Requirements-based execution time prediction of a partitioned real-time system using I/O and SLOC estimates

Abstract

Predicting computer processing requirements of a completed system early in the design and development lifecycle of that system is challenging. Software requirements and avionic or hardware systems often mature in parallel, and, in early stages of design, uncertainty over processing requirements makes determination of processing architecture difficult. Later in the design process, as details become finalized and prototypes are developed, estimations become increasingly more accurate. However, waiting until later in the lifecycle to make architectural changes causes those changes to be more costly and introduces schedule and technical risks. The sooner processing needs are determined and the corresponding system architecture is established, the more easily an appropriate processing platform can be incorporated into the design. This paper describes a novel approach for estimating processor utilization early in the lifecycle of a large, real-time software development project: NASA's Orion Crew Exploration Vehicle flight software. The approach uses available requirements documentation as a basis of the estimate and decouples input/output (I/O)- and computation-based processing by estimating each separately then combining the results. This approach is unique because it can be used to estimate the execution time for unwritten or partially specified software, in addition to giving a specific contribution for I/O. The methodology for estimating I/O processing is based on quantifying data, while the methodology for estimating algorithmic processing is based on approximated code size. Results can be used as an aid to predict target processor types and quantities, allocate software to processors, predict communication bandwidth utilization, and manage processor margins.