Reducing the Implementation Overheads of IPCP and DFP (RTSS)

Most resource control protocols such as IPCP (Immediate Priority Ceiling Protocol) require a kernel system call to implement the necessary control over any shared data. This call can be expensive, involving a potentially slow switch from CPU user-mode to kernel-mode (and back). In this paper we look at two anticipatory schemes (IPCP and DFP - Deadline Floor Protocol) and show how they can be implemented with the minimum number of calls on the kernel. Specifically, no kernel calls are needed when there is no contention, and only one when there is. A standard implementation would need two such calls. The protocols developed are verified by the use of model checking. A prototype implementation is described for POSIX pThreads (thus opening up improvements to a range of programming approaches). Experimental results demonstrate the effectiveness of the scheme, showing average case savings of 86%.