1. A computer-implemented method comprising: receiving, by one or more computing systems of a program execution service, one or more requests from a client over a public network, the received one or more requests indicating an operating system to execute within a virtual machine; selecting, by the one or more computing systems, one or more computing nodes of the program execution service to use for execution of the operating system from computing nodes located in a first geographical location, wherein the program execution service includes computing nodes located in a plurality of geographical locations; and causing, by the one or more computer systems, the one or more computing nodes to execute one or more instances of the operating system within one or more virtual machines.