Carbon Compromises: Minimising the Carbon-Cost and Power-Use of Grid Sites

This paper presents an overview of ways to reduce the carbonimpact of the Worldwide LHC Computing Grid (WLCG) along with specific results demonstrating the potential benefits of CPUs based on the ARM architecture. The choice of CPUs during procurement depends on balancing costs, performance, and the carbon-impact (both the embodied and operational) and decisions made within the context of site, fundingagency, and national preferences or policies. To aid this, we have developed a web-based utility that allows comparison of various options, once actual costs have been established. Operating CPUs at frequencies lower than their maximum, can improve their carbon-efficiency, doing the same work for less power/carbon but more slowly. In this paper we show that the effect is quite significant for CPUs based on the ARM architecture and discuss the compromises involved.