11. A system for generating ranked search results, comprising: one or more processors configured to: receive a plurality of matching information items that match a search request; rank at least some of the plurality of matching information items using a linear ranking model that linearly combines a first plurality of feature values for the at least some of the plurality of matching information items to obtain a first set of ranked results; rank at least some of the first set of ranked results using a nonlinear ranking model that nonlinearly combines a second plurality of feature values for at least some of the first set of ranked results to obtain a second set of ranked results; and provide a search response based at least in part on the second set of ranked results; one or more memories coupled to the one or more processors, configured to provide the processors with instructions.
11. A system for generating ranked search results, comprising: one or more processors configured to: receive a plurality of matching information items that match a search request; rank at least some of the plurality of matching information items using a linear ranking model that linearly combines a first plurality of feature values for the at least some of the plurality of matching information items to obtain a first set of ranked results; rank at least some of the first set of ranked results using a nonlinear ranking model that nonlinearly combines a second plurality of feature values for at least some of the first set of ranked results to obtain a second set of ranked results; and provide a search response based at least in part on the second set of ranked results; one or more memories coupled to the one or more processors, configured to provide the processors with instructions. 18. The system of claim 11 wherein the nonlinear ranking model is determined by training.