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Concurrency: Mutual Exclusion

and Synchronization

Chapter 5

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Concurrency

Communication among processes

Sharing of and competing for resources

Synchronization of multiple processes Allocation of processor time

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Concurrency

Multiple applications

Multiprogramming

Structured applicationApplication can be a set of concurrent

processes

Operating-system structure

Operating system is a set of processes or

threads

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Difficulties with Concurrency

Sharing global resources

Management of allocation of resources

Programming errors difficult to locate

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A Simple Example

void echo()

{

in = getchar();chout = in;

putchar(chout);

}

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A Simple Example

Process P1 Process P2

. .

in = getchar(); .

. in = getchar();

. chout = in;

. putchar(chout);

chout = in; .

putchar(chout); .

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Race condition

A situation in which multiple threads or

processes read and write a shared data

item and the final result depends on the

relative timing of their execution.

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Operating System Concerns

Keep track of active processes

Allocate and deallocate resources

Processor time

Memory Files

I/O devices

Protect data and resources

Result of process must be independent of thespeed of execution of other concurrentprocesses

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Process Interaction

Processes unaware of each other:

Independent process not intended to work

together. Competition.

Processes indirectly aware of each other:

Share access to some object, such as an I/O

buffer. Cooperation.

Process directly aware of each other:

Communicate each other by PID and are

designed to work jointly. Cooperation.

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Competition Among

Processes forR

esources Three control problems:Mutual Exclusion

Critical sections

Only one program at a time is allowed in its critical

section

Example only one process at a time is allowed to

send command to the printer

DeadlockStarvation

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Mutual exclusion

Requirement that when one process is in

a critical section that accesses shared

resources, no other process may be in a

critical section that accesses any of those

shared resources.

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Critical section

Section of code within a process

Requires access to shared resources

May not be executed while anotherprocess is in a corresponding section of

code.

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Deadlock

Two or more processes are unable to

proceed because each is waiting for one

of the others to do something.

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Starvation

Runnable process is overlooked

indefinitely by the scheduler.

Although it is able to proceed, it is neverchosen.

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Cooperation Among Processes

by Sharing Processes interact with other processeswithout being explicitly aware of them.

Writing must be mutually exclusive Critical sections are used to provide data

integrityentercritical()

exitcritical()

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Cooperation Among Processes

by Communication Messages are passedMutual exclusion is not a control

requirement

Possible to have deadlockEach process waiting for a message from

the other process

Possible to have starvation

Two processes sending message to eachother while another process waits for amessage

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Requirements for Mutual

Exclusion Only one process at a time is allowed inthe critical section for a resource

A process that halts in its non-criticalsection must do so without interfering

with other processes

No deadlock or starvation

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Requirements for Mutual

Exclusion A process must not be delayed access toa critical section when there is no other

process using it

No assumptions are made about relative

process speeds or number of processes

A process remains inside its critical

section for a finite time only

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First Attempt

Busy Waiting

Process is always checking to see if it can

enter the critical section

Process can do nothing productive until it

gets permission to enter its critical section

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Second Attempt

Each process can examine the others statusbut cannot alter it

When a process wants to enter the critical

section it checks the other processes first If no other process is in the critical section, it

sets its status for the critical section

This method does not guarantee mutual

exclusion Each process can check the flags and then

proceed to enter the critical section at the sametime

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Correct Solution

Each process gets a turn at the critical

section

If a process wants the critical section, it

sets its flag and may have to wait for its

turn

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Semaphores

Special variable called a semaphore is

used for signaling

If a process is waiting for a signal, it is

suspended until that signal is sent

Wait (Proberen) and signal (Verhogen)

operations cannot be interrupted

Queue is used to hold processes waiting

on the semaphore

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Semaphores

Semaphore is a variable that has an

integer value

May be initialized to a nonnegative number

Wait operation decrements the semaphore

value, if the value becomes negative, the

process is blocked.

Signal operation increments semaphorevalue. If the value is less than or equal to

zero, then a process blocked by semWait is

unblocked.

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Semaphores

Binary semaphore or mutex

May be initialized to 0 or 1

Wait operation checks the semaphore value.

If the value is 0 the process is blocked. If

the value is 1 then the value is changed to 0

and the process continues.

Signal operation checks to see if anyprocess are blocked on this semaphore. If

so, then a process is unblocked. If no

processes are blocked, then the value of the

semaphore is set to 1.

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Semaphores

A semaphore whose definition includes

the FIFO policy is called strong

semaphore.

A weak semaphore does not specify the

order in which the processes are

removed from the queue.

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Producer/Consumer Problem

One or more producers are generating

data and placing these in a buffer

A single consumer is taking items out of

the buffer one at time

Only one producer or consumer may

access the buffer at any one time

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Producer

producer:

while (true) {

/* produce item v */b[in] = v;

in++;

}

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Consumer

consumer:

while (true) {

while (in

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Monitors

Monitor is a software module

Chief characteristics

Local data variables are accessible only bythe monitor

Process enters monitor by invoking one of

its procedures

Only one process may be executing in themonitor at a time

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Monitors

Monitor supports synchronization by the

use of conditional variables that are

contained within the monitor and

accessible only within the monitor.

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Monitors

Condition variables are operated by:

cwait(c): Suspend execution of the calling

process on condition c. The monitor is

available for use by another process.

csignal(c): Resume execution of some

process blocked after a cwait on the same

condition.

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Monitors

Advantages:

All of the synchronization functions are confined tothe monitor.

Easier to verify that the synchronization has been

done correctly.

Easier to detect bugs.

Once a monitor is correctly programmed, access tothe protected resource is correct for access from all

process. Contrast: With semaphores, resource access is correct

only if all of the process that access the resource areprogrammed correctly.

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Message Passing

Enforce mutual exclusion

Cooperating processes may need to

exchange information

Lends itself to implementation in

Distributed systems, shared-memory

multiprocessor and uniprocessor

send (destination, message)

receive (source, message)

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Synchronization

Sender and receiver may or may not be

blocking (waiting for message)

Blocking send, blocking receive

Both sender and receiver are blocked until

message is delivered

Called a rendezvous

Tight synchronization.

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Synchronization

Nonblocking send, blocking receive

Sender continues processing such as

sending messages as quickly as possible

Receiver is blocked until the requested

message arrives

Nonblocking send, nonblocking receive

Neither party is required to wait

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Addressing

Direct addressing

send primitive includes a specific identifier

of the destination process

receive primitive could know ahead of time

from which process a message is expected

receive primitive could use source

parameter to return a value when the receiveoperation has been performed

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Addressing

Indirect addressing

messages are sent to a shared data structure

consisting of queues

queues are called mailboxes

one process sends a message to the mailbox

and the other process picks up the message

from the mailbox

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Message Format

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Dining Philosophers Problem