The Illustrated Guide to Semaphores

You are the receptionist at a very fancy Michelin-star restaurant. Due to COVID-19 restrictions, you can only allow 10 people in at a time, and if there are ever more than 10 people in the restaurant at the same time, the restaurant loses its license and shuts down.

How do you enforce this? One way you can do it is by using semaphores. To the receptionist, a semaphore is like a counter.

Initially, the semaphore starts at 10, representing that there are 10 empty spots in the restaurant. When someone enters the restaurant, they are “acquiring” the semaphore. Each time someone acquires the semaphore, they get to enter the restaurant, and the semaphore decrements by one.

As soon as the semaphore hits 0, that means that the restaurant is full, and so we can’t allow any more people in. At this point, anyone who tries to enter the restaurant by acquiring the semaphore will block. Remember that each person can act independently (ie; they’re separate threads), so this means that as long as the semaphore is 0, everyone who tries to acquire it, even if they cut in line, will inevitably have to wait.

So what happens when someone leaves the restaurant when the semaphore is at 0? When someone leaves, they “release” the semaphore. Ordinarily, this will increase the semaphore by 1, but if someone is blocked on the semaphore, like the two customers above, one of the two customers will get unblocked by the release, allowing one (and only one) of them to get in. When this happens, the semaphore remains at 0 since there are still only 10 people in the restaurant (one left, one entered, so there is no change).

Now here comes the interesting part. There are two types of semaphores. The first type is a “fair” semaphore. A fair semaphore acts like a normal line. If the semaphore is 0, the first person who finishes the acquire call and gets blocked, is also the first person to get unblocked when a customer leaves and releases the semaphore. Thus, it acts like a standard queue.

A tiny caveat is that it’s the first person who finishes the acquire call, not the first person who makes the acquire call. If Bob calls acquire first, then John, but John’s acquire call resolves before Bob’s, then John will be the first to get unblocked.

The other type of semaphore is an unfair semaphore. An unfair semaphore is different in that it doesn’t guarantee that the first person who blocks is also the first person to unblock. With an unfair semaphore, as soon as one person (thread) exits, it’s like a mad crowd of people all trying to get in at once, with no sense of order or fairness in sight.

Because of this, it’s not exactly a good idea to use unfair semaphores if you’re waiting in line at a restaurant. Suppose there was a man in line, and he failed to get into the restaurant after someone left. What would happen if he was super unlucky, and failed to get in even after a really long time?

This situation is called starvation, and it occurs when a thread (person) is continuously unable to do some sort of work due to being blocked or forced to wait. In this case, a customer is unluckily unable to enter the restaurant, because they never get chosen to get unblocked by the semaphore.

In the example image above, the semaphore counter is currently 0. Bob is the one wearing the blue hat, and he is blocked.

A new person arrives in step one and blocks on the semaphore. Then in step 2, someone leaves, and releases the semaphore. In step 3, the newly arrived person gets unblocked by the semaphore, allowing them to enter.

This leaves poor Bob in step 4 to starve. Then, it loops back to step 1, and the whole process repeats over and over again, guaranteeing that Bob never gets into the restaurant. In this scenario, Bob starves, both literally and in the programming sense.

Now, this is a very particular scenario, and it’s highly unlikely that Bob will continuously get blocked over and over again for an infinite amount of time. Depending on how unlucky Bob is, and how many people Bob is competing with, Bob could be stuck waiting for months or even years.

Conclusion

So based off these results, you’re probably thinking something like, “Oh, that sounds awful. Guess I’ll just use fair semaphores instead of unfair semaphores.”

Unfortunately, it’s not always best to choose fair semaphores. If fair semaphores were faster than unfair semaphores, and had better utility, then no one would ever use unfair semaphores.

When using a fair semaphore, you have additional overhead because you need to remember the exact ordering of who called acquire first, which makes them slower than unfair semaphores who will just randomly pick the first free thread that it sees.

The main reason to use a semaphore is when your problem has a limited resource, typically some sort of resource pool, that needs to be shared with other threads. You want the semaphore to be the size of the number of resources so that it blocks when all of the resources are gone, and unblocks when some thread releases their piece of the resource pool.

Lastly, remember that a binary semaphore (a semaphore whose value initializes at 1) is not the same as a mutex lock, and you generally shouldn’t use a binary semaphore in place of a mutex. Mutex locks can only be unlocked by the thread that locked it, whereas semaphores can be released by any thread, since it has no sense of ownership.

For most intents and purposes, a binary semaphore can basically act as a lock. However, you really shouldn’t say that a mutex lock is just a binary semaphore, because saying that is like saying that a car is an umbrella. It’s technically true, since a car covers you from the rain, but any normal person would look at you as if you had two heads.

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