Enhancement of Generalized Earliest Deadline First Policy

摘要

Scheduling transactions in real-time database systems (RTDBSs) is more complex than scheduling tasks in real-time systems. In fact, the RTDBS must guarantee the database logical consistency, on one hand, and it must schedule the transactions in order to meet their deadlines, on the other hand. The main policy used to schedule transactions in RTDBSs is Earliest Deadline First (EDF). However, it is well-known that EDF is not efficient for scheduling transactions in overload conditions. Consequently, different scheduling protocols: AED, AEVD, APP, AEDF-Co, GEDF... were proposed to improve the system performances in firm RTDBSs, i.e. late transactions are considered useless. Generalized Earliest Deadline First (GEDF) is a new scheduling protocol in which transaction priority is assigned according to both deadlines and a parameter, called SPriority, which expresses the importance of transactions. In this paper, an RTDBSs analysis is presented. The accuracy of GEDF scheduling policy and the influence of database workload on the system performances is investigated. This study enabled us to describe the complete behavior of the transaction success ratio. Moreover, based on intensive simulations, we have derived the optimal values of the system parameters which improve the success ratio without modifying GEDF protocol.