Tracing with a minimal number of probes

When a program execution must be observed, probes are injected to trace its execution. Probes are intrusive, causing execution overhead and modifying a program's real time properties. It is therefore desirable to place them efficiently, so that their negative effects are mitigated. This paper presents a new algorithm for efficiently allocating probes at vertices in the control flow graph of a program in a way that makes it possible to reconstruct program traces. Despite of the intractability of finding minimal vertex probe placement, a fact proven in this paper, the presented algorithm is capable of identifying a locally minimal set of vertices, which reduces the set of needed probes beyond the current state-of-the-art. First, the paper shows the advantages of placing probes at vertices, rather than edges. Next, the known methods for reducing the set of probes needed to profile execution are proven to be applicable to reduce the set needed to trace execution. Further, the new algorithm is described along with the algorithm for reconstructing the executed path from a trace file. In addition, the new algorithm is combined with methods for placing profiling probes to reduce the execution overhead even further. Finally, the new probe placement algorithm is compared with known algorithms to elaborate on its benefits.