A hierarchical coordination language for interacting real-time tasks

Abstract

We designed and implemented a new programming language called Hierarchical Timing Language (HTL) for hard realtime systems. Critical timing constraints are specified within the language, and ensured by the compiler. Programs in HTL are extensible in two dimensions without changing their timing behavior: new program modules can be added, and individual program tasks can be refined. The mechanism supporting time invariance under parallel composition is that different program modules communicate at specified instances of time. Time invariance under refinement is achieved by conservative scheduling of the top level. HTL is a coordination language, in that individual tasks can be implemented in "foreign" languages. As a case study, we present a distributed HTL implementation of an automotive steer-by-wire controller.

The Evolution of Real-Time Programming

“…So-called compositional scheduling aims at reducing the need to reconsider already scheduled tasks. Compositional scheduling techniques have only recently received more attention [32][20] [11]. The ZET model also benefits from the results in scheduling hybrid sets of real-time and non-real-time tasks [6], which have also made it into real-time operating systems.…”

“…Accurate and precise timing of task I/O requires mechanisms for event handling similar to the mechanisms needed for ZET programming. There are four LET programming languages, called Giotto [17], xGiotto [12], HTL [11], and TDL [36]. The notion of LET programming was introduced with Giotto, which supports the possibly distributed implementation of periodic tasks whose logical execution times must be equal to their periods.…”

“…Thus LET programming can be seen as a generalization of the early PET programming style to handle concurrent tasks while maintaining temporal accuracy and precision of the individual tasks' I/O. Note that LET programs may also implement conditional timing behavior while still being input-determined such as, e.g., changing the frequencies of tasks or even replacing tasks by others at runtime [11]. An important advantage of Giotto is that time safety of Giotto programs can be checked in polynomial time with respect to the size of the programs although there might be exponentially many conditional timing behaviors [18].…”

“…An important advantage of Giotto is that time safety of Giotto programs can be checked in polynomial time with respect to the size of the programs although there might be exponentially many conditional timing behaviors [18]. Checking time safety of programs written in more general LET languages such as HTL may only be approximated with less than an exponential effort [11]. The LET semantics is abstract in the sense that platform details such as actual execution times and scheduling strategies are hidden from the programmer.…”

“…LET programming therefore also enables model-based design in the sense that models may directly be translated into semantically equivalent executable programs [21]. Finally, LET programs written in Giotto [20], HTL [11], and TDL [9] have been shown to run on distributed systems with support for clock synchronization.…”