Synchronization III:Summary

CPE 261403 - Operating Systemshttp://www.e-cpe.org/moodle

Synchronization Topics

Critical Section

Limiting Concurrent Resource Access

Sequencing Events

What’s wrong the code on the left?int Mutex = 1;

while (Mutex == 0);

Mutex--;

// Critical Section

Mutex++;

Semaphore Mutex = 1;

Wait (Mutex);

// Critical Section

Signal(Mutex);

Non-Atomic instruction

Register1 = MutexDecrease register1Mutex = register1

Register2 = MutexDecrease register2Mutex = register2

Register1 = MutexRegister2 = MutexDecrease register2Mutex = register2Decrease register1Mutex = register1

Semaphore

Wait(s) Signal(s)

Busy Waiting

while (Mutex == 0); Wait (Mutex);

Very high CPU load!

Semaphore vs Monitor

Critical Section

Semaphore mutex=1;

int count=0;

void add() {

wait(mutex);

count++;

signal(mutex);

}

int count=0;

Monitor doStuff {

Void add() {

count++;

}

}

Limiting Concurrent Resource AccessSemaphore db=3;

void connectDb() {

wait(db);

connect();

signal(db);

}

int MaxDb=3;

Monitor doStuff {

Void connectDb() {

if (MaxDb > 0){

MaxDb--;

connect();

MaxDb++;

}

}

}

Sequencing Events

Semaphore buffer=0;

void consume() {wait(buffer);

removeItem();}

Void Produce() {addItem();signal(buffer);

}

Monitor doStuff {

Condition buffer;

Void consume() { if (bufferLen==0)

buffer.wait(); removeItem();}

Void Produce() { addItem(); buffer.signal()}

}