Longitudinal user and usage patterns in the XSEDE user community

The XSEDE user community is often assumed to be dominated by a (mostly) fixed set of users in a largely static pecking order. This assumption is based primarily on anecdotal experience but often the only quantitative data are the consistent patterns of overall resource consumption observed at many time scales. The XSEDE accounting system offers a unique opportunity to study consumption patterns over time by project teams and individuals. This analysis shows some tendency for larger-scale consumers to remain among the larger-scale consumers; however, the XSEDE user community is much more dynamic than often assumed. In addition, small-scale user behavior over time differs distinctly from large-scale user behavior, with the "long tail" more often comprised of short-lived projects. We define a number of metrics for describing these patterns and consider their implications for the outreach activities and user support within XSEDE and other HPC environments.