International Encyclopedia of Systems and Cybernetics

In a parallel processing net, numerous independent programs need to communicate in a coordinated way. However they need also to obtain access to those parts which are of common use for the whole system and thus start to compete for general memory facilities or peripherics time.

J.P. SANSONNET writes: "(As) these interactions are asynchronmous, this turns un predictable their global behavior… It may happen, for example, that different tasks compete for common resources in a disordered manner and in this way very quickly critical situations appear, such as those called "Famine", or "Fatal embrace".

"Famine: Some tasks never obtain the resource they need – for ex. never accede to the printer to produce their output.

" Fatal embrace: Two tasks wait indefinitely for each other" (1988, p.1304).

Organizing the net and making compatible its interconnections is thus a fundamental requirement for correct and efficient operation. SANSONNET writes that, to avoid access problems "each processor may be endowed with some local memory. Each pair processor/ memory thus becomes a small and relatively independent computer" (p.1303).

It is noteworthy to observe, when simultaneity replaces sequentiality, how the need for general organization rules pops up immediately.

→ Contention; Parallelism; Processing: simultaneous or sequential; transputer