A system, comprising: a virtual compute system comprising one or more hardware computing devices executing specific computer-executable instructions and configured to at least: maintain a plurality of virtual machine instances (152, 154, 156. 158, 159) on one or more physical computing devices, the plurality of virtual machine instances comprising a pool of one or more idle virtual machine instances; monitor incoming code execution requests to execute program code on the virtual compute system, wherein to monitor incoming code execution requests comprises determining the time at which one or more of the incoming code execution requests are received, and determining one or more execution parameters associated with the incoming code execution requests, the one or more execution parameters comprising one or more of a user account identifier with known execution settings, a program code identifier, a requested duration, an actual duration, or a memory consumption; determine, based at least in part on the time at which the one or more of the incoming code execution requests are received, whether at least some of the incoming code execution requests exhibit periodicity; in response to determining that the at least some of the incoming code execution requests exhibit periodicity, and based at least in part on the one or more execution parameters, cause a reduced amount of idle compute capacity to be maintained in the pool of the virtual compute system by causing one or more existing virtual machine instances to be spun down or removed from the pool and/or by refraining from adding one or more additional virtual machine instances to the pool until the amount of idle capacity in the pool has reached the reduced amount by allocating virtual machine instances in the pool to further incoming code allocation requests, the reduced amount determined based on the periodicity; and cause an additional virtual machine instance to be pre-initialized based on the one or more execution parameters before an additional periodic request exhibiting the same periodicity as the at least some of the incoming code execution requests is received by the virtual compute system. maintain a plurality of virtual machine instances (152, 154, 156. 158, 159) on one or more physical computing devices, the plurality of virtual machine instances comprising a pool of one or more idle virtual machine instances; monitor incoming code execution requests to execute program code on the virtual compute system, wherein to monitor incoming code execution requests comprises determining the time at which one or more of the incoming code execution requests are received, and determining one or more execution parameters associated with the incoming code execution requests, the one or more execution parameters comprising one or more of a user account identifier with known execution settings, a program code identifier, a requested duration, an actual duration, or a memory consumption; determine, based at least in part on the time at which the one or more of the incoming code execution requests are received, whether at least some of the incoming code execution requests exhibit periodicity; in response to determining that the at least some of the incoming code execution requests exhibit periodicity, and based at least in part on the one or more execution parameters, cause a reduced amount of idle compute capacity to be maintained in the pool of the virtual compute system by causing one or more existing virtual machine instances to be spun down or removed from the pool and/or by refraining from adding one or more additional virtual machine instances to the pool until the amount of idle capacity in the pool has reached the reduced amount by allocating virtual machine instances in the pool to further incoming code allocation requests, the reduced amount determined based on the periodicity; and cause an additional virtual machine instance to be pre-initialized based on the one or more execution parameters before an additional periodic request exhibiting the same periodicity as the at least some of the incoming code execution requests is received by the virtual compute system.