**6.2 Preprocessing task: Selecting the most appropriate CEs**

A production infrastructure such as EGI involves tens of thousands of resources from hundreds of sites, involving tens of countries and a large human team. Since it is a general-purpose platform, and although there is a common middleware and a recommended operating system, the heterogeneity in the configuration and operation of the resources is inevitable. This heterogeneity, along with other social and human factors such as the large geographical coverage and the different skills of operators introduces a significant degree of uncertainty in the infrastructure. Even considering that the service level required is around 95%, it is statistically likely to find in each large execution sites that are not working properly. Thus, prior to beginning the experiments, it is necessary to do empirical tests to define a group of valid computing resources (CEs) and this way facing resource setup problems. These tests can give some real information like computational speed, primary and secondary memory sizes and I/O transfer speed. These data, in case there are huge quantities of resources, will be helpful to establish quality criteria choosing resources.
