Computer implemented method, system, and computer usable program code for simulating processor operation in a data processing system. An instruction trace is generated, wherein the instruction trace includes markers specified by a user for identifying interval boundaries for at least one interval of the instruction trace. The instruction trace is divided into a plurality of intervals in consideration of the markers, and the plurality of intervals are formed into a plurality of interval clusters, wherein each interval cluster represents one phase of execution of the instruction trace. At least one interval from each of the plurality of interval clusters is selected as a trace sample to provide a plurality of trace samples, wherein each selected interval is of at least a minimum size, a simulation is performed using the plurality of trace samples, and a result of the simulation is provided to the user.