2024-05-11
Semaphores are similar to mutexes as in they allow to control who has access to a particular resource in a scenario where multiple threads are trying to mutate shared data.
However, mutexes are used when only one thread can access a particular resource, whereas semaphores are more general, and allow a number N of threads (but no more) to access a particular resource.
Suppose a physical library has ten identical study rooms, to be used by one
student at a time. Students must request a room from the front desk. If no
rooms are free, students wait at the desk until someone relinquishes a room.
When a student has finished using a room, the student must return to the desk
and indicate that the room is free.
In the simplest implementation, the clerk at the front desk knows only the
number of free rooms available. This requires that all of the students use
their room while they have signed up for it and return it when they are done.
When a student requests a room, the clerk decreases this number. When a
student releases a room, the clerk increases this number. The room can be
used for as long as desired, and so it is not possible to book rooms ahead of
time.
In this scenario, the front desk count-holder represents a counting
semaphore, the rooms are the resource, and the students represent
processes/threads. The value of the semaphore in this scenario is initially
10, with all rooms empty. When a student requests a room, they are granted
access, and the value of the semaphore is changed to 9. After the next
student comes, it drops to 8, then 7, and so on. If someone requests a room
and the current value of the semaphore is 0,[3] they are forced to wait until
a room is freed (when the count is increased from 0). If one of the rooms was
released, but there are several students waiting, then any method can be used
to select the one who will occupy the room (like FIFO or randomly picking
one). And of course, a student must inform the clerk about releasing their
room only after really leaving it.
The paradigm is especially powerful because the semaphore count may serve as
a useful trigger for a number of different actions. The librarian above may
turn the lights off in the study hall when there are no students remaining,
or may place a sign that says the rooms are very busy when most of the rooms
are occupied.