Colin Snow
When a thread tries to acquire a mutex and it is unavailable, the thread is put to sleep and wakes up when the mutex is available. On the other hand, spinlocks are used in multi-core CPUs and are useful when protecting a tiny portion of the critical section. Spinlocks are typically used in contexts that cannot sleep, such as interrupts. While mutexes and spinlocks serve similar purposes, they have distinct use cases, and the choice depends on the specific scenario and architecture in question. For example, in a multi-core/multi-CPU system with many short-lived locks, the constant transitioning of threads between sleeping and running states in mutexes may decrease runtime performance, whereas spinlocks allow threads to take advantage of their full runtime quantum, leading to higher processing throughput. However, spinlocks can be wasteful on single-core systems as a thread continuously polling for a lock consumes CPU cycles, and no other thread can execute.
| Characteristics | Values |
|---|---|
| Mutex | When a thread tries to acquire a mutex and it’s unavailable, the thread is put to sleep. It wakes up when the mutex is available again. |
| Spinlock | Instead of sleeping, a thread will continuously poll until the lock is available. This can waste CPU time, especially on single-core systems. |
| Use Case | The choice between a mutex and a spinlock depends on the specific scenario and architecture in question. Mutexes are generally safer and are used when a resource should be locked. Spinlocks are faster and are useful for protecting a tiny portion of the critical section. |
| Performance | Mutexes are faster than spinlocks in most cases, especially when there is high contention. Spinlocks can perform better for short critical sections and when there are multiple cores and locks held for a very short amount of time. |
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What You'll Learn
Mutexes are better for single-core systems
Mutexes and spinlocks have distinct use cases, and the choice between them depends on the specific context and system architecture. Spinlocks are typically used in multi-core CPU systems, where multiple CPU cores can access the same resource simultaneously. In such cases, if a lock is held for a very short amount of time, using a mutex can decrease runtime performance due to the overhead of constantly putting threads to sleep and waking them up.
However, in a single-core system, mutexes are generally a better choice. Spinlocks are not very efficient in this context because, while a thread is busy spinning and polling for a lock, no other thread can execute, and the lock cannot be unlocked. This wastes CPU time and can lead to a situation where the system becomes deadlocked.
Mutexes, on the other hand, put a thread to sleep when a lock is unavailable, allowing another thread to run immediately. While the overhead of transitioning a thread between sleeping and running states can be high, especially for short lock times, mutexes can provide better performance in single-core systems by allowing other threads to execute and potentially unlock the lock.
Additionally, mutexes are generally considered safer and are recommended in most applications. They are also suitable for protecting larger critical sections with multiple threads accessing them. While spinlocks are faster for tiny critical sections, they can lead to wasted CPU cycles and poor performance if not used carefully, especially in single-core systems.
In summary, while spinlocks have their use cases, mutexes are often a better choice for single-core systems due to their ability to manage threads efficiently, avoid deadlocks, and provide safer and more predictable performance.
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Spinlocks are faster for short critical sections
Spinlocks are generally considered to be a better option for short critical sections. When a thread tries to acquire a mutex and it is unavailable, the thread is put to sleep and wakes up when the mutex is available. This transition between sleeping and running states has overhead. If a mutex is held only for a brief period, the overhead of waking up a sleeping thread can outweigh the benefit.
On the other hand, spinlocks are best used when the resource being contested is usually not held for a significant number of cycles, meaning the thread that has the lock is likely to give it up soon. Spinlocks are wasteful as a thread continuously polls for a lock, consuming CPU cycles. However, on multi-core systems, if a lock is held briefly, spinning can avoid the overhead of putting threads to sleep and waking them up, potentially increasing performance.
Spinlocks should not spin for longer than a few microseconds. They are best used for code that holds a lock to quickly perform memory read/writes and not for code that does calculations or other expensive operations while holding the lock.
In conclusion, spinlocks are faster for short critical sections as they avoid the overhead of putting threads to sleep and waking them up, but they can be wasteful if used for longer critical sections.
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Spinlocks are useful for multi-core CPUs
Spinlocks and mutexes serve similar purposes but have distinct use cases. Spinlocks are particularly useful for multi-core CPUs.
Spinlocks are used to stop the process from rescheduling. In a single-core CPU, it is pointless to use a spinlock to prevent context switching as no other thread can execute. However, in a multi-core CPU, two or more CPUs can access the same resource at once, and a thread on another processor may release the lock without context-switching.
Spinlocks are also useful in contexts that cannot sleep, such as interrupts. They are faster and useful for protecting a tiny portion of a critical section. They can also be used anywhere, even in a uni-processor system, whereas mutexes can only be used when sleeping is allowed.
On a multi-core CPU with several locks held for a short time, constantly putting threads to sleep and waking them up again may decrease runtime performance. Spinlocks allow threads to take advantage of their full runtime quantum, leading to higher processing throughput.
However, spinlocks can be wasteful as a thread continuously polling for a lock consumes CPU cycles. If the lock is held for a long time, it will waste a lot of CPU time. Therefore, spinlocks are generally not efficient on a single-core CPU.
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Mutexes are safer for most applications
Mutexes and spinlocks, while serving similar purposes, have distinct use cases. The choice between the two depends on the specific scenario and the architecture in question. Mutexes are safer for most applications, and here's why:
When a thread tries to acquire a mutex and it's unavailable, the thread is put to sleep. It wakes up when the mutex becomes available. This is similar to receiving a notification on your phone. Polling on a spinlock, on the other hand, wastes CPU time as a thread continuously polls until the lock is available. This is like constantly scrolling on your phone, wasting time and consuming CPU cycles.
Using spinlocks on a single-core/single-CPU system is usually inefficient. If a thread is busy spinning, no other thread can execute, and the lock won't be unlocked. Spinlocks are more commonly used in multi-core/multi-CPU systems, where they can be advantageous for locks held for a very short time. In such cases, spinlocks allow threads to take advantage of their full runtime quantum, leading to higher processing throughput.
However, if the lock is held for a longer period, the CPU time wasted by polling on a spinlock can be significant. In these cases, it would be better if the thread was sleeping, allowing another thread to run and possibly unlock the lock. Additionally, the operations of putting threads to sleep and waking them up again can be expensive, requiring a lot of CPU instructions and time.
While spinlocks have their use cases, for most applications, mutexes are the safer bet. They allow for better scheduling, even with short critical sections, and help avoid the wasted CPU time associated with spinlocks. Mutexes introduce context-switch overhead, making them suitable for protecting a larger amount of critical section with multiple threads accessing it.
In summary, while spinlocks have their advantages in specific scenarios, mutexes are generally safer for most applications due to their ability to conserve CPU resources, handle longer lock times efficiently, and provide better scheduling and context-switch capabilities.
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Spinlocks are counterproductive for single-core systems
Spinlocks and mutexes are both used for synchronisation purposes, but they have distinct use cases. While spinlocks can be useful in certain scenarios, they are generally counterproductive for single-core systems.
On a single-core system, if a thread is busy spinning on a spinlock, no other thread can execute. This is because spinlocks continuously poll until the lock is available, consuming valuable CPU cycles. As a result, the lock won't be unlocked, and the CPU time is wasted without any real benefit.
In contrast, a mutex puts the thread to sleep when it is unavailable, allowing another thread to run immediately. The thread wakes up when the mutex becomes available. While putting threads to sleep and waking them up incurs overhead, this approach is generally more efficient on single-core systems, as it ensures that the CPU resources are utilised effectively.
For example, if a mutex is held only for a brief period, the overhead of waking up a sleeping thread can be justified by the increased system performance. On the other hand, a spinlock in the same scenario would waste CPU time and potentially decrease overall system performance.
Therefore, while spinlocks can be useful in certain contexts, such as when locks are held briefly on multi-core systems, they are generally not the best choice for single-core systems due to their counterproductive nature in such environments.
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Frequently asked questions
Sleeping mutexes are preferable when the mutex is held for a long time, as the overhead of waking a sleeping thread can be outweighed by the benefits of allowing another thread to run.
Spinlocks are preferable when the mutex is held for a very short time, as the overhead of putting threads to sleep and waking them may decrease runtime performance.
In a single-core system, spinlocks are not efficient as they waste CPU time. If a thread is put to sleep instead, another thread can run, possibly unlocking the lock and allowing the first thread to continue.
In a multi-core system, spinlocks can increase performance by allowing threads to take advantage of their full runtime quantum.
