1. A computer-implemented method executed by one or more processors, the method comprising: initializing a virtual machine instance on a particular computing device, the particular computing device including a plurality of physical processors; determining a maximum number of the physical processors available to be associated with the virtual machine instance to execute instructions provided by the virtual instance; initializing a number of virtual processors for use by the virtual machine instance, wherein the number of virtual processors initialized is the same as the maximum number of the physical processors available to be associated to the virtual machine instance, and wherein each virtual processor is configured to receive instructions for execution from the virtual machine instance and execute the received instructions using the physical processors associated with the virtual instance; associating the virtual machine instance with a number of the physical processors, the number being less than the maximum number of the physical processors available to be associated to the virtual machine instance; executing the virtual machine instance on the particular computing device; during execution of the virtual machine instance: identifying a change in a demand metric associated with the virtual machine instance, the demand metric indicating a prediction of future usage of the physical processors by the virtual machine instance; and adjusting the number of the physical processors associated with the virtual machine instance based on the identified change in the demand metric while maintaining the number of virtual processors.