1. A method, comprising: obtaining allocation data for a set of advertisements to which slot inventory are allocated, the allocation data specifying: allocation constraints for the set of advertisements; current allocations of the slot inventory to the set of advertisements; determining, based on the allocation constraints and by one or more computers, that the current allocations include allocation, to the set of advertisements, of additional slot inventory beyond a minimum portion of the slot inventory that is required to be allocated to satisfy the allocation constraints; for each of one or more current allocations of the additional slot inventory: determining, based on current revenue generated by allocating the additional slot inventory and by one or more computers, a minimum price at which the additional slot inventory will be allocated through an auction; and re-allocating, by one or more computers at least one of the current allocations of the additional slot inventory through an auction, the re-allocation being based on the minimum price.