A virtual machine management service obtains a request to instantiate a virtual machine image (VMI) to implement a virtual network function (VNF). The request specifies a set of processor requirements corresponding to instantiation of the VMI. In response to the request, the service identifies, from a server comprising a set of processor cores, available processor capacity. The service determines, based on the available processor capacity and the set of processor requirements, whether to instantiate the VMI on to a subset of processor cores of the server. Based on this determination, the service instantiates the VMI on to the subset of processor cores to implement the VNF.