Swift: Mutex benchmark

When designing concurrent programs, correctness is always the most important factor, but you should also consider performance factors as well. Code that executes correctly under multiple threads, but slower than the same code running on a single thread, is hardly an improvement. In this post, I benchmark the overhead of the acquisition/release time of different mutexes available on the Apple platform. The results of this benchmark may be useful when you find yourself faced with choosing the right synchronization primitive.

Intro

It is unlikely that a developer writing concurrent (multithreaded) code has not dealt with mutexes (or locks), which are the optimal tool for synchronizing multiple threads. To refresh the memory, in a multithreaded, program access to a shared resource is organized through a limited region of code, also known as a critical section and a mutex is a program object that prevents simultaneous access to that region of code, i.e., guarantees mutual exclusion.

The Little Book of Semaphores gives a mutex metaphor: it is a token that is passed from one thread to another. In an alternative metaphor there, we think of the critical section as a room, and only one thread is allowed to be in the room at a time. In this metaphor, mutexes are called locks, and a thread is said to lock the mutex before entering and unlock it while exiting. The thread that acquires (locks) the mutex is the owner of the mutex. Only after it releases (unlocks) the mutex, another thread can acquire the mutex and become the owner.

An ordinary mutex will deadlock if the thread-owner tries to lock the mutex again without first unlocking it. But a recursive mutex can be locked multiple times by a single thread.

Mutex overview

On the Apple platform, a bunch of mutexes is available:

  • os_unfair_lock: a low-level effective lock, which is the replacement for the deprecated OSSpinLock, but instead of spinning on contention, it yields (i.e. it waits in the kernel to be woken up by an unlock). Its state currently fits in a 32-bit integer, and the thread ownership information it comprises allows the system to resist priority inversions. As the name suggests it is unfair, meaning, a thread-unlocker can potentially immediately reacquire the lock before a woken up waiting thread gets an opportunity to attempt to acquire the lock. This is advantageous for performance reasons, but also makes starvation of waiting threads a possibility.
    (There also exists a recursive version, os_unfair_recursive_lock, but it’s currently in a private API).
  • pthread_mutex_t: a comprehensive POSIX Threads (Pthreads) mutex, which is provided by the lower-level C API. The size of its state structure is considerably larger than that of os_unfair_lock, 64 bytes. By default it is configured for maximum speed at the expense of correctness: not only is there no check of the owner and can any thread unlock it (as if it were a binary semaphore). No attempt of priority inversion resolution and fairness control is done either. But it can be configured to respect fairness and perform error checking and priority inheritance if you prefer correctness over speed. A Pthreads mutex can also be configured for recursion. Finally, Pthreads mutex does support condition variables, that, among other things, allows you to establish a wait timeout period.
    In the benchmark, pthread_mutex_t comes as PthreadMutex.
  • pthread_rwlock_t: a Pthreads implementation of a read-write lock. A read-write lock is an advanced mutex that allows concurrent access for read-only operations, while write operations require exclusive access, which is efficient in many scenarios. It has different operations for read locking and for write locking. The size ofpthread_rwlock_t state structure is as much as 200 bytes.
    In the benchmark, pthread_rwlock_t comes as PthreadRWLock_WriteLock and PthreadRWLock_ReadLock for read and write locking respectively.
  • NSLock: a Foundation-provided Objective-C class for the most general-purpose mutex. Along with it come the classes NSRecursiveLock, NSConditionLock, NSCondition, NSDistributedLock, each targeting its own set of problems. Essentially, they all are abstractions based on Pthreads mutex (on macOS/iOS) and therefore do nothing that cannot be done with Pthreads mutex but are easier to use.
    Foundation mutexes use the default (speed-oriented) Pthreads mutex configuration, but they are still somewhat slow. One reason they are slower than the non-Objective-C alternatives is the extra cost of having to go through Objective-C messaging system. Another reason is that they use a combination of synchronization primitives, for example, they have to internally handle a condition variable to support functionality such as wait timeouts, which results in some overhead even when wait timeouts are not used.
  • The DispatchSemaphore based binary semaphore: although it is not a full-fledged mutex (because it doesn’t allow for ownership) it still could be useful in some cases and it looks promising in terms of performance:

A dispatch semaphore is an efficient implementation of a traditional counting semaphore. Dispatch semaphores call down to the kernel only when the calling thread needs to be blocked. If the calling semaphore does not need to block, no kernel call is made. Read more

The binary semaphore usage:

let biSema = DispatchSemaphore(value: 1)
biSema.wait() // lock
// critical section
biSema.signal() // unlock

In the benchmark, it comes as BiSemaphore.

Mutex overhead

Mutexes are indispensable when it comes to multithreaded synchronization, but they also have overhead. Basically, any form of synchronization comes with a performance hit.

Major sources of mutex overhead are:

  • the range of functionality provided by mutex implementation, such as recursion, death detection, priority inheritance, wait timeouts, control of thread ownership, etc (read more)
  • need to enter the kernel (read more: 1, 2)
  • Objective-C messaging (objc_msgSend calls) on the Apple platform (read more)
  • hardware-related causes (read more)

Mutex overhead is the subject of interest of this benchmark. It can vary depending on what functionality the mutex has to use. For example, in the case of an uncontended call made from single process threads, the mutex may behave as a lightweight mutex and not to enter the kernel. Otherwise, a kernel-mode transition and a wait list enqueuing will be involved. Performance may also be affected by the need to handle fairness and prioritize threads. Being able to determine that a single thread is running allows the control flow to switch to a trivial code path.

Analyzing how mutexes behave under different circumstances and what overheads arise is valuable for concurrent software developers, as the knowledge gained will help improve software quality.

Benchmark

In this benchmark, I compare the performance of mutexes in the absence of contention with a single thread is running. To get the cleanest possible results, the benchmark was performed from a command line tool application with the Release build configuration (for maximum optimization) and with the Exclusive Access to Memory runtime check disabled.

The following chart shows the benchmark results I got on a 3.1GHz Dual-Core Intel Core i7 with 16GM of RAM running macOS Catalina v10.15.3, deployment target 10.15. Time is given in nanoseconds.

The Pthreads read-write lock (for both read and write locking) is a bit slower in the diagram. In principle, this is expected because of the complexity of its tasks.

The performance of the ordinary Foundation lock, NSLock, virtually matches that of the recursive Pthreads mutex, PthreadRecursiveMutex, but its recursive companion NSRecursiveLock is the slowest. In general recursive versions are somewhat slower than their basic versions.

I have noticed that some items, such as the top two, sometimes tend to change their relative position from time to time. I assume that since these measurements are conducted at the limit of the processor’s capability, side factors such as interrupts have a noticeable effect on the measurement results. Therefore, in order not to speculate, absolute values in the benchmark should be taken in moderation.

Summary

To summarize, I find that the top three items can be thought of as equivalent in performance and overall the performance of all mutexes in the absence of contention is quite high.

I also benchmarked pairs of contended calls to the mutexes, and the overhead turned out to be 2 orders of magnitude higher! And for a relatively small number of measurements, it even jumped up to 3 orders of magnitude! Since the arrangement of items in the benchmark varied quite chaotically from time to time, I decided not to present a diagram for them. If you are curious, you can download my benchmark project and play with it yourself. Still, some items behaved steadily as opposed to the others, like the BiSemaphore and OSUnfairLock: the measurements (the total time of the lock operation immediately followed by the unlock operation) for them mostly varied in the range of 8-10 microseconds. Such a drastic degradation in performance is caused by the need for a thread to enter the kernel, go to the sleeping state, and the subsequent awakening.

You can find the benchmark Xcode project in this repository.

Thanks for reading 🎈

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