Lecture 12 Synchronization and mutual exclusion

Semaphore

• Semaphore is a method provided by the operating system to coordinate access to shared resources
• It was introduced by Dijkstra in the 1960s
• Semaphore was one of the primary synchronization mechanisms in early OS
  

Semaphore

• Semaphore is an abstract data type consisting of an integer variable (sem) and two atomic operations
  • P(): Prolaag in Dutch, meaning "try to decrease"
    • $sem = sem - 1$
    • If sem < 0, enter waiting; otherwise, continue
  • V(): Verhoog in Dutch, meaning "increase"
    • $sem = sem + 1$
    • If $sem \le 0$, wake up one waiting
      

Semaphore

• Semaphore is protected integer variable
  • Once initialized, it can only be modified through P() and V() operations
  • The OS ensures that PV operations are atomic
• P() may block, while V() does not
• Semaphore is typically assumed to be "fair"
  • Threads are not indefinitely blocked by P() operations
  • Semaphore waits are assumed to be first-in, first-out

Can spinlocks implement first-in-first-out?

Semaphore

Conceptual implementation of semaphore

Semaphore

There are two types of semaphores

• Binary semaphore: Resource count of 0 or 1
• Counting semaphore: Resource count of any non-negative value
• The two types are equivalent and one can be implemented using the other

Uses of semaphore

• Mutual exclusion and conditional synchronization

Semaphore

Example of mutual exclusion

• Set a semaphore with an initial value of 1 for each critical section
• Use P() and V() operations in pairs
  • P() operation ensures mutually exclusive access to resources
  • V() operation releases resources after use
  • The sequence of PV operations must be correct, non-repetitive, and not missed

Semaphore

Example of conditional synchronization

• Set a semaphore with an initial value of 0 for each condition

Semaphore

Example of producer-consumer problem

• Description of the bounded buffer producer-consumer problem
  • One or more producers place data in a buffer
  • A single consumer retrieves data from the buffer
  • At any time, only one producer or consumer can access the buffer
    

Semaphore

Example of producer-consumer problem

• Problem analysis
  • Only one thread can operate on the buffer at a time (mutual exclusion)
  • When the buffer is empty, the consumer must wait for the producer (conditional synchronization)
  • When the buffer is full, the producer must wait for the consumer (conditional synchronization)
• Use semaphores to describe each constraint
  • Binary semaphore mutex
  • Count semaphore fullBuffers
  • Count semaphore emptyBuffers

Semaphore

Examples of producer-consumer problems: Does the order of P() and V() operations matter?

Semaphore

• Reading/developing the code can be challenging
• It is prone to errors
  • Using occupied semaphore
  • Forgetting to release semaphore
  • It cannot prevent deadlocks
  • It requires a higher level of proficiency from programmers