PAD First Lecture

Types of queues

Serial

A → B → C → D

Concurrent

A [--------] B [_______] C

Context switch

Process Thread → shared memory Actor model → message passing Coroutines

• thread - code sequence executed as part of a program. They depend on CPU cores.
• shared memory - shared resources between threads, either written or read.

Example

x = x + 1

1. citire
2. incrementation
3. scriere

for (int i = 0; i < 100; i++) {
    x = x + 1
}
print(x);

1 single thread - 100 is printed 2 →

Race-conditions

• [~] când avem read/modify/write → x + 1
• [~] check-then-act ⇒ if (x) then

Solutions:

1. Mutex

for (int i = 0; i < 100; i++) {
    m.lock();
    x = x + 1;
    m.unlock();
}
print(x);

Functions: lock/unlock It is an object ce poate fi eliberat de obiectul de l-a preluat

Semaphore

wait/signal Queue ce așteptă procesele + o variabilă care arată la câte procese poți accessa (atomic) Poate fi eliberat de oricine

2. Deadlock

• Dead-lock - îmbrățișare fatală

Coffman conditions (for a deadlock to occur):

• Mutual admission
• Hold & wait
• No preemption
• Circular wait

1. Dining philosopher problem
2. Priority inversion
3. Reader/Writers problem

3. Starvation

Busy waiting - o variabilă care se verifică

CAUTION

DO NOTT USE

      while (x < 10) {
          sleep(10)
      }