Scheduling Coprocessor for Enhanced Least-Laxity-First Scheduling in Hard Real- Time Systems

Scheduling time impact on system performance increases especially when using dynamic priority algorithms, because of the enlarged computational effort at runtime. This overhead can be reduced by using dedicated hardware that does the time consuming computations necessary for scheduling. This can be a coprocessor capable of implementing dynamic scheduling algorithms which are, until now, rarely used because of their complex computations at schedule time. One of these algorithms is Least-Laxity-First (LLF). This is an optimal scheduling methodology that allows detection of time constraint violations ahead of reaching a tasks deadline, but has the disadvantage of showing poor runtime behavior in some special situations (“thrashing”). In this paper, we present a universal deterministic scheduling coprocessor that implements the newly developed Enhanced Least-Laxity-First-algorithm (ELLF) which eliminates this disadvantage of LLF. Computation time of this device is rather a matter of time resolution than of the number of tasks. 1