Synchronization III: Summary

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