Mechanisms for User-Invoked Retraction of Events in Time Warp

We examine a primitive that allows application programs to explicitly retract previously scheduled events in Time Warp. Specifically, a simple mechanism for retracting messages is defined, and a correctness proof is presented. Various optimizations to the proposed mechanism are also described. The proposed mechanism is intended to be implemented within the kernel of the Time Warp runtime executive. It is also possible to implement event retraction within the application program itself, without kernel support. Empirical data is presented to compare the performance observed in these two approaches. Based on these studies, we argue that support for a retraction primitive within the Time Warp kernel can significantly improve performance for programs that use event retraction to a moderate or heavy degree.