Timing constraint remapping to avoid time discontinuities in distributed real-time systems

We propose a dynamic constraint transformation technique for ensuring timing requirements in a distributed real time system, possessing periodically synchronized distributed local clocks. Traditional discrete clock synchronization algorithms that adjust local clocks instantaneously yield time discontinuities. Such time discontinuities lead to the loss or the gain of critical time points such as task release times and deadlines, thus raising run-time faults. While continuous clock synchronization is generally suggested to avoid the time discontinuity problem, it incurs too much run-time overhead to be implemented in software. The proposed constraint transformation for equi-continuity (CTEC) technique can solve this problem without modifying discrete clock synchronization algorithms. The CTEC working as an added component of discrete clock synchronization moves timing constraints out of correction intervals. In doing so, it makes use of a mapping derived from continuous clock synchronization in order to exploit the continuity property of continuous clock synchronization. We formally prove the correctness of CTEC by showing that the CTEC with discrete clock synchronization generates the same task schedule as continuous clock synchronization. In order to show the effectiveness of CTEC, we have implemented it on a distributed platform based on the CAN bus, and performed extensive experiments. The experimental results indicate that time discontinuities present a consistency problem to real world systems. They also show that CTEC is an effective solution to the problem, while incurring little run-time overhead