Parallel processing in computational linguistics

The topic to be discussed by the panel is new and at present very much under debate. Paralleli~im is developed in a large variety of approaches. The panel will make an attempt to clarify the underlying concepts, the differences <~f approach, the perspectives and general tt!ndencies, and the difficulties to be expected. Some differences of approach will be iAlustrated with examples from the work of the panelists. The ~ommon context of our approaches is the following:Standard computational linguistics tries to solve its problems by programmiltg a yon Neumann computer. The execution of the programs is inherently sequenti~l. This is implied by the fact that there is only one central processing unit (CPU) executing the program. In contrast to this, pal~allel processing defines the solution of p~oblems in terms of sets of computational units which operate concurrently and interactively, unless sequentialized for simulation ~urposes. Various approaches to parallelism differ in the computational power they assume for the concurrently active units. The differences may be outlined as follows: Massively parallel systems are usually systems whose units are, intuitively speaking, purely reactive units, i.e. mathematically defined by a specific function relating the state and output of a unit to its inputs. They could also be called connectionist systems in the wide sense; connectionist systems in the narrow sense are those whose function~ are based on weighted sums of input activities. In contrast to these systems, the units may be themselves complicated systems which compute their states and outputs depending on the messages and control signals which they receive. The units cooperate in solving the problem. In typical cases, each unit may be a central processor unit or even a complete computer. Systems with cooperative processors (computing "agents") are usually considered to be non-massively parallel. These distinctions suggest different metaphors used in informal talk about the systems: the neural net metaphor on the one hand and the society of minds (demons) meta~ phor on the other.