Implications of non-constant clock drifts for the timestamps of concurrent events

To support the development of efficient parallel codes on cluster systems, event tracing is a widely used technique with a broad spectrum of applications ranging from performance analysis, performance prediction and modeling to debugging. Usually, events are recorded along with the time of their occurrence to measure the temporal distance between them and/or to establish a total event ordering. Obviously, measuring the time between concurrent events requires a global clock, which often, however, is not available on clusters. Assuming that potentially different drifts of local clocks remain constant over time, linear offset interpolation can be applied postmortem to map local onto global timestamps. In this study, we investigate the robustness of the above assumption using different timers and show that the error of timestamps derived in this way can easily lead to a misrepresentation of the logical event order imposed by the semantics of the underlying communication substrate. We conclude that linear offset interpolation alone may be insufficient for many applications of event tracing and discuss further options.