Empirical measurements of overheads in conservative asynchronous simulations

• Authors: Mary L. Bailey;Michael A. Pagels
• Venue: ACM Transactions on Modeling and Computer Simulation (TOMACS)
• Year: 1994

In order to make formal and analytic models more realistic, overheads which were previously ignored or vastly simplified must be included. We consider the feasibility of characterizing the overheads in conservative asynchronous simulations for such models, and we focus on a single communication structure (i.e., meshes) and use both multicomputer programs and a queueing network as example applications. We find that the two most important issues for modeling are to understand how to estimate the time spent in sending null messages and how to account for the resulting overhead due to the input waiting rule. For null messages, we estimate both the number of messages sent as well as the cost per null message. The number of messages sent can often be estimated by the application, although irregularities in communication structure and edge effects in communication can affect these estimates. A constant is valid as a first-order approximation for the time per null message, but there are secondary factors that one may wish to model, such as load balancing and communication irregularities. The overhead attributable to the input waiting rule depends on several factors: communication structure, communication frequency, and processor load balancing. Irregularity in any of these dimensions can adversely affect the performance of the conservative strategy. It appears feasible to use the factors contributing to the overheads (i.e. context switch costs; null-message costs; percentage of looping due to the conservative synchronization) in a formal model to estimate the cost of the conservative overheads.