User level scheduling of communicating real-time tasks

Abstract

Unique challenges are present when one tries to build distributed real-time applications using standard off-the-shelf systems which are in common use but are not necessarily designed specifically for real-time systems. In particular, to realize end-to-end predictability when, say, a process on one node sends data to a process on another node, several issues must be addressed: (1) mapping application real-time requirements into requirements imposed on the system schedulable entities (tasks), (2) ensuring predictable execution of the tasks in the face of possible priority inversions, limited OS level real-time scheduling support, and limited number of priorities, and (3) integrating real-time and non-real-time tasks in the same platform. In this paper, we propose solutions to these challenges. In particular, we present user-level scheduling schemes for communicating tasks. These solutions are practical and are based on simple primitives that can be found in most of today´s commonly used operating systems. To validate our design and to examine the feasibility of user-level scheduling in actual systems, we have implemented our solutions in MidART running on PCs with Windows NT operating system over UDP/IP and Fast Ethernet LANs. This paper contributes to further our understanding of how to build real-time systems using commercially available off-the-shelf components.