Simulation of Feedback Scheduling Dan Henriksson, Anton Cervin and Karl-Erik Årzén Department of Automatic Control

Outline TrueTime: A simulator for distributed real-time control systems Feedback scheduling Simulation example

TrueTime Matlab/Simulink based tool Computer and network blocks written in C-MEX Co-simulation of process dynamics, task execution and network transmissions

Features Event-based simulation Computer block with a flexible real-time kernel –arbitrary scheduling policy: RM, EDF, static scheduling... –periodic or aperiodic (externally triggered) tasks –tasks modeled as a series of code segments –constant, random or data-dependent execution times –interrupt handling, timers, monitors, context switches… Network block –arbitrary scheduling policy –preemptive or non-preemptive transmissions –constant, random or data-dependent transmission times

Motivation Study the impact of timing non-determinism on control performance –varying execution and transmission times –scheduling-induced delays (CPU and network) –COTS hardware and software Simulate event-based control systems Develop flexible compensation schemes –feedback scheduling: dynamic scheduling based on measurements of actual execution times or delays –on-line adjustment of controller parameters

Feedback Scheduling View the scheduler as a controller Controlled variables –CPU or network utilization –round-trip delay Control signals –sampling periods –execution time of controllers (anytime algorithms) Objectives –higher utilization, avoid overload, even distribution of resources, better control performance –relaxes assumptions on hard deadlines, fixed sampling periods and known WCETs

Simulation example Distributed control system –three I/O nodes connected to three open-loop unstable first-order processes –one central controller node executing three P-controllers –one disturbance node generating high-priority network traffic CAN-type network –priority-based scheduling of messages –rate-monotonic priority assignment

Feedback scheduler Controls round-trip delay by adjusting the sampling periods of the controllers Implemented in the controller node –the I/O nodes measure the delay from sampling to actuation and sends this information to the scheduler –if the worst-case delay is larger than the setpoint the sampling periods are decreased –the new sampling periods are sent back to the I/O nodes with the control signals