Intel Threading Building Blocks

Mutex Flavors

Connoisseurs distinguish various attributes of mutexes. It helps to know some of these, because they involve trade-offs between generality and efficiency. Picking the right one often helps performance. Mutexes can be described by the following qualities, also summarized in Table 7-1.

Table 7-1. Traits and behavior of mutexes

Mutex	Scalable	Fair	Reentrant	Sleeps	Size
mutex	OS-dependent	OS-dependent	No	Yes	Three or more words
spin_mutex	No	No	No	No	One byte
queuing_mutex	Yes	Yes	No	No	One word
spin_rw_mutex	No	No	No	No	One word
queuing_rw_mutex	Yes	Yes	No	No	One word

Scalable

Some mutexes are called scalable. In a strict sense, this is not an accurate name because a mutex limits execution to one task at a time and is therefore necessarily a drag on scalability. A scalable mutex is rather one that does no worse than forcing single-threaded performance. A mutex actually can do worse than serialize execution if the waiting tasks consume excessive processor cycles and memory bandwidth, reducing the speed of tasks trying to do real work. Scalable mutexes are often slower than nonscalable mutexes under light contention, so a nonscalable mutex may be better. When in doubt, use a scalable mutex.

Fair

Mutexes can be fair or unfair. A fair mutex lets tasks through in the order they arrive. Fair mutexes avoid starving tasks. Each task gets its turn. However, unfair mutexes can be faster because they let tasks that are running go through ...