Real-Time for the Real World ™
Work-in-progress preview of a work-in-progress book:
Fundamentals of Timeliness and Predictability
in Dynamically Real-Time Systems [Jensen 2020]
Informally, a system is a real-time one if its core properties of timeliness and predictability of timeliness are integral to its logic, not just performance measures. In general, those properties are dynamic due to inevitable kinds and degrees of imprecisions and uncertainties in open-world system models. Such systems constitute the spectrum of dynamically real-time ones. Traditional (e.g., “hard,” “firm”) real-time computing systems are narrow special (albeit often important) cases whose system models and core properties are predominantly static—or are treated as being (e.g., worst-case execution times). They have very limited (albeit often important) applicability. Dynamically real-time systems are wide-spread, many being in application domains outside of the traditional real-time computing field, and unknown by its researchers and practitioners. However, dynamically real-time systems suffer from the insufficiency of coherent foundations for expressing and achieving application-specific timeliness and predictability. This book introduces one partial approach to providing that. Timeliness is dynamically expressive using my time/utility (née time/value) functions and utility accrual (née value-based) scheduling paradigm [Jensen 77] [Jensen+ 85]. In this approach, system model parameter imprecision is handled with fuzzy set theory. Scheduled accrued utility and predictability of its timeliness requires reasoning about them with some formal theory of uncertainty. Orthodox probability theory cannot deal with ignorance and paradoxical information, which frequently occur outside of closed-world system models. This book surveys some popular uncertainty theories which can do so, and focuses on applying belief-based theories (i.e., Dempster-Shafer theory and its subsequent elaborations). The foundation introduced here has been successfully employed in a multiplicity of different application-specific real-time contexts having wickedly dynamic system models and core properties, where traditional static real-time perspectives and traditions were inadequate and counter-productive.
N.B. There still are pages from the previous (c. 2008-2012) version of this site which I have not yet updated and integrated (or removed).