Differences Between Mutex and Spinlock

1. Introduction

In this tutorial, we’ll examine the differences between mutexes and spinlocks.

Both are mechanisms used in parallel and distributed computing systems to organize and synchronize multiple cores and threads when running in parallel.

2. Why Do We Need Synchronization in Parallel Computing?

Parallel computing involves the simultaneous execution of multiple threads or processes to improve performance and efficiency. This approach leverages multiple processing units to perform tasks concurrently, significantly speeding up computations and optimizing resource utilization when applicable.

When these threads need to access shared resources or data, synchronization is crucial to avoid race conditions, data corruption, and inconsistent states. Without proper synchronization, two or more threads could simultaneously modify shared data, leading to unpredictable results and difficult-to-debug issues.

Synchronization primitives like mutexes and spinlocks provide the control to coordinate access to shared resources. Ensuring that only one thread can access a critical section of code at a time, these tools help maintain data integrity and program stability.

3. Mutex

3.1. Description

A mutex is a mechanism that locks critical sections of code and only allows one thread to acquire the lock and use the resources at a time.

The word “mutex” comes from “mutual exclusion.” Essentially, the programmer marks a code section as critical, meaning only a single thread can run it at a time and place a mutex around it. The mutex locks the critical section and only allows a single thread to enter it. If another thread tries to run the critical section while it’s locked, the OS puts this thread to sleep:

The above diagram shows a mutex over a critical code section (the green square). Threads 1 and 2 are running in parallel, and both need to run this critical section. Thread 2 reaches the section first and asks for the lock (top left subfigure) and then acquires the lock (bottom left). After some time, Thread 1 also reaches the critical section, but Thread 2 is still using it; therefore, the mutex remains locked (top right subfigure). Thread 1 then attempts to run the lock, but the OS puts Thread 1 to sleep since the lock it’s trying to access is unavailable (bottom right).

3.2. Characteristics

Let’s take a look at some characteristics of mutexes:

• A mutex is a blocking mechanism. If a thread attempts to acquire a mutex but a different thread currently holds it, the operating system puts it to sleep, freeing up CPU resources for other tasks
• The blocking mechanism involves context switches, which can be costly in terms of performance but prevent CPU wastage
• Many mutex implementations include features like priority inheritance to avoid priority inversion and ensure fair access
• However, mutexes incur overhead due to context switching and ensuring fairness. This is the price to pay for having consistent and reliable synchronization mechanisms

4. Spinlocks

4.1. Descriptions

A spinlock is a synchronization primitive that uses busy-waiting. When a thread attempts to acquire a spinlock that another thread already holds, it continuously checks the lock in a loop (spins) until the lock becomes available. Unlike mutexes, spinlocks don’t put the thread to sleep, avoiding context switches.

Contrary to a mutex, a spinlock is a non-blocking mechanism. Let’s see it with an example:

In this example, Thread 1, which tries to acquire a held lock, will continue trying to get it until it is freed from the thread holding it. This is why this state is called “busy waiting.” This behavior differs from what a mutex would do (putting the thread to sleep).

4.2. Characteristics

Let’s look through the characteristic features of spinlocks:

• Threads continuously check (spin) for the lock, avoiding the overhead of context switches, making spinlocks a non-blocking mechanism
• However, spinning consumes CPU cycles, which can be wasteful if the task holds the lock for a long time
• Spinlocks can be more efficient than mutexes for very short critical sections where the overhead of blocking and waking up threads outweighs the cost of busy waiting
• Spinlocks typically lack mechanisms for ensuring fairness, which can lead to starvation and priority inversion issues

5. Comparison of Mutex and Spinlocks

5.1. Performance

When it comes to performance, mutexes involve context switches, which can introduce significant overhead, especially if the critical section is short. This overhead arises because the operating system needs to put the thread to sleep and later wake it up.

In contrast, spinlocks don’t use context switches. Therefore, they are faster for short critical sections, but this speed comes at the cost of potentially wasting CPU cycles if the wait time for the lock to become available is longer.

5.2. CPU Usage

Mutexes are efficient regarding CPU usage because blocked threads don’t consume CPU resources. Once a thread sleeps while waiting for a mutex, the CPU can execute other tasks.

Spinlocks, on the other hand, can lead to high CPU usage due to their busy-waiting nature. While spinning, the thread continuously checks the lock in a loop, which is inefficient, primarily when the lock is held for long durations. But even with shorter durations, the busy waiting loop is unavailable to perform other tasks for the system.

5.3. Fairness

In terms of fairness and priority handling, mutexes offer mechanisms that can prevent lower-priority threads from indefinitely blocking higher-priority ones.

In contrast, spinlocks generally lack built-in fairness mechanisms, which can lead to starvation. Some threads might never acquire the lock, or lower-priority threads may hold the lock while higher-priority threads wait.

5.4. Complexity & Overhead

Finally, considering complexity and overhead, mutexes are more complex due to the need for operating system support for blocking and waking up threads, ensuring fairness, and handling priority inversion.

Spinlocks are simpler in implementation because they don’t require the operating system to manage thread states (although this lower complexity results in poorer performance and resource utilization).

6. Which One for Which Scenario?

Both primitives shine in different applications:

We should remember that performance and CPU usage depend on each application and hardware setting. Fast or slow performance also relies heavily on the critical section size, the CPU’s frequency, the number and type of cores available to the system, etc.

Mutexes are ideal when the critical section may be held longer. They are handy when threads need to wait for I/O operations or other long-running tasks. The blocking mechanism prevents the waste of CPU cycles, making mutexes suitable for applications where efficiency and fairness are crucial.

Spinlocks are best suited for scenarios where the critical section is very short and the expected wait time to acquire the lock is minimal. They are commonly used in low-level system programming and when the overhead of context switching is too high. Spinlocks are also useful in RTOSes (real-time operating systems), where predictable behavior is part of the system’s design.

7. Conclusion

In this article, we discussed spinlocks and mutexes, their differences, and which scenarios best fit each synchronization primitive.

Mutexes are blocking and work better with long-running processes, whereas spinlocks are non-blocking and more efficient when expected wait times are very short.