Reducing Monitoring Overhead by Integrating Event- and Time-Triggered Techniques
Runtime verification is a formal technique used to check whether a program under inspection satisfies its specification by using a runtime monitor. Existing monitoring approaches use one of two ways for evaluating a set of logical properties: (1) event-triggered, where the program invokes the monitor when the state of the program changes, and (2) time-triggered, where the monitor periodically preempts the program and reads its state. Realizing the former is straightforward, but the runtime behaviour of event-triggered monitors are difficult to predict. Time-triggered monitoring (designed for real-time embedded systems), on the other hand, provides predictable monitoring behavior and overhead bounds at run time. Our previous work shows that time-triggered monitoring can potentially reduce the runtime overhead provided that the monitor samples the program state at a low frequency. In this paper, we propose a hybrid method that leverages the benefits of both event-and time-triggered methods to reduce the overall monitoring overhead. We formulate an optimization problem, whose solution is a set of instrumentation instructions that switches between event-triggered and time-triggered modes of monitoring at run time; the solution may indicate the use of exactly one mode or a combination of the two modes. We fully implemented this method to produce instrumentation schemes for C programs that run on an ARM Cortex-M3 processor, and experimental results validate the effectiveness of this approach.