1. An apparatus comprising: an arbitration unit configured to merge a plurality of coherent memory request streams received from a first plurality of memory requesting agents into at least a first coherent memory request stream; and a memory controller unit configured to select a memory request for placement in a merged memory request stream from a merged non-coherent memory request stream and a first serial memory request stream based on the first coherent memory request stream, the merged non-coherent memory request stream being based on an indicator of a previous memory request selected for placement in the merged memory request stream.