc

…

signal(mutex);

signal(empty);

…

consume the item in nextc

…

} while (1);

Readers-Writers Problem

Shared data

semaphore mutex, wrt;

Initially

mutex = 1, wrt = 1, readcount = 0

Readers-Writers Problem - Writer Process

wait(wrt);

…

writing is performed

…

signal(wrt);

Readers-Writers Problem - Reader Process

wait(mutex);

readcount++;

if (readcount == 1)

wait(wrt);

signal(mutex);

…

reading is performed

…

wait(mutex);

readcount--;

if (readcount == 0)

signal(wrt);

signal(mutex):

Dining-Philosophers Problem

Shared data

semaphore chopstick[5];

Initially all values are 1

Dining-Philosophers Problem

Philosopher i:

do {

wait(chopstick[i])

wait(chopstick[(i+1) % 5])

…

eat

…

signal(chopstick[i]);

signal(chopstick[(i+1) % 5]);

…

think

…

} while (1);

Dining-Philosophers Problem

Limitations: It has the possibility of creating a deadlock.Suppose that all five philosophers become hungrysimultaneously, and each grabs her left chopstick. All theelements of chopstick will now be equal to 0. When eachphilosopher tries to grab her right chopstick, she will be delayedforever.

Solution to the dining-philosophers problem that ensuresfreedom from deadlocks are:

Allow at most four philosophers to be sitting simultaneously at thetable

Allow a philosopher to pick up her chopsticks only if bothchopsticks are available

an odd philosopher picks up first her left chopstick and then herright chopstick, whereas an even philosopher picks up her rightchopstick and then her left chopstick.

Reference

A. Silberschatz, P.B. Galvin & G. Gagne, “Operating system

concepts”, Sixth Edition John Wiley, 2005.