A system for providing low-latency computational capacity from a virtual compute fleet is provided. The system may be configured to maintain a plurality of virtual machine instances on one or more physical computing devices, wherein the plurality of virtual machine instances comprises a first pool comprising a first sub-pool of virtual machine instances and a second sub-pool of virtual machine instances, and a second pool comprising virtual machine instances used for executing one or more program codes thereon. The first sub-pool and/or the second sub-pool may be associated with one or more users of the system. The system may be further configured to process code execution requests and execute program codes on the virtual machine instances of the first or second sub-pool.