Synchronous Signals

Abstract

In Ada 2012, the language expanded its support for concurrency with the addition of the Synchronous Barriers library package to the Real-Time Systems annex[1]. This package provides a mechanism to synchronize a group of tasks after the number of blocked tasks reaches a specified count value. One use for this feature is to interleave sequential processing with concurrent or parallel processing. For this usage, two syncrhonous barrier objects can be utilized where one barrier manages the transition from parallel to sequential code, and the other barrier manages the transition back from sequential to parallel code. In general, performance can be improved by minimizing the amount of synchronization in an application. The more that threads of execution can proceed independentally without interference with other threads, the more likely that the available CPUs can focus on completing the independent tasks rather than spending time synchronizing with the other threads. A Synchronous Signal is a synchronization primitive that provides a similar abstraction as a Synchronous Barrier, except it can reduce the amount of synchronization needed by a factor of two. In addition, only one object is needed, to manage both transitions instead of two. In this paper, the abstraction is explored and an interface to use the abstraction is presented. Two forms of the abstraction are considered; a blocking form and a non-blocking form, and the performance measurements are reported and compared against Synchronous Barriers usage. Finally, these examples are also compared and considered for use in a Ravenscar environment.