Task Management for Soft Real-Time Applications Based on General Purpose Operating Systems

Abstract

Many embedded systems, such as those found in planes, trains, cars, robots and machine tools, exhibit specific timeliness, predictability and precedence constraints that must be respected. In many cases they are built on top of COTS microprocessor boards, possibly PCcompatibles, e.g., PC104, SBCs and Mini-ITX, and using multi-tasking operating systems or kernels, both real-time (RTOS) and general purpose (GPOS) despite the profound architectural and functional differences exhibited by these two classes of software infrastructures In fact, GPOS are typically time-shared multi-processing systems, optimized to manage heterogeneous classes of resources such as CPU, memory, disk, network interface, etc. The performance criteria for these systems are mostly associated with average throughput and fairness, the typical applications are not strictly time-constrained and may exhibit unpredictable blocking and execution times and activation latencies. Conversely, RTOS favor the timely execution of the activities they support. However the predictability delivered by this class of OSs comes at a price, which usually takes the form of additional information and constraints on the application tasks, e.g., bounded worst-case execution time, minimum inter-arrival time or activation period, relative phases and precedence constraints, and on operating system primitives, e.g., bounded blocking times, predictable synchronization primitives, suitable schedulers and admission control. The architectural dichotomy presented by RTOS and GPOS leads to significant differences at the application implementation level, and thus the choice of using an RTOS or a GPOS may not be completely dictated by the timeliness and predictability aspects, only. For example, in soft real-time applications, which tolerate occasional failures in the time domain, GPOS may be preferred since they deliver sufficient real-time performance and generally outperform RTOS in practical aspects like hardware support, price, availability and diversity and quality of development tools. Nevertheless, GPOS lack some features that are commonly required by embedded applications, e.g., support for automatic activation of recurrent tasks with enough precision, phase control and precedence constraints. These difficulties have been perceived in the scope of the CAMBADA middle-size robotic soccer team (Cambada), developed at the