Арбитр: Устройство управления доступом к общим ресурсам

Electronic device that allocates access to shared resources
Arbiters are electronic devices that allocate access to shared resources.

An important form of arbiter is used in asynchronous circuits to select the order of access to a shared resource among asynchronous requests. Its function is to prevent two operations from occurring at once when they should not. For example, in a computer that has multiple CPUs or other devices accessing computer memory, and has more than one clock, the possibility exists that requests from two unsynchronized sources could come in at nearly the same time. "Nearly" can be very close in time, in the sub femtosecond range. The memory arbiter must then decide which request to service first. Unfortunately, it is not possible to do this in a fixed time [Anderson 1991].

Arbiters break ties. Like a flip flop circuit, an arbiter has two stable states corresponding to the two choices. If two requests arrive at an arbiter within a few picoseconds (today, femtoseconds) of each other, the circuit may become meta stable before reaching one of its stable states to break the tie. Classical arbiters are specially designed not to oscillate wildly when meta stable and to decay from a meta stability as rapidly as possible, typically by using extra power. The probability of not having reached a stable state decreases exponentially with time after inputs have been provided. A reliable solution to this problem was found in the mid 1970s. Although an arbiter that makes a decision in a fixed time is not possible, one that sometimes takes a little longer in the hard case (close calls) can be made to work. It is necessary to use a multistage synchronization circuit that detects that the arbiter has not yet settled into a stable state. The arbiter then delays processing until a stable state has been achieved. In theory, the arbiter can take an arbitrarily long time to settle (see Buridan's principle), but in practice, it seldom takes more than a few gate delay times. The classic paper is [Kinniment and Woods 1976], which describes how to build a "3 state flip flop" to solve this problem, and [Ginosar 2003], a caution to engineers on common mistakes in arbiter design. This result is of considerable practical importance, as multiprocessor computers would not work reliably without it. The first multiprocessor computers date from the late 1960s, predating the development of reliable arbiters. Some early multiprocessors with independent clocks for each processor suffered from arbiter race conditions, and thus unreliability. Today, this is no longer a problem.

Arbiters are used in synchronous contexts as well in order to allocate access to a shared resource. A wavefront arbiter is an example of a synchronous arbiter that is present in one type of large network switch.