The DEDOS On-line Scheduler

Abstract

The basic mechanisms of the on-line scheduler for the Dependable Distributed Operating System (DEDOS) are presented. DEDOS supports the reliable execution of time-critical applications such as the control of a production system. An application consists of a time-critical hard real-time (HRT) part and a less time critical soft real-time (SRT) part. The HRT part is the backbone of the application and consists mainly of periodic tasks. This part of the application is scheduled off-line to guarantee that all deadlines are met. The off-line scheduler takes all resources into account including the network. On each processor in the distributed system, an on-line scheduler (OLS) enforces this schedule. The off-line scheduler divides the HRT tasks into non-preemptable scheduling blocks and calculates a start time and a worst-case execution time for each block. The OLS starts the execution of these blocks at the prescribed time. Programs can contain alternative statements. Therefore, a scheduling block can have several alternative successors (data-dependent block). Only one of the alternative successor blocks is executed at run time. Therefore, the OLS determines at run time which alternative was taken and the corresponding part of the schedule is enforced. The OLS computes the next block to execute in a time proportional to N+l. where N is the number of data-dependent tasks on a processor. The OLS supports exception handling. Tasks are removed from the schedule after an exception and reinserted into the schedule after a reconfiguration. It is also possible to execute exceptional schedules or to change to a new schedule. The OLS delects violations of user-defined deadlines. Timing violations of network accesses arc also detected because the network and receiver schedules may not be compromised.