Integrating real-time and partial-order information in event-data displays

The events occurring in the execution of a distributed or parallel application are related by a partial, rather than a total, order. We have developed prototype software that collects such events during program execution and produces a graphical display consistent with the partial order. Such a display can be very helpful in understanding and debugging distributed and parallel applications. However, using only partial-order information does not allow the performance characteristics of an application to be understood. Integrating real-time information with the partial order can provide a display that is useful for understanding both functional and performance aspects of the application. An algorithm is required to adjust the collected real-time information, to ensure that real times are consistent with the partial order. Lamport's clock algorithm provides such an adjustment, but can significantly distort the real-time values. It was necessary to develop a more complex algorithm, using the same basic principles, that minimises such distortions. We have extended existing prototype software for displaying event data, so that either a purely partial-order display or a real-time display may be obtained. The real-time facilities can be used in multiple target environments, such as OSF/DCE, Hermes, and SR.