A computing system (102; 200) comprising: one or more computers having memory and at least one data processing apparatus configured to implement: a computing resource that is shared among a plurality of online entities for executing activities of the plurality of online entities; an online entity manager that determines relative priorities of the plurality of online entities based on prioritization criteria; an optimization engine that: determines a set of reservation zones for the plurality of online entities, each reservation zone designating a portion of unused capacity of the computing resource that online entities having relative priorities at or below a corresponding threshold priority for the reservation zone are permitted to reserve; determines zone limits that define boundaries between reservation zones in the set of reservation zones based on: (i) forecasted levels of use of the computing resource by online entities in the plurality of online entities; and (ii) the relative priorities of the online entities; and updates the zone limits based on changes in at least one of: (i) the forecasted levels; or (ii) the relative priorities of the online entities; and a resource manager that manages allocation of a capacity of the computing resource to the plurality of online entities based on whether any requested amount of the capacity of the computing resource from a given online entity is less than an amount of the portion of unused capacity of the computing resource that is designated by the corresponding reservation zone for the given online entity. a computing resource that is shared among a plurality of online entities for executing activities of the plurality of online entities; an online entity manager that determines relative priorities of the plurality of online entities based on prioritization criteria; an optimization engine that: determines a set of reservation zones for the plurality of online entities, each reservation zone designating a portion of unused capacity of the computing resource that online entities having relative priorities at or below a corresponding threshold priority for the reservation zone are permitted to reserve; determines zone limits that define boundaries between reservation zones in the set of reservation zones based on: (i) forecasted levels of use of the computing resource by online entities in the plurality of online entities; and (ii) the relative priorities of the online entities; and updates the zone limits based on changes in at least one of: (i) the forecasted levels; or (ii) the relative priorities of the online entities; and determines a set of reservation zones for the plurality of online entities, each reservation zone designating a portion of unused capacity of the computing resource that online entities having relative priorities at or below a corresponding threshold priority for the reservation zone are permitted to reserve; determines zone limits that define boundaries between reservation zones in the set of reservation zones based on: (i) forecasted levels of use of the computing resource by online entities in the plurality of online entities; and (ii) the relative priorities of the online entities; and (i) forecasted levels of use of the computing resource by online entities in the plurality of online entities; and (ii) the relative priorities of the online entities; and updates the zone limits based on changes in at least one of: (i) the forecasted levels; or (ii) the relative priorities of the online entities; and (i) the forecasted levels; or (ii) the relative priorities of the online entities; and a resource manager that manages allocation of a capacity of the computing resource to the plurality of online entities based on whether any requested amount of the capacity of the computing resource from a given online entity is less than an amount of the portion of unused capacity of the computing resource that is designated by the corresponding reservation zone for the given online entity.