Fault-Tolerant Scheduling of Primary-Alternate Version Based on Variable Workload

Task workload analysis for primary-alternate fault-tolerance is usually performed based on WCET (Worst-Case Execution Time), which causes excessive redundancy of processor resource reservation in conventional real-time systems. To address this issue, a novel primary-alternate fault-tolerant model based on a variable workload is built, theorems for both the alternate schedulability and the primary execution success ratio is proposed, and their correctness are proved. Motivated by these theorems, a new algorithm called BCEVW (BCE with Variable Workload) is proposed through improving the algorithm BCE. Further, a new error probability model for the primary version is proposed, which is closer to the practice than does the conventional model. Using two metrics including schedulability of a task set and execution success ratio of the primary version, simulation results show that (1) for scheduling task sets in the alternate version, backwards-EDF algorithm performs better than backwards-RM algorithm, and the necessary and sufficient conditions for its schedulability is that the processor utilization is no more than 1; (2) in the case of variable workload, BCEVW algorithm can significantly improve the schedulability of task sets in the primary version; (3) for the new error probability model, the results of the proposed scheduling algorithm show that there is an obvious linear relationship between the error probability and execution success ratio of the primary version, and further the greater processor utilization of the primary version is, the more significant linear relationship is.