In an example, a method of controlling an additive manufacturing apparatus to generate a three dimensional object comprises processing successive layers of build material so as to form successive layers of a three dimensional object, wherein the processing of each layer is performed within a predetermined layer processing time by a plurality of components. The method may further include selecting, by at least one processor, from a plurality of ancillary tasks, at least one ancillary task to be performed in relation to at least one component of the plurality of components. The method may further include scheduling, by at least one processor, the at least one ancillary task to be performed in relation to the at least one component within the predetermined layer processing time for a single layer or for multiple layers of the build material.