The controlled logical clock--a global time for trace-based software monitoring of parallel applications in workstation clusters

Event tracing and monitoring of parallel applications are difficult if each processor has its own unsynchronized clock. A survey is given on several strategies to generate a global time, and their limits are discussed. The controlled logical clock is a new method based on Lamport’s logical clock and provides a method to modify inexact timestamps of tracefiles. The new timestamps guarantee the clock condition, i.e. that the receive event of a message has a later timestamp than the send event. The corrected timestamps can also be used for performance measurements with pairs of events in different processes. The controlled logical clock is motivated and it is analyzed in detail by computer simulations. No additional protocol overhead is needed for the new method while tracing an application. It can be implemented as a filter for tracefiles or it can be integrated into monitor tools for parallel applications.