13. A machine learning system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a plurality of examples, separable by feature into at least two classes, for distribution to a plurality of workers in a mapreduce process, each worker only receiving examples associated with a first class or a second class, wherein the first class is a positive class and the second class is a negative class, and wherein a worker is selected from the group consisting of a mapper and a reducer; determining whether each example is either associated with the first class or associated with the second class; distributing an example associated with the first class to a first worker of the plurality of workers in the machine learning system, the first worker receiving only examples associated with the first class; and distributing an example associated with the second class to a second worker of the plurality of workers in the machine learning system, the second worker receiving only examples associated with the second class.
13. A machine learning system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a plurality of examples, separable by feature into at least two classes, for distribution to a plurality of workers in a mapreduce process, each worker only receiving examples associated with a first class or a second class, wherein the first class is a positive class and the second class is a negative class, and wherein a worker is selected from the group consisting of a mapper and a reducer; determining whether each example is either associated with the first class or associated with the second class; distributing an example associated with the first class to a first worker of the plurality of workers in the machine learning system, the first worker receiving only examples associated with the first class; and distributing an example associated with the second class to a second worker of the plurality of workers in the machine learning system, the second worker receiving only examples associated with the second class. 14. The system of claim 13 , wherein the plurality of examples are from a learned training dataset.