A Priority Ceiling Protocol with Dynamic Adjustment of Serialization Order

The difficulties of providing a guarantee of meeting transaction deadlines in hard real-time database systems lie in the problems of priority inversion and of deadlocks. Priority inversion and deadlock problems ensue when concurrency control protocols are adapted in priority-driven scheduling. The blocking delay due to priority inversion can be unbounded, which is unacceptable in the mission-critical real-time applications. Some priority ceiling protocols have been proposed to tackle these two problems. However, they are too conservative in scheduling transactions for the single-blocking and deadlock-free properties, leading to many unnecessary transaction blockings. In this paper, we analyze the unneccessary transaction blocking problem inherent in these priority ceiling protocols and investigate the conditions for allowing a higher priority transaction to preempt a lower priority transaction using the notion of dynamic adjustment of serialization order. A new priority ceiling protocol is proposed to solve the unnecessary blocking problem, thus enhancing schedulability. We also devise the worst-case schedulability analysis for the new protocol which provides a better schedulability condition than other protocols.