Mod 3: Semaphores

Question 1

What is “busy waiting”?

Answer 1

(1) A thread is spinning in a loop waiting for a resource to become available, instead of waiting for it to be signaled
(2) Synchronization technique that can be quite wasteful of resources

Question 2

What is a semaphore?

Answer 2

a Synchronization method

(1) Counter
(2) Abstract data type, not integer
(3) Shared between threads

Question 3

What is a semaphore used for?

Answer 3

Semaphores are used to synchronize access to shared resources, and can solve a wide range of synchronization issues

Question 4

Which to atomic operations does semaphores use?

Answer 4

Wait( ) (or P, Down)
Signal( ) (or V, Up)

Question 5

How does a semaphore work?

Answer 5

It’s a counter that keeps tabs on available resources, when a thread requests access to resource the counter is decremented, and when the threads release the semaphore the counter is incremented. If the counter is negative, the thread has to wait

Question 6

What are the attributes of a semaphore?

Answer 6

(1) Value, a integer
(2) A queue, holding blocked threads, not accessible for the programmer

Question 7

What are the methods/operations of a semaphore?

Answer 7

(1) Initialize(s), where s is number of available resources
(2) P or Wait, called by threat who want to enter the CS
(3) V or Signal, called by thread who want to exit the CS, if the in a blocked thread, it is unblocked once the counter is incremented

Question 8

Does a semaphore have history?

Answer 8

Yes, a semaphore remember which thread has released the ressource

Question 9

Does the thread owns the semaphore if it uses it?

Answer 9

No, a semaphore is not owned by a thread

Question 10

What are some real-life examples of semaphore uses?

Answer 10

• Traffic light,
• Parking slots in a parking lot,
• Counters in an bank office (nbr of service ppl available)

Question 11

What are the rule for number of semaphore required?

Answer 11

One for each constraint

Question 12

What are the two types of semaphore?

Answer 12

(1) Counting semaphore - used for a resource with many units or when several threat are allowed to access the resource (eg. for reading)
(2) Binary semaphore - can only have value 0 (wait) or 1 (signal), used as lock mechanism or where only on thread can access a resource

Question 13

Which values can a counting semaphore have?

Answer 13

0, and positive or negative value (-1 means no resources are available).

Question 14

What is a binary semaphore used for?

Answer 14

Mutual Exclusion, ME

Question 15

What are the main differences between a lock and a binary semaphore?

Answer 15

(1) Locking usually disables interrupts inside CS, Semaphore does not

Question 16

What is the difference between a semaphore and mutex?

Answer 16

Mutex is a locking mechanism, while a semaphore is a signaling mechanism

Question 17

What is the difference between a lock and mutex?

Answer 17

Mutex can be shared by different threads, a lock cannot (a lockable object can be)

Question 18

What are the main advantages of using semaphores?

Answer 18

(1) Efficiently synchronization technique,
(2) Use CPU more efficiently

Question 19

What are the main disadvantages of using semaphores?

Answer 19

(1) Not structured
(2) Difficult to develop and maintain
(3) Programmer must have good control over calls to P( ) and V( )
(4) No connection between the semaphore and the data it controls

Question 20

What can be used instead of semaphores?

Answer 20

(1) Built-in language support
(2) Monitor