Outline

• Basic Synchronization Principles - continued– Review

• Bounded-Buffer problem

• Readers-writers problem

– Dining-philosophers problem– Semaphore implementations– Issues with semaphores– Monitors

- Please pick up Homework #2 from the front

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Bounded-Buffer using Semaphores

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Bounded-Buffer using Semaphores – cont.

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Bounded-Buffer using Semaphores – cont.

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The Readers-Writers Problem

• A resource is shared among readers and writers– A reader process can share the resource with any

other reader process but not with any writer process

– A writer process requires exclusive access to the resource whenever it acquires any access to the resource

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First and Second Policy

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Readers-writers with active readers – cont.

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Readers-writers with an active writer – cont.

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Readers-writers with an active writer – cont.

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Dining-Philosophers Problem

• Shared data var chopstick: array [0..4] of semaphore;(=1 initially)

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Dining-Philosophers Problem - cont.

• Philosopher i:repeat

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

…eat …

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

…think …

until false;

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Semaphore Implementation

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Semaphore Implementation – cont.

• Binary semaphore through test-and-set instruction

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Semaphore Implementation – cont.

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Semaphore Implementation – cont.

• Semaphores implemented using interrupt disabling and test-and-set require busy waiting– This type of semaphores is often called spinlock

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Semaphore Implementation – cont.

• Define a semaphore as a structuretypedef struct {

int value; queue L;

} semaphore;• Assume two simple operations:

– block suspends the process that invokes it.– wakeup(P) resumes the execution of a blocked

process P.

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Semaphore Implementation - cont.

• Semaphore operations now defined as

P(S): S.value = S.value – 1;

if S.value < 0

then begin

add this process to S.L;block;

end;

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Semaphore Implementation - cont.

V(S):

S.value = S.value + 1;

if S.value 0

then begin

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

end;

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Semaphore Implementation - cont.

• Semaphores are resources in the above implementation

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Constraints of Acceptable Solutions

• An acceptable solution to the critical section problem needs to meet the following constraints– Mutual exclusion– Progress: If a critical section is free, a set of processes are

trying to enter the critical section, only those processes participate in the selection of the next process to enter the critical section and the selection cannot be postponed indefinitely

– Bounded waiting: After a process requests entry into its critical section, only a bounded number of other processes may be allowed to enter their related critical sections before the original process enters its critical section

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Issues Using Semaphores

• 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 1P0 P1

P(S); P(Q);P(Q); P(S);

V(S); V(Q);V(Q) V(S);

• Starvation – indefinite blocking– A process may never be removed from the semaphore queue

in which it is suspended.

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Active and Passive Semaphores

• There is a subtle problem related to semaphore implementation– Bounded waiting may not be satisfied

• when the passive V operation is used where the implementation increments the semaphore with no opportunity for a context switch

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Active and Passive Semaphores – cont.

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Active and Passive Semaphores – cont.

• Active semaphores– In contrast to passive semaphores, a yield or

similar procedure will be called after incrementing the semaphore for a context switch in a V operation implementation

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Monitors

• High-level synchronization construct that allows the safe sharing of an abstract data type among concurrent processes.

class monitor {

variable declarations semaphore mutex = 1;

public P1 :(…) { P(mutex); ........ V(mutex);

}; ........

}

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Monitors – cont.

• To allow a process to wait within the monitor, a condition variable must be declared, as

condition x, y;

• Condition variable can only be used with the operations wait and signal.

– The operation

x.wait;means that the process invoking this operation is suspended until another process invokes

x.signal;

– The x.signal operation resumes exactly one suspended process. If no process is suspended, then the signal operation has no effect.

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Bounded buffer monitor

monitor BoundedBufferType {private: BufferItem * buffer; int NumberOfBuffers; int next_in, nextout; int current_size; condition NotEmpty, NotFull;public: BoundedBufferType( int size ) { buffers = new BufferItem[size]; NumberOfBuffers = size; next_in = 0; next_out = 0;

current_size = 0; }

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Bounded buffer monitor – cont.

void Put( BufferItem item ) { if( current_size == NumberOfBuffers ) wait( NotFull ); buffer[next_in] = item; next_in = (next_in+1) % NumberOfBuffers; if( ++current_size == 1 ) signal( NotEmpty ); } BufferItem Get( void ) { if( current_size == 0 ) wait( NotEmpty ); BufferItem item = buffer[next_out]; next_out = (next_out+1) % NumberOfBuffers; if( --current_size == NumberOfBuffers-1 ) signal( NotFull ); return item; }}

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Using a bounded buffer monitor

BoundedBufferType BoundedBuffer;

int main() { // the Producer while( 1 ) { BufferItem item = ProduceItem(); BoundedBuffer.Put( item ); }}

int main() { // the Consumer while( 1 ) { BufferItem item = BoundedBuffer.Get(); ConsumeItem( item ); }}

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Readers-writers through monitor

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Readers-writers through monitor – cont.

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Readers-writers through monitor – cont.

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Traffic Synchronization

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Dining-philosopher Monitor

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Dining-philosopher Monitor – cont.

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Thread Synchronization in Java

• Synchronized – Only one synchronized method for a particular

object can be called– Each object contains a monitor, which is

automatically part of an object

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Summary

• Processes that share data need to be synchronized– Otherwise, a race condition may exist

• Semaphores are the basic mechanism underlying synchronization– can be used to solve process synchronization problems

– need to be implemented carefully

• Monitor is an abstract data type– For mutual exclusion