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Difference between Spinlock and Semaphore

In this article, you will learn the difference between the Spinlock and Semaphore. But before discussing the differences, you must know about the Spinlock and Semaphore.

What is Spinlock?

It is a locking mechanism. It enables a thread to wait for the lock to become ready, i.e., the thread can wait in a loop or spin until the lock is ready. It is only held for a short time, and it is useful in a multiprocessor system. The thread holds the spinlock until it is released after acquiring the lock. In some implementations, the spinlock is automatically released if the thread holding the lock is blocked or goes to sleep state.

A spinlock also avoids the overhead caused by OS process rescheduling or context switching. Furthermore, the spinlock is an effective method to block the threads temporarily. As a result, spinlocks are used in most of the operating system kernels. However, if a thread keeps a spinlock for an extended period of time, it may prevent other threads from executing. In this case, the other threads repeatedly try to acquire the lock, while the thread holding the lock doesn’t begin to release it. Generally, it may mainly occur in single-processor systems.

Advantages and disadvantages of spinlock

There are various advantages and disadvantages of a spinlock. Some of the advantages and disadvantages of the spinlock are as follows:

Advantages

1. It does not require a context switch because it is busy waiting, and the thread is not sleeping.
2. If the critical section (CS) is smaller, it is helpful.

Disadvantages

1. Spinlock needs busy waiting.
2. When the lock is unavailable, it wastes a CPU cycle and repeatedly checks for it to be accessible.

What is Semaphore?

A semaphore is a variable that is shared by multiple processes. The major goal of using a semaphore is process synchronization and access control for a common resource in a concurrent environment. There are two wait() and signal() methods that can modify the semaphore value in semaphore. When one process changes the semaphore value, other processes may not change the semaphore value at the same time.

Moreover, the operating system divides semaphores into two types:

1. Counting Semaphore
2. Binary Semaphore

Counting Semaphore

In counting semaphore, multiple resources are handled. They are initialized with the count function, and resources are allocated until the count reaches 0. The task requesting the resources will be blocked, and they have a non-restricted value domain. When several resources are added, the count value increases, and when the number of resources reduces, the count is reduced.

Binary Semaphore

In binary semaphore, the semaphore value ranges from 0 to 1. It’s similar to mutex lock, with the difference that mutex is a locking method and semaphore is a signalling method. When a process needs to use a binary semaphore resource, it invokes the wait() method, which decreases the semaphore’s value from 1 to 0.

When the process releases the resource, it uses the signal() method to increase the semaphore value to 1. When a process needs to access a resource, and the semaphore value is 0, it uses the wait() method to block until the current process that is using the resource releases it.

Advantages and disadvantages of semaphore

There are various advantages and disadvantages of semaphore. Some of the advantages and disadvantages of semaphore are as follows:

Advantages

1. It doesn’t permit various processes to enter in the critical section.
2. There is no waste of process time or resources because of the busy waiting in semaphore. The processes are only permitted to access the crucial area if a certain condition is satisfied.
3. It allows flexible management of resources.
4. It allows multiple threads to access in the critical section.

Disadvantages

1. The wait and signal method must be executed to avoid the deadlock.
2. A procedure can become blocked if it is used incorrectly. Such a situation is called Deadlock.
3. It can cause priority inversion, which means that low-priority processes can access the critical section first, and high-priority processes can access it later.

Main Differences between the Spinlock and Semaphore

Here, you will learn the main differences between the Spinlock and Semaphore. Various differences between the Spinlock and Semaphore are as follows:

1. Spinlock may be used for mutual exclusion. In contrast, semaphores may be used either for mutual exclusion or a counting semaphore.
2. Spinlocks permit only a single process at any particular time to access the critical section. In contrast, semaphore allows multiple processes at any particular time to access the critical section.
3. Spinlock may have only two values which are Locked and Unlocked. In contrast, in semaphore, the mutex will have a value 1 or 0, but if used like a semaphore, it may have different values.
4. Spinlock allows only a single thread at a time to acquire the lock and proceed it with a critical section. In contrast, semaphore allows multiple threads to access the critical section.
5. Spinlock is a low-level synchronization mechanism. In contrast, the semaphore is a signaling mechanism.
6. A process waiting for a lock in spinlock will have instant access to the critical section since the process will poll for the lock continually. In contrast, a process waiting for a lock in semaphore may not enter the crucial section as soon as the lock is released since the process has gone to sleep and will enter the critical section when it is woken up.
7. Spinlock is a busy-wait process. In contrast, the semaphore is a sleep wait process.

Head-to-head Comparison between the Spinlock and Semaphore

Here, you will learn the head-to-head comparison between the Spinlock and Semaphore. Various differences between the Spinlock and Semaphore are as follows:

Spinlock	Semaphore
It may be used for mutual exclusion.	It may be used either for mutual exclusion or a counting semaphore.
Spinlock is a low-level synchronization technique.	It is a signaling mechanism.
If a spinlock is held for a long time, it may be wasteful.	There is no resource wastage of resources and process time.
Spinlocks are very effective because they are blocked only for a short period of time.	It is held for a longer time, and it uses spinlock to get access to its control structure.
It permits only a single thread at a time to acquire the lock and proceed it with a critical section.	It permits multiple threads to access the critical section.
It is a busy-wait process.	It is a sleep wait process.
Spinlocks are ineffective in uniprocessor systems because they keep the processor busy each time it polls the lock and prevents any other process from functioning.	Semaphore is helpful in uniprocessor systems since they do not keep the processor busy while waiting for the lock.
It may have only two values which are Locked and Unlocked.	In semaphore, the mutex will have a value 1 or 0, but it may have different values if used as a counting semaphore.
It permits only a single process at a time to access in the critical section.	It permits several processes at any particular time to access the critical section.
While holding a spinlock, it is recommended to disable the interrupts.	It may be locked with interrupt enabled.
It is valid for only a single process.	It may be used to synchronize between many processes.
A process waiting for a lock in spinlock will have instant access to the critical section since the process will poll for the lock continually.	A process waiting for a lock in semaphore may not enter the crucial section as soon as the lock is released since the process has gone to sleep and will enter the critical section when it is woken up.

Conclusion

Spinlock is a low-level synchronization method. It’s simple and quick to install. However, it wastes system resources. On the other hand, semaphores provide a more advanced solution to the process synchronization problem. They don’t waste system resources as they put the waiting processes to sleep. Still, if semaphores are used carelessly, they might cause deadlocks.

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