1. A system, comprising: a plurality of computing devices configured to implement a plurality of computing resources of a provider network, wherein each computing resource has associated performance characteristics; and one or more computing devices configured to implement a resource manager, wherein the resource manager is operable to: receive a task specification and one or more execution constraints for execution of a task, wherein the execution constraints comprise a completion time for the execution of the task; select a plurality of parameter values and one or more of the plurality of computing resources for the execution of the task based at least in part on an execution history for a plurality of prior tasks performed for a plurality of clients and based at least in part on the associated performance characteristics of the one or more of the plurality of computing resources, wherein the associated performance characteristics comprise a modeled completion time for the execution of the task, and wherein the selected parameter values and the selected computing resources optimize the one or more execution constraints for the execution of the task; configure the selected computing resources with the selected parameter values; and initiate the execution of the task using the selected computing resources configured with the selected parameter values.