Is a bird in the hand worth more than two in the bush? Limitations of priority cognizance in conflict resolution for firm real-time database systems
After a "boom" period from the late 80s to the early 90s, there appears to have been a reduction In the amount of work published on Concurrency Control (CC) in real-time database systems (RTDBS) in general and firm RTDBS in particular. This may be because existing paradigms (e.g., Optimistic CC) have been pushed to their limits and it is difficult to extract additional meaningful performance. One of the last unresolved bastions of real-time CC is the successful incorporation of priority cognizance. Researchers have speculated that priority cognizant optimistic concurrency control (OCC) algorithms, if designed well, could outperform priority insensitive ones in real-time database systems. So far, however, there is a distinct lack of conclusive proof available on this topic and the priority cognizant OCC algorithms that have appeared so far in the literature cannot claim unilateral superiority over their priority insensitive relatives. We thus surmise that successful incorporation of priority cognizance may lead to an increase in the performance of OCC protocols in firm RTDBSs. Based on this premise, we analyze the issue of priority cognizance and Identify a critical condition that must hold for priority cognizant conflict resolution to work. The condition is that, on the average, the conflict sets of validating transactions should have a "large" number of transactions: We call this a bird in hand more than two in the bush phenomenon. Subsequently, we design a smart priority cognizant OCC variant, which we call OCC-APR, and analyze its performance, as well as that of several other concurrency control algorithms across a wide range of resource contention and system loading parameters. Surprisingly, it turns out that it is very difficult for priority cognizance to work as the above mentioned condition does not, usually, hold. We explain why this occurs and conclude that priority cognizance does not appear to be a promising technique to increase real-time CC performance. The contribution of this paper, thus, is to have laid to rest the "priority cognizance" issue with regard to real-time CCMs.
real-time database systems concurrency control performance evaluation