1. A method for generating ranked search results, comprising: receiving a plurality of matching information items that match a search request; ranking 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; ranking 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 providing a search response based on the second set of ranked results.