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Synchronization III: Summary

Synchronization III: Summary. CPE 261403 - Operating Systems http://www.e-cpe.org/moodle. Synchronization Topics. Critical Section Limiting Concurrent Resource Access Sequencing Events. int Mutex = 1; while (Mutex == 0); Mutex--; // Critical Section Mutex++;. Semaphore Mutex = 1;

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Synchronization III: Summary

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  1. Synchronization III:Summary CPE 261403 - Operating Systems http://www.e-cpe.org/moodle

  2. Synchronization Topics • Critical Section • Limiting Concurrent Resource Access • Sequencing Events

  3. int Mutex = 1; while (Mutex == 0); Mutex--; // Critical Section Mutex++; Semaphore Mutex = 1; Wait (Mutex); // Critical Section Signal(Mutex); What’s wrong the code on the left?

  4. Non-Atomic instruction • Register1 = Mutex • Decrease register1 • Mutex = register1 • Register2 = Mutex • Decrease register2 • Mutex = register2 • Register1 = Mutex • Register2 = Mutex • Decrease register2 • Mutex = register2 • Decrease register1 • Mutex = register1

  5. Wait(s) Signal(s) Semaphore

  6. Busy Waiting while (Mutex == 0); Wait (Mutex); Very high CPU load!

  7. Semaphore vs Monitor

  8. Semaphore mutex=1; int count=0; void add() { wait(mutex); count++; signal(mutex); } int count=0; Monitor doStuff { Void add() { count++; } } Critical Section

  9. Semaphore db=3; void connectDb() { wait(db); connect(); signal(db); } int MaxDb=3; Monitor doStuff { Void connectDb() { if (MaxDb > 0){ MaxDb--; connect(); MaxDb++; } } } Limiting Concurrent Resource Access

  10. 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() } } Sequencing Events

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