Priority Inheritance and Ceilings for Distributed Mutual Exclusion

The contributions of this paper are threefold. First, a solution to the problem of prioritized mutual exclusion in a distributed system is proved correct. This protocol is based on fewer requirements than prioritized extensions of other protocols and outperforms other protocols with an average complexity of (log n) and a worst-case complexity of O(n) messages for n nodes. Second, the concept of relative fairness is introduced, which quantifies the relation between parallel events in terms of their ordering in the absence of synchronized clocks. This concept is applied to the protocol in order to determine a requirement to guarantee a certain order between events when message delays are bounded. Third, the protocol is extended to prevent priority inversion by incorporating the priority inheritance and the priority ceiling protocols. The extensions are shown to integrate well with the original protocol. They impose the same message overhead as mentioned before for each dynamically raised priority due to resource contention while early priority boosting, such as for the priority ceiling emulation protocol, requires no additional overhead at all.